FNA Annual Report 2010 - Technische Universiteit Eindhoven

tue.nl

FNA Annual Report 2010 - Technische Universiteit Eindhoven

TU e

Technische

Universiteit

Eindhoven

University of Technology

Group Physics of Nanostructures

Department of Applied Physics

Group Physics of Nanostructures

Department of Applied Physics

TU/e

Technische

Universiteit

Eindhoven

University of Technology

Annual Report

2010

Where innovation starts


Annual Report 2010

Physics of Nanostructures group (FNA)

Department of Applied Physics

Eindhoven University of Technology

Physics of Nanostructures

NLe 1.06

Den Dolech 2

P.O. Box 513

5600 MB Eindhoven

The Netherlands

Tel.: 040 2475778

Fax: 040 2475724

E-mail: c.a.m.jansen@tue.nl

Website: http://www.fna.phys.tue.nl


2 FNA Annual Report 2010 0.Table of Contents

Editors:

M. Hoeijmakers BSc.

T. Weekenstroo BSc.


FNA Annual Report 2010 0.Table of Contents 3

Table of Contents

1 Introduction....................................................................................................................................... 4

2 Acknowledgments ........................................................................................................................... 7

3 Group Members ............................................................................................................................... 9

3.1 List of Group Members ...................................................................................................................... 9

3.2 Photos of Group Members ............................................................................................................... 11

3.3 Group Photo ....................................................................................................................................... 12

4 Equipment ....................................................................................................................................... 13

4.1 EUFORAC deposition and in-situ analysis facility ...................................................................... 13

4.2 Scanning Probe Microscopy and Laser laboratories .................................................................... 14

4.3 Ex-situ structural, electrical, and magnetic characterization ...................................................... 16

4.4 Nanostructuring ................................................................................................................................ 17

5 Research Projects ............................................................................................................................ 19

5.1 NanoMagnetism ................................................................................................................................ 19

5.2 Spintronics .......................................................................................................................................... 20

5.3 Ultrafast spin dynamics .................................................................................................................... 21

6 Results .............................................................................................................................................. 23

6.1 Nano Magnetism ............................................................................................................................... 24

6.2 Spintronics .......................................................................................................................................... 34

6.3 Ultrafast Spin Dynamics ................................................................................................................... 48

7 Output .............................................................................................................................................. 61

7.1 Publications ........................................................................................................................................ 61

7.2 Presentations ...................................................................................................................................... 62

7.3 Chapters .............................................................................................................................................. 65

7.4 Guest Lectures ................................................................................................................................... 66

7.5 Posters ................................................................................................................................................. 66

7.6 PhD Theses ......................................................................................................................................... 67

7.7 Master Theses..................................................................................................................................... 67

7.8 Internship Reports ............................................................................................................................. 68

7.9 Publicity .............................................................................................................................................. 69

8 Social Events ................................................................................................................................... 75


4 FNA Annual Report 2010 0.Table of Contents


FNA Annual Report 2010 1.Introduction 5

1 Introduction

Dear colleagues and friends of FNA,

This Annual report of our group Physics of Nanostructures (FNA) at the Eindhoven University of Technology

covers facts & figures as well as highlights of the year 2010. In particular it provides a review over research

progress in the fields we are active in, viz. nanomagnetism, spin-polarized transport and ultrafast magnetic

processes. It has become a tradition that two of our master students take the lead in the editorial process – while

all group members contribute to the contents. With the final results we hope to present you an overview of not

only our scientific achievements, but also providing you a glimpse of the atmosphere in our group.

Within the department of Applied Physics, the past year was a special one in which we celebrated our 50 th

anniversary. Many activities were organized, such as a grand party for all members and students of the

department, and an ‘open day’ to which many of our group members contributed. It attracted a large number of

visitors from all ages to our department. Nano-quizzes in which enthusiastic kids were confronted with the

amazing world of nanotechnology, demos of our electromagnetic launching-gear (‘coil-gun’) – shooting quite a

few trays with high precision, and a nano-slide show accompanied by a 3 byte-harddisk demo where young

kids took care of ultimate spinning speeds. Finally, the main star was our EUFORAC (Eindhoven University

nano-Film depOsition Research and Analysis System). Lighted by disco lights (see cover) the audience was

enjoying explanations about atom-by-atom deposition and its applications in spintronics.

The year 2010 was also a year of several lively international meetings organized by the group. End of June we

hosted the joint Dutch-Korean meeting on Spintronics. The lectures in the Guus Hiddink hall, and a trip to the

catacombs of the PSV-stadium (where quite a few Korean players were starring at the exhibitions), may have

been particularly memorable for our guests. Early September, we organized (co-chaired by Peter Bobbert from

our partner group at the department) the 3 rd International Conference on Spins in Organic Semiconductors in

the historical Trippenhuis along the canals in downtown Amsterdam. The conference marked the end of the

first phase of our Vici-program on organic spintronics. It attracted a record number of participants from all over

the world, and illustrated the enormous development in the new field of research – combining issues in

spintronics and organic electronics.

As to scientific progress, the year 2010 showed new exciting results in fields of research we are traditionally

strong in over the past years, but also important breakthroughs in new areas. Here, in particular, I want to

mention our work in domain wall motion, and first successes in writing truly ferromagnetic nanostructures

using our focused electron-beam induced nano-pencil. Thus, students produced our group’s logo with a 300 nm

wide font of free standing ferromagnetic letters (see cover); results obtained in intense collaboration with FEI

company. Among other new research programs, the national FOM program on ‘Controlling Spin Dynamics in

Magnetic Nanostructures’ got a real start, our collaboration with Hitachi Global Storage Technologies (San Jose,

CA) got formalized in the framework of a joint research effort, and our collaboration with our TU/e

distinguished professor Stuart Parkin (IBM Almaden Research Center at San Jose, CA) will gain momentum

again with a new PhD project. Finally, after several years of preparation, we’re particularly pleased about

having obtained new funding opportunities for nano-research at TU/e within the Dutch NanoLab, and to

welcome the final start-up of the new Dutch nanotechnology research program NanoNextNL. The latter is the

successor of the NanoNed program, which may be familiar to some of you.

Looking back the year, we saw a lot of new PhD/master/bachelor students coming and leaving the group, each

of them leaving social and scientific footprints. In particular we had three PhD students getting their doctoral

degree, Wiebe Wagemans, Francisco Bloom, and Reinoud Lavrijsen (early 2011) – each of them producing really

high quality PhD theses and impressive publications scores. Without having the opportunity mentioning all of

the other students, researchers, non-scientific staff members, as well as external collaborators, I do want to

acknowledge their important contributions to our successes.


6 FNA Annual Report 2010 1.Introduction

Finally, the news of 2010/2011 that is going to have the most significant impact on the future of the group

regards our housing. Early 2011, the final decision was announced that our department will move to an entirely

new building that will be constructed in the framework of the ‘Campus 2020’ project. Although beyond doubt it

is something that is going to ask a lot of efforts from all our group members, it will also provide us with unique

opportunities. While you are reading this Annual report, we are shaping our group’s future infrastructure. We

are looking forward continuing our stimulating interaction with you also within the future setting.

Best regards,

Bert Koopmans


FNA Annual Report 2010 2.Acknowledgments 7

2 Acknowledgments

Department of Applied Physics of TU/e (Peter Bobbert, René Janssen, Martijn Kemerink, Paul

Koenraad, Ton van Leeuwen, Thijs Meijer, Leo van IJzendoorn, Menno Prins, Peter Nouwens)

Department of Chemical engineering and Chemistry of TU/e (Martijn Wienk)

Department of Electrical engineering of TU/e (Siang Oei, Erik Jan van Geluk, Barry Smalbrugge, Meint

Smit, Tjibbe de Vries)

AGH Krakow (Maciej Czapkiewicz, Tomasz Stobiecki)

CEMES-CNRS Toulouse (Etienne Snoeck)

CNRS Paris (Gregory Malinowski)

FEI Company (Hans Mulders, Piet Trompenaars)

FIAT Research (Daniele Pullini)

Hitachi Global Storage Technologies, San Jose (Paul van der Heijden, Jeff Childress, Young-Suk Choi)

Holst Centre (Herman Schoo, Jo De Boeck)

IBM Almaden Research Center (Stuart Parkin, Luc Thomas)

IFW Dresden (Sabine Würmehl)

Indian Institute of Science Bangalore, India (Anil Kumar)

Inha University (Chun-Yeol You)

IMEC at Leuven (Wim Van Roy, Liesbet Lagae, Jan Genoe, Koen Weerts)

ISMN-CNR Bologna (Alek Dediu)

Kavli Institute of NanoScience at Delft (Emile van der Drift, Arnold van Run, Anja van Langen)

Korea University (Kungwon Rhie)

Kyoto University (Teruo Ono)

Leeds University (Chris Marrows)

LLG Micromagnetics Simulator (Mike Scheinfein)

Max-Planck-Institut für metallforschung (Manfred Fähnle, Daniel Steiauf)

MIT Cambridge (Jagadeesh Moodera)

MESA+, University of Twente (Ron Jansen, Vishwas Gadgil)

NXP (Friso Jedema, Michiel van Duuren, Fred Roozeboom)

Omicron NanoTechnology GmbH (Marcus Maier, Dieter Pohlenz, Joerg Seifritz)

Philips Research (Reinder Coehoorn, Hans Boeve, Menno Prins, Dago De Leeuw, Denis Markov)

Polish Academy of Sciences at Warsaw (Tomasz Story, Grzegorz Karczewski)

Radboud University at Nijmegen (Rob de Groot, Gilles de Wijs, Jisk Attema, Theo Rasing, Andrei

Kirilyuk, Alexey Kimel)

Royal Institute of Technology, Stockholm (Stefano Bonetti, Johan Åkerman)

Russian Academy of Science, St. Petersburg (Yurii Vladimirovich Trushin)

SmartTip Probe Solutions (Roeland Huijink, Daan Bijl)

Sogang University (Myung-Hwa Jung)

SPECS Nanotechnology (Ad Ettema)

Tecnische Universität Dresden (Karl Leo)

TNO Science and Industry, Delft (Emile van Veldhoven, Diederik Maas)

University of Alabama (Patrick LeClair, Tim Mewes)

University of Antwerp (Etienne Goovaerts, Hans Moons)

University of Goettingen (Markus Muenzenberger)

University of Iowa (Markus Wohlgenannt)

University of Kaiserslautern (Burkard Hillebrands, Martin Aeschlimann, Mirko Cinchetti, Tobias Röth)

University of Konstanz (Mathias Kläui, Philipp Möhrke, Ulrich Rüdiger)

University of Leeds (Serban Lepadatu, Chris Marrows, Brian Hickey)

University of Mainz (Claudia Felser, Gerhard Fecher, Benjamin Balke, Christian Blum)

University of Zaragoza (Rosa Córdoba Castillo, José Maria De Teresa, Manuel Ricardo Ibarra García,

Frank Schoenaker)

Uppsala Unitversity (Björgvin Hjörvarsson)

Utrecht University (Rembert Duine, Erik van der Bijl)

Yacht (Veronica den Bekker-Tiba)


8 FNA Annual Report 2010 2.Acknowledgments


FNA Annual Report 2010 3.Group Members 9

3 Group Members

3.1 List of Group Members

Scientific Staff

Prof. dr. B. (Bert) Koopmans (group leader)

Prof. dr. ir. H.J.M. (Henk) Swagten

Dr. J.T. (Jürgen) Kohlhepp

Dr. O. (Oleg) Kurnosikov

Prof. dr. W.J.M. (Wim) de Jonge (emeritus)

Guests

Prof. S.S.P. (Stuart) Parkin (Distinguished professor at TU/e)

Technical Staff and Operators

G.W.M. (Gerrie) Baselmans

Ir. J. (Jeroen) Francke

Ing. J.J.P.A.W. (Jef) Noijen

Dr. B. (Beatriz) Barcones Campo

Secretary

C.A.M. (Karin) Jansen

Post-Doctoral Staff

Dr. E. (Elena) Murè since 01-06-‘10

Dr. D. (Daowei) Wang since 01-09-‘10

PhD Students

Ir. W. (Wiebe) Wagemans until 14-06-‘10

F. (Francisco) Bloom MSc. until 22-11-‘10

Ir. P. (Paul) Janssen

Ir. K.C. (Koen) Kuiper

Ir. R. (Reinoud) Lavrijsen

A.J. (Sjors) Schellekens MSc. since 01-03-‘10

J.H. (Jeroen) Franken MSc. since 15-03-‘10

Master Students

A.J. (Sjors) Schellekens until 11-02-‘10

J.H. (Jeroen) Franken until 12-02-‘10

F.J. (Frank) Schoenaker until 17-02-‘10

C.O. (Can) Avci until 05-08-‘10

M.J.M. (Mathijs) van Schijndel until 18-10-‘10

R. (Rik) Paesen until 19-10-‘10

P.E.D. (Paul) Soto Rodriguez MSc. until 03-11-‘10

G.C.F.L. (Geerit) Kruis until 16-12-‘10

T.H. (Tim) Ellis

N. (Niels) de Vreede

M. (Matthijs) Cox since 01-02-‘10

S. (Sükrü) Hasdemir since 04-02-‘10

M. (Mark) Hoeijmakers since 15-06-‘10

T. (Tim) Weekenstroo since 01-09-‘10


10 FNA Annual Report 2010 3.Group Members

Bachelor Students

M. (Mark) Herps until 26-01-‘10

F.H.A. (Frank) Elich from 22-04-’10 until 11-10-‘10

W. (Wouter) Verhoeven from 11-05-‘10 until 07-12-‘10

Ing. C. (Christiaan) Otten since 07-09-‘10

J. (Jeroen) de Groot since 11-10-‘10


FNA Annual Report 2010 3.Group Members 11

3.2 Photos of Group Members

Beatriz Barcones Campo Gerrie Baselmans Francisco Bloom Jeroen Francke

Jeroen Franken Karin Jansen Paul Janssen Wim de Jonge

Jürgen Kohlhepp Bert Koopmans Koen Kuiper Oleg Kurnosikov

Reinoud Lavrijsen Elena Murè Jef Noijen Henk Swagten


12 FNA Annual Report 2010 3.Group Members

Sjors Schellekens Wiebe Wagemans Daowei Wang

3.3 Group Photo


FNA Annual Report 2010 4.Equipment 13

4 Equipment

Research in the group Physics of Nanostructures is aimed at the engineering and investigation of functional

nanostructures. Current emphasis is on structures and devices with application potential in the field of

spintronics, (magnetic) data storage, and sensors. A state-of-the-art infrastructure for preparation and

manipulation, as well as in-situ and ex-situ characterization of nanostructures, is available.

4.1 EUFORAC deposition and in-situ analysis facility

The group Physics of Nanostructures is equipped with a state-of-the-art deposition and analysis facility

EUFORAC, the Eindhoven University nano-Film depOsition Research and Analysis Center.

In EUFORAC a complementary cluster of ultra-high vacuum (UHV) deposition and analysis facilities are

present, and exists of:








MEMULA: Vacuum Generators V80 M MBE: The MEMULA (MEtallic MUltiLAyers) is a general purpose

MBE for deposition of (magnetic) metallic multilayer systems. It features: a base pressure below 10 -11 mbar;

7 deposition sources (4 Knudsen cells, 3 e-guns); and wedge growth and shadow mask evaporation

(roughly < 50 micron resolution)

CARUSO: Kurt J. Lesker UHV sputter facility: CARUSO, the Chamber for ARtificial Ultra-high vacuum

Sputtered nanOstructures, is a dedicated, computer-controlled sputter coater manufactured at Kurt J.

Lesker Co. Vacuum Products, and is connected to the Vacuum Generators V80 M MBE (MEMULA) as

shown on the left side of the picture. The system is configured for sputter-down deposition, using oil-free

diaphragm, molecular drag, and cryopumps.

XPS, AES, RHEED, LEED: UHV chambers for in-situ analysis of the deposited layers with XPS, AES,

RHEED, and LEED.

Omicron-1 STM: A chamber for room-temperature scanning tunneling microscopy using the standard

Omicron-1 system.

Plasma oxidation: A chamber for DC/RF glow discharge oxidation of metal layers (such as Al), in

particular to serve as a barrier in magnetic tunnel junctions. The system includes in-situ differential

ellipsometry and an automized dosage system for reproducible oxidation of ultrathin films.

Organic layer deposition: A UHV chamber for deposition and optical thickness control of hybrid

nanostructured systems of metallic and organic materials.

Glovebox: Facility for measuring (organic) samples in an oxygen and water free environment with options

to measure with a modulated magnetic field and with temperatures down to 10 K.


14 FNA Annual Report 2010 4.Equipment

4.2 Scanning Probe Microscopy and Laser laboratories

A series of inter-communicating labs for low-temperature STM and UHV deposition, basic AFM and MFM,

femtosecond pulsed laser set-ups covering a broad spectral range, and a variety of magneto-optical and other

optical characterization techniques, including SNOM.

Multiprobe LT STM surface analysis UHV system

The UHV system is used for sample preparing, thin film deposition and in-situ analysis in low temperature

STM. The LT-STM is also used for a magnetic characterization using spin-polarized tunneling as well as for

single atom manipulation. It consists of:




Omicron LT-STM: With a basic pressure of 5.10 -11 mbar and can reach temperatures down to 4.5 K, with a

magnetic field at the sample position up to 200 Oe. Also, there is optical access to the sample.

Omicron MBE chamber: Consisting of three e-beam evaporation cells for MBE deposition, LEED, and a

manipulator with sample heating-cooling facilities. Options to equip the chamber with AES/XPS, RHEED,

and MOKE are being investigated.

Preparation chamber: The chamber consists of a tip preparation tool, an ion sputtering gun, and a

manipulator with sample heating facility (up to 1200 K). In future, the chamber will be equipped with

sputtering deposition sources and k-cells.

TSUNAMI ultra-short pulse lab

High power CW Spectra Physics Millennia V pump laser for pumping the:

Spectra Physics TSUNAMI fs/ps Ti:Sapphire laser: Mode-locked Ti:Saphire pulsed laser (80 MHz, sub 50

fs, and 700 - 850 nm optics set), with picosecond option.

AvTech electrical pulse generator: Generator for pulses with risetime < 100 ps and repetition rate up to 80

MHz.


Set-ups for TR-MOKE: A controllable time delay between the pump (current/field) pulse and the probing

laser pulse gives us a time-resolved measurement of the polarization change of the incident laser beam due

to the change of magnetization of the reflecting ferromagnetic structure. The TR-MOKE consists of a 2x300

mm mechanical delay line (delay up to 3 - 6 ns); 50 kHz Photo-elastic modulator; and double-modulation

configurations for < 10 -7 rad polarization sensitivity.


FNA Annual Report 2010 4.Equipment 15

TSUNAMI - OPAL lab

High power (10 W) CW Spectra Physics Millennia X pump laser for pumping the:





Spectra Physics TSUNAMI fs Ti:Sapphire laser: 700-1000 nm optics set and 70 fs pulse duration; lock-toclock

option for active synchronization with OPAL or second TSUNAMI in Ne 0.05.

Spectra Physics OPAL laser: Optical Parametic Oscillator with 1050 -1350 nm signal optics set.

Spectra Physics Frequency Doubler: Extending the wavelength range to 350 - 1350 nm.

TiMMS set-up: For measuring picosecond spin-dynamics in semiconductors, with a 100 mm mechanical

delay line and a 50 kHz photo-elastic modulator. Experiments can be performed at 5 K when performed in

Ne 0.09.

MOKE lab





Homemade RT MOKE magnetometer:

For measuring the Magneto Optical Kerr

Effect of thin magnetic films at room

temperature. With the use of a microscope

objective the measurement of magnetic

structures less than 1μm in size is possible.

Photo- and electroluminescence:

Oriel spectrometer with cooled Andor

CCD camera (- 70 o C).

Oxford flow cryostat for variable

temperature MOKE and TiMMS:

Cryostat for performing experiments at

temperatures down to 5 K.

Evico Magnetics wide-field Magneto-

Optical Kerr-Microscope:

Room-temperature visualization of

magnetic domains down to the resolution

limit of optical microscopy.

MFM/AFM

Variable temperature for measuring the

temperature dependence of magnetic domains.

Magnetic field for applying an external

magnetic field during MFM measurements.

Wide-field Magneto-Optical Kerr-Microscope

.


16 FNA Annual Report 2010 4.Equipment

4.3 Ex-situ structural, electrical, and magnetic characterization

Stand-alone equipment for ex-situ characterization, which includes X-ray diffraction, various cryostats (He3,

He4) for electrical conductance and magnetoresistance measurements, SQUID and MOKE magnetometry, NMR,

and Mössbauer spectroscopy.

Sorption-pumped He-cryostat

Oxford Heliox VL for current-voltage measurements at 0.3 K in fields up to 8 T.

NMR and SQUID laboratory



SQUID: Superconducting Quantum Interference Device (Quantum Design), to measure the magnetic

moment of a sample for temperatures between 5 and 400 K at fields up to 5 T.

NMR: The group FNA runs a sensitive, home-built Nuclear Magnetic Resonance (NMR) apparatus,

dedicated to research the structural properties of ferromagnetic materials (especially cobalt). The set-up is

phase-coherent, frequency tuned from 100-400 MHz, uses fields of 0-5.5 T, can measure at temperatures

down to 2 K, and has a sensitivity of better than 0.1 monolayers of Co. Setup is comprised of two cryostats;

one high magnetic field bath cryostat and one variable temperature flow cryostat.

Magnetoresistance laboratory

MR-setup: Home-built magnetoresistance setup operating at temperatures down to 2 K, fields up to 1.2 T,

and suitable to measure a wide range of input impedances (up to 10 giga-ohms).


Probe station: Easy access setup to perform room-temperature magnetoresistance measurements on nonstandard

sample geometries. Adaptable to perform Current-In-Plane Tunneling (CIPT) experiments for

quick characterization of (new) materials for MTJs.

X-Ray laboratory

The X-Ray Diffraction (XRD) setup is a commercial Philips X'Pert system, using Bragg diffraction of Cu K-alpha

radiation for determination of the lattice parameters and texture of thin films. It is also used for calibrating film

thicknesses by reflection of X-rays coming in at very low angles to the sample surface.

ESR, Domain wall motion setup, and "mini-MR/MOKE" laboratory




ESR: Variable temperature Electron Spin Resonance is used to investigate a variety of magnetic materials,

such as thin film magnetic layers and magnetic insulators, focusing mostly on magnetic anisotropy.

Domain wall motion setup: Variable temperature (2 – 400 K) and high frequency (up to 6 GHz) magneto

resistance (MR) measurement setup equipped with an electromagnet capable of applying fields up to 1.2 T

at a variable angle relative to the sample plane. The setup uses chip carriers which are wire bonded to

structured samples. The setup is used to measure domain wall velocities in perpendicular magnetized

samples and to perform MR measurements in structured samples.

Mini-MR/MOKE: Simple fast-entry setup to quickly determine the resistance of a magnetic thin film

device as a function of magnetic field at room temperature. This setup has been upgraded to measure the

Magneto Optical Kerr effect of thin magnetic films at room temperature.


FNA Annual Report 2010 4.Equipment 17

4.4 Nanostructuring

The nanostructuring facilities consist of a dualbeam system (FEI Nova NanoLab 600i) with focused ion and

electron beams and an Ion Beam Miller (Unilab IonSys 500). Both devices belong to the NanLab network. The

group also has access to the Spectrum Cleanroom where other fabrication techniques are available.

See http://web.phys.tue.nl/en/the_department/department_staff/clean_room/ for more information.

FEI Nova 600i

Inside the vacuum chamber of this system, characterization, fabrication and manipulation can be performed in

the nanoscale regime. Among other typical experiments, the machine can perform x-sections for direct

inspection or X-ray spectroscopy analysis, TEM lamella fabrication, circuit modification and mask repair.








Scanning Electron Microscope: Field emission gun filament, resolution of 1.1 nm at 15 kV, accelerating

voltage range 200V-30 kV, and maximum current 20 nA. The electron source can be either used for imaging

or for deposition of different precursors (EBID).

Focused Ion Beam: Ga liquid ion source, resolution of 7 nm, accelerating voltage range 2kV-30kV and

maximum current 20 nA. The ion source is used for imaging, patterning and deposition of different

precursors (IBID).

Electron Dispersive X-Ray detector: Si(Li) type detector with SUTW. Energy resolution of 136 eV capable

of detection of every element down to and including Be. Combined with the scanning unit, can be used not

only to acquire single spectra but also to produce elemental mappings.

Gas Injection Systems: Different gases can be introduced in the vacuum chamber for the purpose of

deposition or to help ion sputtering. At the moment only deposition gases are installed in the system,

mostly metal based for low resistivity pad fabrication.

MeCpPt(IV)Me3 - Pt deposition

W(CO6) - W deposition

TEOS - insulator deposition for high resistivity pads and lines, device edit and electrical isolation

C10H8 - C deposition for protective layers and large area coating

Fe2(CO)9 - Fe deposition for magnetic device fabrication

Kleindieck Nanomanipulators: Two small motors can be installed inside the vacuum chamber allowing

small elements to be manipulated and insitu electrical measurements to be performed.

Electron Beam Lithography: A RAITH system coupled to the dualbeam permits the use of the scanning

electron microscope as an electron beam lithography tool. This system adds to the dual beam extra tools

like a CAD software editor and alignment software which permits the writing of complicated features and

the possibility to fabricate designs with multi lithographic steps. The added system can work either with

the electron or the ion beam.

Detectors: The interaction of the electron and ion beams with matter produce different products, in the

system both secondary and backscattered electrons can be collected to produce an image. Furthermore,

secondary ions can be directly imaged with a CDEM detector. It is also possible to study TEM lamellas with

an STEM detector, producing Bright Field, Dark Field and High Angle Dark Field images.


18 FNA Annual Report 2010 4.Equipment

Unilab IonSys 500

With this system it is possible to etch by Ar ion milling or deposit SiO2 layers, transferring patterns previously

defined with a mask by Electron Beam or UV Lithography.





Ion beam source: For ion beam milling using inert gases. Maximum RF power up to 300 W. Homogeneity

of collimated ion beam +/-10 % over 30 mm diameter.

Magnetron sputter source: For SiO2 deposition.

Cryo thermostat: Temperature range -25


FNA Annual Report 2010 5.Research Projects 19

5 Research Projects

The research at FNA can be categorized into three main fields: Nanomagnetism, Spintronics, and Ultrafast Spin

Dynamics. Within each of these areas a number of projects are active, supported by a wide range of

organizations. In this chapter a brief overview of the projects is listed. Some highlights of recent results can be

found in Chapter 6.

5.1 NanoMagnetism

‚Focused electron-beam induced deposition of Fe‛ (Subproject under EMM.6474)

TU/e research priorities

PhD student: R. Lavrijsen EBID of ferromagnetic nanostructures

‚Sensing and switching of 'hidden' nanomagnets‛ (09PR2715)

FOM projectruimte 2009

Staff member: O. Kurnosikov Nanomagnetism & STM

PhD student:

Vacancy

‚Nanowire manipulation for sensing and spintronics‛

Nanonext NL 2010, Program advanced nano-electonic devices

PhD student: Vacancy Collaboration with Holst Centre

Student projects

MSc projects: C.O. Avci Near surface quantum wells induced by buried nanoparticles in

metals

T.H. Ellis

F.J. Schoenaker

Electron Beam Induced Deposition of Iron (collaboration with FEI)

Exploring the fabrication of ferromagnetic nanostructures by EBID

(collaboration with FEI)

T. Weekenstro NMR on Heusler alloys (collaboration with Hitachi: Global data

storage)


20 FNA Annual Report 2010 5.Research Projects

5.2 Spintronics

‚Engineering of nanostructured magnetic multilayers for generic MR devices‛ (EMM.6474)

Flagship NanoSpintronics with NanoNed / NanoImpulse

PhD student: R. Lavrijsen Co/Pt based structures for Spin-Transfer Torque devices

‚Engineering and integration of electrical domain wall memory devices‛

Nanonext NL 2010, Program advanced nano-electronic devices

PhD. student: Vacancy Collaboration with IMEC Leuven

‚Spin engineering in molecular devices‛ (ETF.6628)

NWO-Vici 2005 (Koopmans)

PhD students: F. Bloom Investigating organic magnetoresistance

W. Wagemans Organic spintronics

P. Janssen Novel device options in organic spintronics

‘Chasing the spin in organic spintronics’

NWO - Nano 2011

PhD student: Matthijs Cox Chasing spin in organic spintronics (starting April 2011)

‚Novel methods to drive domain walls‛ (08SPIN10)

FOM Program 109 - Controlling spin dynamics in magnetic nanostructures

PhD student: J. Franken Novel methods to drive domain walls

Student projects:

MSc projects: A.J. Schellekens Exploring spin interactions in organic semiconductors

M.J.M. van Schijndel Modelling spin transport through organic layers

J. Franken Domain wall motion in perpendicularly magnetized ultrathin

Pt/CoFeB/Pt films

G.C.F.L. Kruis

LLG simulations of racing domain walls

N. de Vreede Spin torque oscillators

P.E.D. Soto Rodriguez Nano-stencil devices for spin-transfer torque switching

M. Cox Tuning spin interactions in organic semiconductors

M. Hoeijmakers Domain wall resistance in perpendicular magnetized materials

BSc projects: C. Otten The flip-chip: A novel way of fabricating layered organic

devices

W. Verhoeven Frequency dependence of organic magnetoresistance


FNA Annual Report 2010 5.Research Projects 21

5.3 Ultrafast spin dynamics

‚Femtosecond spin transfer‛ (08PR2654)

FOM projectruimte 2008

Postdoc: D.W. Wang Ultrafast magneto-optics and spin transfer

PhD student: K.C. Kuiper Ultrafast magneto-optics and spin transfer

‚Ultrafast spin dynamics‛ (08SPIN06)

FOM Program 109 - Controlling spin dynamics in magnetic nanostructures

PhD student: A.J. Schellekens Ultrafast spin-transfer torque dynamics in nanomagnets

‚Dynamics of magnetic domain walls‛

Funded by EPFL Lausanne

Postdoc: E. Murè Ultrafast domain wall dynamics

‚Magnetic logic devices‛

External project at IBM Almaden Research Center, San Jose (CA) (supervisor Prof. Stuart Parkin)

PhD student: R. van Mourik Magnetic logic (Starting march 2011)

Student projects:

MSc projects: R. Paesen Ultrafast domain wall dynamics

BSc projects: M. Herps Gilbert damping in Ga irradiated Pt/CoFeB/Pt

F. Elich Gilbert damping in Pt/Co/

J. de Groot Ultrafast magnetization dynamics in Pt/Co/Pt/Co/Pt multi-layers


22 FNA Annual Report 2010 5.Research Projects


FNA Annual Report 2010 6.Results 23

6 Results

6.1 Nano Magnetism ............................................................................................................................ 24

6.1.1 Diffraction of internal electrons from an atomically-ordered nano-interface in Cu(110) 24

6.1.2 Reduced DW pinning in patterned strips of ................................................. 26

6.1.3 Controlling domain walls by anisotropy engineering using focused He and Ga beams . 28

6.1.4 Focused electron beam induced deposition of Fe – domain wall pinning ......................... 30

6.1.5 Local formation of a Heusler type structure in CoFe-Al current perpendicular to the

plane GMR spin-valves ............................................................................................................. 32

6.2 Spintronics....................................................................................................................................... 34

6.2.1 Tunable Rashba effect: spin-orbit-torque-assisted field-driven domain wall creep ......... 34

6.2.2 Spin–spin interactions in organic magnetoresistance probed by angle-dependent

measurements ............................................................................................................................. 36

6.2.3 Frequency dependence of organic magnetoresistance .......................................................... 38

6.2.4 Exploring Organic Magnetoresistance: An investigation of microscopic and device

properties – PhD Thesis ............................................................................................................. 40

6.2.5 Plastic Spintronics – PhD Thesis .............................................................................................. 42

6.2.6 Microscopic modeling of spin-dependent interactions in organic semiconductors ......... 44

6.2.7 Tuning Spin Interactions in Organic Semiconductors .......................................................... 46

6.3 Ultrafast Spin Dynamics ............................................................................................................... 48

6.3.1 Theory of femtosecond laser-induced magnetization dynamics ......................................... 48

6.3.2 Magnetism and dynamics of Pt / Co / for domain wall devices ................................ 50

6.3.3 Experiments and simulations on femtosecond laser-induced magnetization dynamics . 52

6.3.4 Towards ultrafast studies of current induced domain wall motion ................................... 54

6.3.5 Resolving the genuine laser-induced ultrafast dynamics of exchange interaction in

ferromagnet/antiferromagnet bilayers .................................................................................... 56

6.3.6 Magnetization dynamics in racetrack memory ...................................................................... 58


24 FNA Annual Report 2010 6.Results

6.1 Nano Magnetism

6.1.1 Diffraction of internal electrons from an atomically-ordered nano-interface in Cu(110)

O. Kurnosikov, H.J.M.Swagten, B. Koopmans and C.O. Avci

General Introduction

Recently, it has been shown that a bulk subsurface impurity can induce spatial oscillations of the 3-D electron

local density of states (LDOS), as observed at a surface by scanning tunneling microscopy/spectroscopy (Fig. 1).

In an analogy with an advanced 2-D system such as a quantum corral, it would be also expected that a

structured ensemble of bulk scattering centers may enhance the LDOS oscillations in some locations or in some

directions. A simplest ensemble of scattering centers forming buried nanocluscters can provide an atomically

ordered interface which can induce a specific scattering of electron waves. Although, mainly a flat interface

parallel to a surface is expected to contribute in the variation of LDOS at the surface, the electron wave

scattering from the atomic structure at the inclined interface may also induce extra LDOS variation if the

condition for electron diffraction is fulfilled. Even if the structured interface is buried several nanometers below

the surface, the diffraction effect can lead to the observation of resonances similar to the resonances induced by

reflections from a flat parallel nanofacet (Fig. 2). Such a possibility can be realized for a metallic system

displaying particular bulk electronic properties and a specific interface structure.

Results 2010

We investigated the electron scattering from faceted Ar-filled nanocavities buried several nanometers below a

Cu(110) surface. In this system we observed simultaneously two kinds of resonances: (i) QW resonances formed

by the reflection of electrons from the upper parallel flat facet of the nanocavity, which is consistent with

previous observations, and (ii) initially unexpected resonances originating from electron diffraction from the

interface at the sides of the nanocavity. This inclined interface is intrinsically nanostructured by atomic chains

(Fig. 3), inducing diffraction of electrons back to the probing point. The intensity and sharpness of the effect are

greatly enhanced by exploiting the phenomenon of electron focusing, which in copper is very efficient along the

direction.

We developed a model including a realistic band structure of the Cu substrate and describing the process of

internal electron diffraction from the faceted and atomically ordered nanocavities. Our simulations qualitatively

reproduce all observed features, and elucidate the role of the diffraction process (Fig. 4).

Output

Internal electron diffraction from subsurface atomically-ordered nanostructures in metals

O. Kurnosikov, H.J.M.Swagten, B. Koopmans

Physical Review Letters

(submitted 7 May 2010, under consideration)


FNA Annual Report 2010 6.Results 25

Fig. 1 (a) scheme of experiment; (b) differential

conductance map (46.5 × 42 nm 2 ) measured at 400 mV

and showing many spots of deviating conductance

across the surface induced by subsurface nanocavities;

(c) schematic drawing of the group of the spots

appearing together in (b).

Fig. 2 (a) A typical example of the central and satellite spots in the SDC map of 20×20 nm 2 . The color of the satellite

spots can be different from spot to spot and varies with the bias voltage; (b) the side view of the facetted

nanocavity. The (110) facet (encircled) parallel to the surface induces the central spot. Other locations inducing the

satellite spots are also encircled; (c) Normalized plots of differential conductance measured in the marked points of

(a). Curves 2-4 are shifted for better visibility.

Fig. 3 (a-c) Top parts: different shapes of a subsurface nanocavity, represented by the first atomic layer of Cu at

the interface, top view; bottom parts: corresponding SDC maps (20×20 nm 2 ) simulated with the model. The

dashed encircling in (b) indicates the specific location of the atomic arrangement inducing the corresponding

satellite spot; (d-f) Top parts: (e) is a side view of the diffracting atomic structure corresponding to the encircled

location in (b) and (d,f) are the same for slightly different shapes of the nanocavity. In the bottom: simulation of a

satellite spot (one from the four) induced by the corresponding structure in the top, 4.5×4.5 nm 2 .

Fig. 4 (a) Simulated SDC map (20×20 nm 2 )

induced by a nanocavity with a slight shape

asymmetry and distorted from one side; (b)

Simulated normalized differential

conductance plots in the locations encircled

in (a). The curves are shifted for better

visibility.


26 FNA Annual Report 2010 6.Results

6.1.2 Reduced DW pinning in patterned strips of

R. Lavrijsen, M.A. Verheijen, B. Barcones, J.T. Kohlhepp, H.J.M. Swagten and B. Koopmans

General Introduction

Magnetic domain walls (DW) in magnetic nanowires have attracted much attention due to their application in

field- and current-induced DW logic and magnetic memory devices. More recently, the attention is shifting to

materials with high perpendicular magnetic anisotropy (PMA) resulting in an out-of-plane easy-axis. This high

PMA results in narrow, robust and simple Bloch DWs for which current-induced DW motion/depinning is

predicted to be efficient and also reported to show pure current-induced DW motion and depinning.

Fundamentally, these systems are very interesting since the recently observed Rashba effect can lead to large

spin-orbit torques of the current. Furthermore, the first demonstrations of shift registers have appeared showing

the prospect for devices based on high PMA materials.

Results 2010

The robust, narrow and simple Bloch DWs, however, lead to a strong pinning strength of the DWs on structural

imperfections (pattering induced and/or defects). This has lead to many studies concentrating on the depinning

of a DW from a certain pinning site and the so-called creep motion of DWs at low drive fields or currents. A

major challenge to use the ultrathin PMA films for applications therefore lies in the control of the intrinsic and

extrinsic pinning site density and/or strength for DW's. In the past we have shown that by doping cobalt with

boron a significantly decrease in DW pinning strength was obtained in homogenous films (non-patterned). In

our latest work we have studied the DW velocity by a full electrical-transport measurement technique in

patterned 900 nm wide strips of

or

where FM stands for ferromagnetic Co

. In Fig. 1 we show a SEM image of the used devices including the measurement setup.

In Fig. 2 we show the measured DW velocity as function of drive field in a pure Co and CoB film. As predicted,

we observe that the decreased DW pinning strength as was observed in the homogenous films greatly increases

the DW creep velocity in the patterned strips. This proved that the CoB films are excellent candidates for DW

motion devices.

Output

Reduced DW pinning in patterned strips of Pt/Co68B32/Pt

R. Lavrijsen, M.A. Verheijen, B. Barcones, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans

Appl. Phys. Lett. (accepted 2011)


FNA Annual Report 2010 6.Results 27

4

2 2

1

AC/DC Current Source

high

low

ref out

A

B

A

B

Lock-In #1

ref in

Lock-In #2

ref in

out

out

20 µm

3

A

B

Lock-In #3

ref in

out

Out Sync.

Pulse generator

Ch. 1

Oscilloscope

Ch. 2

Trig.

Ch. 3

Fig. 1 SEM image of used devices and electrical connections. The 900 nm wide magnetic strip (1) is connected (2)

to a AC/DC current source. The pulse line (4) is connected to the output of a pulse generator on one side and

grounded on the other. The three Hall probes (3) are differentially connected to individual lock-in amplifiers

that lock in to the reference frequency of the AC current source. The outputs of the lock-ins are connected to a

oscilloscope where the data acquisition is triggered by the pulse generator.

Fig. 2 (a) Average DW velocity

versus applied field for patterned w =

900 nm wide strips of Pt(4

nm)/FM(0.6 nm)/Pt(2 nm) with

. (b) ln(v) versus

for the same data as

presented in (a). The solid lines are a

fit to the creep law which allow us to

deduce the effect strength of the DW

pinning as indicated by in (b).


28 FNA Annual Report 2010 6.Results

6.1.3 Controlling domain walls by anisotropy engineering using focused He and Ga beams

J.H. Franken, M. Hoeijmakers, R. Lavrijsen, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans, E. van Veldhoven

and D.J. Maas 1

1

TNO Science and Industry, Delft, The Netherlands

General Introduction

The motion of magnetic domain walls (DWs) in small magnetic wires is a very interesting topic, because of the

prospect for new information storage devices such as the magnetic racetrack memory. In our group, DW

dynamics is investigated in materials which have their magnetization direction perpendicular to the sample

plane, such as Pt/Co/Pt. The interaction of current with DWs in these materials is not yet understood and leads

to very interesting fundamental physics (section 6.1.2, R. Lavrijsen). However, before studying the motion of

DWs, you first need to create a DW in a controllable way. Although there are ways to achieve this, e.g. by

thermomagnetic writing with a laser spot or using the Oersted field generated by a nearby current pulse line,

these methods pose restrictions to the experimental environment and sample design. We have developed a

controlled DW injection scheme by tuning the critical parameter in these materials, the perpendicular magnetic

anisotropy (PMA). This is achieved by irradiating a small part of a Pt/Co/Pt wire with a focused ion beam,

which locally reduces the anisotropy. By applying a magnetic field, a domain is first created in the area with

reduced anisotropy, which can then be moved into the non-irradiated region at a tunable field strength.

Although first intended only as an experimental trick to create a DW, we made the interesting observation that

the anisotropy boundary acts as a tunable pinning site for the DW. This leads to exciting new device options, for

example in magnetic memory devices where discrete stopping positions for the DW are required.

Results 2010

Using our recently acquired Kerr microscope, we are able to directly visualize the magnetic switching of

irradiated Pt/Co/Pt strips. Snapshots of the magnetic domain structure upon increasing the external magnetic

field from negative to positive saturation are shown in Fig. 1. We see that the left, irradiated part (indicated by

the shaded area) switches first because of the reduced anisotropy. In the shown sample, surprisingly, the DW

that is nucleated in the irradiated part is not able to move into the right part of the sample and remains pinned

at the boundary (center image). By increasing the field strength further, the pinning is overcome and the DW

swipes through the right part of the structure.

10 µm

M

0 mT

DW

7.3 mT

7.7 mT

External

field

Fig. 1 Kerr images of a Ga irradiated strip at increasing field strength. A DW is nucleated at 7.3 mT, but remains

pinned at the boundary between the irradiated (shaded) and non-irradiated areas. A higher field of 7.7 mT is needed

to move the DW into the latter area, which then switches completely as the DW reaches the end of the strip.


FNA Annual Report 2010 6.Results 29

An interesting behavior was found as a function of the amount of ion irradiation: with increasing ion dose, the

switching field of the left part is decreased as expected, but interestingly, the pinning strength at the boundary

is increasing. This is summarized in Fig. 2a (solid circles), where the injection field (needed to switch the right

part), is plotted as a function of the ion dose in the left part. We see that the injection field can be tuned in two

regimes. Especially interesting is the second, increasing regime, which tells us that the pinning strength at the

boundary scales with the anisotropy difference between the two regions. This observation was well supported

by a simple micromagnetic model.

He + dose (10 13 ions/cm 2 )

0 500 1000 1500 2000

(a) 25

20

He + optimal focus

A

C

15

Ga + optimal focus

B

10

5

Ga + beam blur 200 nm

0

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

H in

(mT)

Ga + dose (10 13 ions/cm 2 )

0.5

(b) out-of-plane in-plane


0.4

K eff,0

K eff


30 FNA Annual Report 2010 6.Results

6.1.4 Focused electron beam induced deposition of Fe – domain wall pinning

R. Lavrijsen, T.H. Ellis, F.J. Schoenaker, B. Barcones, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans, J.M.

DeTeresa, C. Magen, M.R. Ibaraa, P. Trompenaars and J.J.L. Mulders

General Introduction

Focused-electron-beam-induced-deposition (FEBID) to locally create electrical contacts is widely used due to its

versatile application as a mask-less technique with nanometer resolution. In contrast, the use of magnetic

materials, is a relative new area of application due to the inherent low metallic content of FEBID deposits

detrimental to the magnetic properties. Recently, reports have appeared on high-quality Fe deposits up to 95

at.% Fe, however, without a thorough magnetic characterization. Another report shows high-quality FEBID Co

nanowires up to 95 at.% Co content showing high quality magneto-transport properties for deposits with a Co

content higher than 80 at.%. This has resulted in a successful device for a field-induced domain wall motion

study [Pacheco et al. Appl. Phys. Lett. 94, 192509 (2009)] exemplifying the widespread application potential for

FEBID fabricated magnetic nanowires in spintronics.

Results 2010

We have investigated the magnetic properties of Fe nanowires grown by focused-electron-beam-induced

deposition (FEBID) using a Fe2(CO)9 precursor on a FEI Nova Nanolab 600i dualbeam system. The Fe wires

contain up to ~80 at.% Fe as measured by in-situ Energy Dispersive X-ray spectroscopy (EDX). The magnetic

properties are investigated using the Anisotropic Magneto Resistance (AMR) effect and Magneto Optical Kerr

Effect (MOKE) as can be seen in Fig. 1 where the magnetic behavior of a large FEBID Fe is investigated.

Furthermore, we have performed the first pilot experiments using Fe-FEBID for domain wall motion

manipulation in perpendicularly magnetized Pt/Co/Pt layers as can be seen in Fig. 2.

This preliminary

demonstration shows that it is feasible to use the stray fields of the nanomagnets grown by Fe-FEBID in

devices/applications. The possibility to grow free-standing 3D structures in virtually any shape allows for many

more possibilities to shape stable stray fields, such as horse-shoe magnets or other magnetic-flux closure

arrangements. For instance, by simply engineering a structure on top or close to the strip a local DW pinning

landscape can be engineered, and more importantly, without changing the properties of the magnetic strip.

Other exciting applications might be in scanning probe microscopy instruments, magnetic-bead based biosensors,

whenever and wherever local magnetic stray fields are required for nano-scale properties and

functionality.

Output

Fe:O:C grown by focused electron beam induced deposition: Electric and Magnetic properties

R. Lavrijsen, T.H. Ellis, F.J. Schoenaker, B. Barcones, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans, J.M. DeTeresa, C.

Magen, M.R. Ibaraa, P. Trompenaars, J.J.L. Mulders

Nanotechnology 22, 025302, (2011)


FNA Annual Report 2010 6.Results 31

(a)

Kerr Intensity (arb. u.)

*

M

M

-15 -10 -5 0 5 10 15

H (mT)

0

Fig. 1 (a) Hysteresis loop obtained by measuring

the light intensity of wide-field Kerr-microscopy

(longitudinal sensitivity) images as function of field

applied along thelong axis of the structure. The

insets show the Kerr images taken when the sample

is saturated in the applied field direction; the

gray/black scale clearly indicates the magnetic

contrast. (b) Longitudinal Kerr sensitivity image

(dotted arrow) taken at remanence (* in (a)). We

have drawn the flux-closure domain structure

indicated by the solid arrows. (c) Transverse Kerr

sensitivity image (dotted arrow) taken at

remanence (* in (a)).

(b)

(c)

Fig. 2 (a) SEM image of a 1 µm wide Pt/Co/Pt strip where Fe-FEBID pillars with different heights have been

deposited on top indicated by the dashed circles. (b) - (e) Kerr microscopy images of the 1 μm wide Pt/Co/Pt strip

as a function of increasing applied field. The box in (b) indicates the area shown in the SEM of (a). The position

and height of the pillars are indicated by the vertical white dashed lines. An expanding domain can be seen

propagating from the left to the right with increasing field. The DW is pinned due to the stray field of the pillars


32 FNA Annual Report 2010 6.Results

6.1.5 Local formation of a Heusler type structure in CoFe-Al current perpendicular to the

plane GMR spin-valves

S. Wurmehl a , P.J. Jacobs, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans, S. Maat b , M.J. Carey b and J.R.

Childress b

a

now at: Institute for Solid State Research, IFW Dresden, Dresden, Germany

b

San Jose Research Center, Hitachi GST, San Jose, USA

General Introduction

Current perpendicular-to-the-plane giant magneto-resistance (CPP-GMR) read heads are considered as a

follow-up technology for tunnel magneto-resistive (TMR) read heads. CPP-GMR heads exhibit low resistancearea

products and no shot-noise which makes them attractive for sub-50 nm track widths required for recording

densities of 1 Tbit/in 2 and beyond. A crucial issue for the success of such devices is to further enhance the GMR

ratio particularly at room temperature. A successful application of spin-polarized materials in such CPP-GMR

devices requires a detailed knowledge of the interplay between the structure and the magnetic and electronic

properties. Recently a significant improvement of the magneto-transport properties of CPP-GMR devices

consisting of ferromagnetic CoFe alloys was demonstrated by addition of up to 25% Al. In order to understand

this improvement, we started a systematic investigation of the changes in the local atomic environment as a

function of the Al content by means of nuclear magnetic resonance (NMR) measurements. NMR is an ideal tool

for such studies because it probes the local hyperfine fields of the active atoms, which strongly depend on the

local environments.

Results 2010

Fig. 1 shows the NMR spectrum of a sample with roughly 25 at.% Al content. This is also

the composition that was found to exhibit the highest GMR values. If the Co, Fe and Al would form an A2

random bcc alloy, only one broad resonance line located around 190 MHz should be expected. Instead, one

clearly observes six distinct lines with a clear substructure in addition to the expected broad resonance line,

which points to the local formation of a higher degree of order than a pure A2 structure in the film. The distinct

resonance peaks found are in good agreement with the peaks found in bulk single crystal B2 type ordered

Co2FeAl Heusler compound samples, where the peak at 190 MHz corresponds to local environments with

4Fe+4Al neighbors, while higher and lower frequency peaks belong to Fe rich and Al rich environments,

respectively. However, a closer evaluation of the NMR data clearly shows that this alloy sample consists of a

mixture of A2, B2, and L21 contributions, and that the additional substructure observed in the main resonance

lines originates in higher order shell effects.

In Fig. 2 the MNR spectra for samples with different Al contents are summarized, and the

clear trend of decreasing NMR frequencies for increasing Al content up to 22 at.% is demonstrated. These

spectra show again, that the addition of Al to CoFe leads to a drastically different local structure than bcc CoFe,

and the bcc CoFe contributions very quickly become negligible. A clear fingerprint of the resonance lines of a

Heusler type ordering is observed for the 22%, 25%, and 28% samples. Thus, with the addition of Al, CoFe has

the tendency to form a Heusler compound.

In Fig. 3 we demonstrate how the structural results found with our NMR investigations are correlated with the

enhanced GMR values found using such alloys in devices. The CPP-GMR ratio and the formation of a highly

spin-polarized Heusler compound seemingly follow a similar trend upon Al addition. In particular, the highest

GMR ratios are obtained for those Al contents that also show high B2 and L21 type contributions.

in the

B2 type structure is predicted to also conserve the high spin polarization which is found for the L21 structure,

and consequently, the bulk spin-scattering asymmetry in the CPP-GMR spin-valves.


FNA Annual Report 2010 6.Results 33

Output

Local formation of a Heusler structure in CoFe-Al alloys

S. Wurmehl, P.J. Jacobs, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans, S. Maat, M.J. Carey, and J.R. Childress

Applied Physics Letters 98, 012506 (2011)

Fig. 1 Left: 59 Co NMR spectrum of a CoFe-

Al sample with 25 at.% Al, also shown is

the corresponding fit with Gaussian lines

for the different next neighbor (Fe, Al)

environments of the Co nuclei.

Right: Comparison between a random

atom model and the relative areas of each

resonance line in the fit, hinting to the

formation of the local formation of a

Heusler type structure.

Fig. 2

59

Co NMR spectra for several studied

samples.

Fig. 3 Comparison between the CPP-GMR properties and the

contribution of the highly spin-polarized Heusler compound both

as a function of the Al content of the

samples.


34 FNA Annual Report 2010 6.Results

6.2 Spintronics

6.2.1 Tunable Rashba effect: spin-orbit-torque-assisted field-driven domain wall creep

R. Lavrijsen, E. van der Bijl, R.A. Duine, J.T. Kohlhepp, H.J.M. Swagten and B. Koopmans

General Introduction

Manipulation of the magnetization of ferromagnets through torques induced by spin-polarized currents is a

rapidly evolving research field. This is due to the prospect of devices with reduced size and energy

consumption. An actively investigated topic is current-induced magnetic domain-wall motion. Specifically,

perpendicularly magnetized materials are of interest due to narrow and simple Bloch-type domain walls

predicted to enhance the interaction with spin-polarized currents. Here, we demonstrate that current-assisted

field-driven domain wall creep in Pt/Co/Pt is influenced by spin torques due to Rashba spin-orbit coupling, that

primarily affect the domain-wall precession. Surprisingly, the Rashba effect can be tuned by simply changing

the Pt layer thicknesses sandwiching the Co layer providing an alternative origin of the Rashba effect in

Pt/Co/AlOx [Miron et al. Nature Materials 9, 230-234 (2010)]. Our findings may explain contradicting reports in

literature. We expect that the tunability of the Rashba effect will pave the way for new experimental and

theoretical spintronic concepts.

Results 2010

We have shown that by simply changing the relative Pt layer thicknesses in a Pt/Co/Pt stack we can tune the

efficiency and direction of the current induced DW velocity as seen in Fig. 1. This can be explained using the

current distribution in the stack which shows an asymmetry due to the different Pt layer thicknesses leading to

an effective Rashba field as shown in Fig. 2. The presence of the Rashba effect is evidenced by measuring the

perpendicularly applied-field-driven current-assisted DW creep velocity by varying an in-plane field.

Output

Tunable Rahba effect: spin-orbit-torque-assisted field-driven domain wall creep

R. Lavrijsen, E. van der Bijl, R.A. Duine, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans

(In Preparation)


FNA Annual Report 2010 6.Results 35

Fig. 1 (a) Average DW velocity based on 20 measurements plotted as a function of the applied field and ,

(bottom and top x-axis, respectively) and current density (J, the error bars indicate the standard deviation. a Results obtained

on Pt(4 nm)/Co(0.6 nm)/Pt(2 nm), (b) Results obtained on Pt(2 nm)/Co(0.6 nm)/Pt(4 nm). The direction of current relative to

the DWM direction is indicated by the arrows. Please note that the top x-axis scale is non-linear, the lines are a guide to the

eye. The inset in b shows a typical device which are used to measure the DW velocity. All results are obtained at a controlled

constant temperature of T = 300 K.

Fig. 2 Origin of the Rashba field (a) – (b) The direction and magnitude of the effective Rashba field is determined

by the local current density asymmetry at the top and bottom interface of the Co layer. The direction switches

sign between Pt(4 nm)/Co(0.6 nm)/Pt(2 nm) in (a) and Pt(2 nm)/Co(0.6 nm)/Pt(4 nm) in (b).


36 FNA Annual Report 2010 6.Results

6.2.2 Spin–spin interactions in organic magnetoresistance probed by angle-dependent

measurements

W. Wagemans, A.J. Schellekens, M. Kemper, F.L. Bloom, P.A. Bobbert and B. Koopmans

General Introduction

In organic devices, considerable changes in the current have been observed when applying a magnetic field, an

effect called organic magnetoresistance (OMAR). OMAR is generally believed to originate from spin

correlations of interacting charge carriers. The spin character of such pairs is mixed by the random hyperfine

fields, which can be suppressed by an external magnetic field. Gaining a better understanding of the physics of

OMAR will improve knowledge of (spin) transport in organic semiconductors and could help towards possible

applications, for instance in adding the possibility of sensing magnetic fields to cheap organic electronic

devices.

So far, in literature, it has been claimed that OMAR is independent of the orientation of the applied magnetic

field. Indeed, the effect is not just observed for a specific angle between the magnetic field and the current (like

the Hall effect). The models suggested for OMAR, like the electron–hole (e–h) pair model and the bipolaron

model, do not predict any angle dependence of OMAR. We show that changing the orientation of the applied

magnetic field, with respect to the applied electric field, results in a small but systematic change in the

magnitude of OMAR. We show that both anisotropic spin–spin interactions and anisotropic hyperfine fields can

explain the observed effects.

Results 2010

We performed experiments on typical OLED-like devices with tris-(8-hydroxyquinoline) aluminum ( ) as

the active layer. The magnetoconductance (MC) observed for parallel alignment of the magnetic field

with respect to the sample normal is smaller than for the perpendicular case (Fig. 1a). The MC for

intermediate angles shows an oscillation as a function of θ on top of a slowly increasing signal (Fig. 1b).

Vertically plotted is , which was obtained from fitting the curves with a typical `non-Lorentzian' that

is commonly seen in OMAR measurements: , where is the MC at infinite

magnetic field and B0 is the half width at quarter maximum. Within the accuracy of the fits, no change in is

observed. The data can be accurately fitted with a dependence. Additional measurements confirm the

dependence and exclude measurement artifacts to be responsible. It is shown that the angle

dependence is intrinsic to OMAR. By changing the angle of the magnetic field, the processes responsible for

OMAR are apparently modified.

Fig. 1 (a) MC(B) curves measured at 12 V for θ = 0° and 90°, with θ as indicated. (b) MC at infinite field, from fitting

MC(B) curves with a non-Lorentzian, as a function of angle, fitted with


FNA Annual Report 2010 6.Results 37

Fig. 2 Simulated MC for the e–h model with a preferential orientation of the pairs. (a) MC for two orientations of

B with dipole-energy prefactor and exchange-energy prefactor (b) Angle

dependence of the MC at infinite field with and without exchange coupling, fitted with –

The combination of two spin-½ particles has four total spin states: one singlet and three triplets. At zero applied

field, the random hyperfine fields allow mixing between all four states. However, on applying a large external

field, the triplets Zeeman-split in energy and the singlet and only one of the triplet states can mix. This reduced

mixing results in the change in current that we call OMAR. As we observe an angle dependence at large fields,

this mixing between the singlet and one triplet state has to be affected. The mixing at high fields can be altered

in two ways. First, the strength of the hyperfine fields can be different for different orientations of the external

field. Second, there could be a small angle-dependent energy difference between the singlet and triplet state,

caused by spin–spin interactions. The two relevant spin–spin interactions are dipole coupling and exchange

coupling.

By modeling OMAR and including the two different origins (energy splitting or hyperfine coupling), we

showed that both can explain the experimentally observed angle dependence. In Fig. 2, we show the results

using the e–h model with angle-dependent energy splitting. Correct line shapes (Fig. 2a) are obtained. With

only an energy splitting from dipole coupling (Fig. 2b,

), additional features in the angle dependence,

absent in the experiments, are observed, which are removed when also a small exchange coupling is included.

These experiments confirm that OMAR is caused by spin–spin interactions. Two possible origins have been

suggested, which could be distinguished by a smart choice of materials. The investigation of the angle

dependence could provide another way of investigating the spin–spin interactions in organic materials and

might help to further understanding of OMAR.

Output

Spin–spin interactions in organic magnetoresistance probed by angle-dependent measurements

W. Wagemans, A.J. Schellekens, M. Kemper, F.L. Bloom, P.A. Bobbert, and B. Koopmans

(Submitted to Phys. Rev. Lett.)


38 FNA Annual Report 2010 6.Results

6.2.3 Frequency dependence of organic magnetoresistance

P. Janssen, W. Wagemans, E.H.M. van der Heijden, W. Verhoeven, M. Kemerink and B. Koopmans

General Introduction

Due to their relative ease of processing, chemical tunability and possible low costs, organic semiconductors

provide exceptional promise for (future) electronic applications. Recently, it was discovered that the current

through an organic semiconductor, sandwiched between two non-magnetic electrodes, can be changed by

applying a small (~10 mT) magnetic field. This large (up to 20%) magnetoresistance effect is called organic

magnetoresistance (OMAR). The effect can be both positive and negative depending on operating conditions.

Up to now, several models have been proposed to explain OMAR, but the exact origin is still debated. Therefore

novel measurements are needed to discriminate between models.

In this work we investigate the frequency dependence of OMAR on Alq3 (small molecule) and MDMO-PPV

(conjugated polymer) based devices. By this we aim to identify the timescales of the processes involved in

OMAR. Doing so, we ultimately hope to be able to distinguish between the different models proposed for

OMAR, because the processes that are used in the models each occur on different timescales.

Results 2010

To study the frequency dependence of OMAR, we use a superposition of an AC and DC magnetic field. The

magnetoconductance

defined as the relative change in current when applying an external

magnetic field, decreases when the frequency of the AC magnetic field is increased, as is illustrated in Fig. 1.

In addition to the response of the current to an AC magnetic field, we measured the response to an AC voltage,

which is the admittance , where is the conductance and the capacitance. The results for the

capacitance as a function of frequency for different DC voltages are shown in Fig. 2. For a negative

capacitance is obtained for a certain frequency range. A negative contribution to the capacitance is attributed to

the presence of minority carriers. This can be more clearly visualized by plotting the normalized differential

susceptance – th the geometrical capacitance, as can be seen in the inset of Fig.

2. A clear peak in the differential susceptance, shifting to higher frequencies with increasing voltage, is

observed. The position of this peak, and point where the negative capacitance disappears, are related to the

inverse transit time of the minority carriers.

To get a quantitative measure, we fit the frequency dependence of the MC with an empirical function

, where is the characteristic cut-off frequency. In Fig. 3, the cut-off frequency for the

, the frequencies where the capacitance is 95% of its low voltage value ( ) and the peak in differential

susceptance ( are plotted as a function of voltage. A clear correlation between the frequencies is observed,

indicating that the (transit time of the) minority carriers play(s) a crucial role in the frequency dependence of

OMAR.


FNA Annual Report 2010 6.Results 39

Normalized MC

10 0 12 V

11 V

10 V

10 -1

10 -2

9 V

8 V

Capacitance (nF)

3

7 V

95%

10 0 10 1 10 2 10 3

7 V 8 9 10 11 12 V

2

Frequency (Hz) 10 0 10 1 10 2 10 3 10 4 10 5

6

5

4

< 7 V

f C

(8 V)

Normalized B (-)

1.0

0.5

0.0

7 V

Frequency (Hz)

8

9

10 11 12 V

10 0 10 1 10 2 10 3 10 4

Frequency (Hz)

Fig 1. Normalized MC as a function of frequency for

different voltages for a 100 nm thick Alq3 device. The

lines are a fit to

,, where

represents the cut-off frequency for the MC.

Fig 2. Capacitance as a function of frequency for

different dc bias voltages. The frequency where C is

95% of the 0V value is indicated for the 8V

measurement. The inset shows the normalized

differential susceptance as a function of frequency

for different voltages.

Frequency (Hz)

10 3

10 2

10 1

10 0

10 4 Alq 3

(100 nm)

f C

f 0

f B

MC decreases

"single carrier"

"double carrier"

7 8 9 10 11 12

Voltage (V)

MC (arb.u.)

10

1

200 nm Alq 3

Measurement

Model

10 0 10 1 10 2 10 3

Frequency (Hz)

Fig 3. Characteristic cut-ff frequency (squares), the

frequency where the capacitance is 95% of its low

voltage value, (triangles), and the frequency for the

peak in differential susceptance (circles) as a

function of voltage for the 100 nm Alq3 device.

Fig 4. A typical measurement for a 200 nm thick

Alq3 device at 16V (open squares) and the simulation

results of our device model (solid line).

In conclusion, we have shown frequency-dependent OMAR measurements using a superposition of a DC and

AC magnetic field. We observe a decrease of MC with increasing frequency. Using admittance spectroscopy we

show that this decrease is related to a transition from a double carrier to single carrier device. Currently device

simulations are performed to gain further insight in the transport in these devices. A typical result for a

simulation where the magnetic field causes a decrease in minority carrier mobility is shown in Fig. 4, yielding a

qualitative match between experiment and model.

Output

Frequency dependence of organic magnetoresistance

W. Wagemans, P. Janssen, E. H. M. van der Heijden, M. Kemerink, and B. Koopmans

Appl. Phys. Lett. 97, 123301 (2010)


40 FNA Annual Report 2010 6.Results

6.2.4 Exploring Organic Magnetoresistance: An investigation of microscopic and device

properties – PhD Thesis

Francisco Bloom

General Introduction

Recently there has been much interest in combining the fields of organic electronics and spintronics. This has

been motivated by the fact that low atomic mass of organic materials are predicted to have long spin lifetimes.

Also, spintronic devices could benefit from the chemical tunability, ease of fabrication, and mechanical

flexibility of organic semiconductors. The nascent field of organic spintronics has already presented many new

phenomena which must be explained with novel physics, here we explore one of these phenomena, organic

magnetoresistance (OMAR).

OMAR is a room temperature spintronic effect in organic devices without any magnetic materials. OMAR is a

large change in resistance (up to 25%) at low magnetic fields (20mT). OMAR represents a scientific puzzle since

no traditional magnetoresistance mechanisms can explain the combination of properties listed above. Another

one of the remarkable properties of OMAR is that the sign of the MR can change based operating conditions of

the device, like temperature and voltage. In this dissertation we focused in particular on resolving the origin of

the sign change since understanding this unique property should be a major step in unraveling the microscopic

origin of OMAR.

Results

We have explored the properties of the sign change experimentally with bipolar semiconducting small molecule

and polymer devices, in which we observed sign changes as functions of voltage and temperature. These

devices showed a strong correlation between the sign change and the onset of minority charge carrier injection

and we could describe the lineshape and MR(V) behavior as a superposition of two MR effects of opposite sign.

From this work we concluded the separate MR effects were from the mobilities of holes and electrons having

different responses to magnetic fields, which is best described by the bipolaron model for OMAR.

To test this conclusion, we employed analytical and

numerical device models assigning separate

magnetomobilities to holes and electrons. The models

show, counter-intuitively, that in the case when the

minority charge carrier contact is injection limited, a

decrease in minority charge carrier mobility increases the

current. This is a result of the minority carrier contact

acting like a constant current source, and of the

compensation of the majority carrier space charge by the

oppositely charged minority carriers. We show that these

models describe the observed MR(V) behavior very well,

and if one assumes the magnetic field acts to reduce the

mobility of electrons and holes, we observe that our

models can reproduce all the sign changes observed in

literature. The device model also predicts how different

device parameters affect the observed MR, to test its

predictions we performed experiments in which we

increased the charge recombination by dye doping the

organic active layer, we also observed how changing the

charge injection by altering the organic semiconductor/

metal contacts experimentally compared with the device

model.


FNA Annual Report 2010 6.Results 41

The fact that the current can increase when the minority carrier mobility decreases may explain the fact that in

experiments the magnitude of the negative MR features has been much larger than the positive MR features,

even though, microscopically, the bipolaron model predicts the opposite. Therefore, the presence of both signs

of magnetoresistance may be related only to the device physics and not to the microscopic mechanism which

causes OMAR.

Output

Organic Magnetoresistance –An investigation of microscopic and device properties

Francisco Bloom

PhD thesis, November 2010


42 FNA Annual Report 2010 6.Results

6.2.5 Plastic Spintronics – PhD Thesis

Wiebe Wagemans

General Introduction

In this thesis, both theoretical and experimental results on organic magnetoresistance (OMAR) and spin

polarized transport have been presented. Contributions have been made to a new model for OMAR and new

type of experiments have been performed that have added further insights to the puzzle of OMAR. The limiting

role of the hyperfine fields on spin-polarized transport has been investigated theoretically, providing an

explanation for the experimentally observed magnetoresistance curves of organic spin valves and providing

suggestions for future experiments.

Results

We investigated the use of organic semiconductors in spintronics applications, where next to the charge of the

electrons, also their spin is utilized. Organic semiconductors have several advantages that make them

interesting for these applications. They are relatively cheap, are easy to process, and can be chemically adapted.

Two different, but related, topics that combine organic materials with spintronics have been studied both

experimentally and theoretically.

The first topic is the recently discovered OMAR effect. OMAR is observed in a wide range of organic materials,

from small molecules to polymers. It is believed that OMAR originates from the interactions of a pair of charge

carriers (for instance, electron–electron, hole–hole, or electron–hole). More specifically, from the relative

orientation of the spins of these two charges. Without an external magnetic field, small intrinsic magnetic fields

in the organic layer (resulting from hyperfine coupling to nuclear spins) randomize the orientations of the two

spins. This allows a change from a spin configuration that is less favorable for the current into a more favorable

configuration. However, applying a magnetic field larger than these hyperfine fields results in a strong

reduction of this spin randomization or spin mixing, causing a pair to remain locked in a less favorable spin

configuration.

Although there is agreement on the crucial role of hyperfine fields, the exact mechanisms behind OMAR are

still heavily debated. Several models were proposed in literature explaining OMAR in terms of different charge

pairs. We investigated a model based on pairs of equal carriers, called the bipolaron model. We used an

elementary model of two neighboring sites, where, depending on the spins, one carrier might be preventing

another one to pass. With this theoretical model we were able to successfully reproduce several characteristics

of OMAR. Both a decrease and an increase in current, as found in experiments, could be obtained and also the

universal shapes of the experimental OMAR curves could be reproduced.

Additionally, we performed new types of experiments to gain better understanding of OMAR. We showed that

when an oscillating magnetic field is applied, OMAR is reduced beyond a certain frequency threshold. This

occurs when the slowest charges can no longer follow the oscillations, as we showed by measuring the

frequency dependence of the capacitance. These findings are in agreement with recent interpretations in which

these slowest carriers are expected to induce the largest OMAR effect.

In literature, it was claimed that OMAR is independent of the orientation of the magnetic field. However, via

sensitive measurements we demonstrated a small but systematic dependence on the angle between the

magnetic field and the sample. We showed theoretically that this angle dependence can be explained in the

different models by including an interaction between the spins. This interaction has to be direction dependent

in order to explain the angle dependence. We identified dipole–dipole coupling or an anisotropy in the

hyperfine fields as the most likely candidates.

Furthermore, we outlined a first exploration of an alternative approach to describe OMAR curves. We

introduced a function that allows us to extract information both about the hyperfine fields and about an


FNA Annual Report 2010 6.Results 43

additional broadening of the curves. Thereby, this approach could allow for a more quantitative analysis of

changes in the OMAR curves resulting from changes in the operating conditions or the material properties.

In the second topic, we investigated spin-polarized

transport through an organic layer, for instance in an

organic spin valve.

The main mechanism for loss of

polarization in most inorganic semiconductors, which is

related to spin–orbit coupling, is negligible in organic

materials. However, there might still be other

mechanisms that cause a smaller but non-zero loss of spin

polarization. We conjectured that the hyperfine fields are

the main source of polarization loss in organic materials,

which results from mixing between the spin-up and spindown

electrons by precession of spins about these

random fields.

We theoretically investigated this effect of the hyperfine

fields on spin polarization. We explicitly included the

hopping transport characteristic for organic

semiconductors. Due to spatial and energetic disorder,

the charges hop from one localized site to another. The

longer the time they spend on a site, the larger the loss of

spin polarization. We showed that an external magnetic

field larger than the typical hyperfine-field strength

reduces the loss of spin polarization. Hence, such an

external field causes the polarization to persist over a larger distance, leading to a magnetic-field dependent

increase of the spin-diffusion length. Using these findings, we could very accurately fit experimental data on the

magnetoresistance of organic spin valves reported in literature. Moreover, we made predictions about the effect

of changing the orientation of the magnetic field, thereby manipulating the spins during transport.

Output

Plastic Spintronics; spin transport and intrinsic magnetoresistance in organic semiconductors

Wiebe Wagemans

PhD thesis, June 2010


44 FNA Annual Report 2010 6.Results

6.2.6 Microscopic modeling of spin-dependent interactions in organic semiconductors

A.J. Schellekens, W. Wagemans, S.P. Kersten, P.A. Bobbert and B. Koopmans

General Introduction

An applied magnetic field can alter the current in organic semiconductors. This relatively large effect (> 10%) is

dubbed ‘Organic Magnetoresistance’ (OMAR) and can be measured even in small applied fields (≈ 10 mT) and

at room temperature. Since its discovery, multiple models to explain the large magnetoresistance have been

proposed by various authors. However, none of them unambiguously explain all the experimental results,

making the origin of OMAR a heavily debated topic in the scientific community.

Results 2010

Although the models for OMAR are based on different reactions, there is also a strong similarity between them.

In the proposed models an applied magnetic field alters the spin-dependent reactions between two particles,

thereby changing the current through the organic devices. Because of this similarity it is possible to perform

microscopic calculations on the different models using a single mathematical framework. To do this we have

used the following master equation:

called the Stochastic Liouville equation (SLE). This master equation governing the system dynamics has been

introduced with considerable success in different fields of research, ranging from delayed fluorescence in

organic semiconducting crystals to laser theory.

As an example of how the SLE works, we show a flow diagram for the so called bipolaron model in Fig. 1 In the

bipolaron model spin pairs are continuously being added to the density matrix by, which is a matrix

proportional to the identity matrix. When a spin pair has a singlet component there is a possibility that a

bipolaron is formed, removing the pair from the density matrix by the projection operator Ʌ. Pairs dissociate by

spin-independent hopping to the environment. The magnetic field dependence of the model enters through the

Hamiltonian

, which determines the coherent evolution in time of the spin states. The applied field suppresses

singlet-triplet mixing by precession around random hyperfine fields, hereby changing the bipolaron formation

rate and the current.

polaron pair formation

Г

coherent interactions

H bipolaron formation

ρ

hopping to environment

Fig. 1 Flow diagram for microscopic calculations on the bipolaron model using the SLE. When a polaron pair is

created by , the spins in the pair states evolve in time according to the Hamiltonian . The pairs can be

removed from by bipolaron formation or hopping to the environment.


FNA Annual Report 2010 6.Results 45

With the SLE it is possible to study the influence of the ratio of the typical hyperfine precession and hop

frequencies , unlike with simple rate-equation models proposed in literature. In Fig. 2 the magneto

conductance (MC) in large applied fields, i.e. – , is plotted as a function of for

different values of the branching ratio b, which is the ratio between the bipolaron formation rate and hopping

rate to the environment. What can be observed is that the MC vanishes when hopping is much faster than the

hyperfine precession frequency. In Fig. 3 MC line-shapes are shown for various hop rates. Here it can be

concluded that not only the magnitude of the MC decreases on increasing the hop rate, but also the line widths

broaden.

0 b = 0

0

hop / hf

= 50

MC (%)

-20

-40

-60

-80

-100

b = 10

b = 100

10 -1 10 1 10 3

hop

hf

MC (%)

-20

hop

/ hf

= 15

-40

-60

hop

/ hf

= 5

-80

-100

hop

/ hf

= 0.5

-20 -10 0 10 20

B (mT)

Fig. 2 MC in large applied fields as a function of ωhop / ωhf

for different values of the branching ratio b.

Fig. 3 MC as a function of B for different values of the hop

rate ωhop / ωhf.

To illustrate the power of the SLE for interpreting experimental data, a measurement of the MC in an Alq3

OLED under illumination is show in Fig. 4. It can be observed that two contributions to the MC are present; a

positive one in small applied fields and a negative one in large applied fields. From a large series of

measurements for varying operating conditions it is concluded that triplet-charge interactions are likely to be

responsible for the effects in large applied fields, while the interactions between electron and holes are likely to

dominate the MC in small fields. This conclusion is supported by the line-shapes in Fig. 5, where the SLE

equation is used to calculate both the effect of an applied field on e-h pair recombination as on the detrapping of

charges by triplet excitons. Line-shapes similar to the experimental ones are obtained from the microscopic

calculations.

Measurement

Calculation

150

150

e-h pairs

MC (%)

100

50

MC (%)

100

50

total

0

0

-50

-400 -200 0 200 400

B (mT)

-50

triplet - charge

-400 -200 0 200 400

B (mT)

Fig. 4 Example of a measurement of the photo-generated

current in an Alq3 OLED.

Fig. 5 Calculated MC line-shape due to e-h pair

recombination and triplet-charge interactions.


46 FNA Annual Report 2010 6.Results

6.2.7 Tuning Spin Interactions in Organic Semiconductors

P. Janssen, M. Cox, M. Kemerink, M.M. Wienk and B. Koopmans

General Introduction

This work focuses on the combination of two new emerging fields of electronics, namely molecular electronics

and spintronics. The field of molecular electronics deals with the study of molecular materials for the

application of electronic devices. Spintronics on the other hand aims at exploiting the spin of the electron to

transport and store information, whereas conventional electronics only use the charge.

A novel effect in these two fields, called organic magneto resistance (OMAR), has recently been discovered in

room temperature non-magnetic molecular materials. It is generally observed in OLED devices, where an

organic semiconducting material is sandwiched between two metal electrodes. The current through these

devices can change up to ∼10% when an external magnetic field in the order of 10 mT is applied. All

contemporary models explaining the OMAR effect are based on the interactions of the spin of the charge

carriers, which can be electrons or holes. However, there is no consensus on the exact origin. Therefore,

extensive research is required to verify the correct underlying model.

The spin interactions that play an important role in these organic semiconductors manifest themselves as pairs

of particles. This includes Coulomb bound electron-hole pairs, but also interactions of particles with the same

charge, such as electron-electron pairs, which are called bipolarons. Furthermore, an electron-hole pair can

convert into a particle called an exciton. These excitons can interact with free unpaired charges again. Most

OMAR models distinguish between these three different spin interactions; bipolarons, electron-hole pairs and

exciton-charge interactions.

In our research the versatility of organic materials has been used test and verify these models. This is done in a

unique way by following the path from an OLED to a blended organic photovoltaic. The presence of different

molecules in an OLED can influence the charge carriers significantly and under the right circumstances a phase

separated network develops, thereby creating separate paths for the electrons and holes to follow and reducing

their interaction with each other. The effect of the changes in spin interactions and morphology on OMAR could

point to the correct underlying model.

Results 2010

In this work, a blend of poly(phenylene-vinylene) (MDMO-PPV), acting as electron donor, with fullerene

(PCBM) molecules, acting as electron acceptor, has been studied. This blend is a well known and extensively

studied organic photovoltaic. The morphology and charge transport through such a blend are schematically

depicted in Fig. 1a. Within a few weight percent PCBM, virtually all excitons can find a PCBM site within their

diffusion length at which rapid charge transfer can take place. At around 20 wt.-% PCBM electrons will be able

to hop through the fullerenes, thereby significantly altering the transport properties of the blends. Finally, phase

separation sets in at around 67 wt.-% PCBM, which reduces the electron-hole interactions, but further enhances

the transport properties.

We have studied the magnetic field effects on the current as a function of applied magnetic field, voltage and

fullerene concentration. Typical results for the magnetoconductance, defined as the relative change in current

when applying a magnetic field, are shown in Fig. 1b-d. Changing the bias voltage and PCBM content causes

the line shapes to change dramatically.


FNA Annual Report 2010 6.Results 47

Fig 1. (a) Schematic representation of the morphology and charge transport in a PPV – PCBM blend. (b)–(d) Examples of

magnetoconductance (MC) traces as a function of magnetic field (B) using different bias voltages and PCBM concentrations.

Two different magnetic field effects have been identified, which behave different with applied voltage and

fullerene concentration. Furthermore, drift-diffusion simulations have been performed to gain insight on the

behavior of the devices as a function of voltage and fullerene concentration. We are able to relate the changes in

the MC to the morphology of the blend and we are trying to unravel the organic magnetoresistance by

comparing the proposed models with our observed data and simulations.

So far we have been able to conclude that exciton-charge interactions are the dominant mechanism behind the

magnetic field effect in PPV devices with little PCBM content. In blended devices with more than 20 wt.-%

PCBM these effects are fully quenched. In these devices the electron-hole pairs have been identified as the

dominant spin interactions causing the magnetic field effects. When phase separation sets in, the bipolaron

mechanism becomes dominant, as can be observed in the magnetoconductance traces.

By tuning the spin interactions in a blend of organic materials we have thus concluded that different

mechanisms are responsible for OMAR. However, which mechanism is dominant is based on the exact material

choice and operating conditions. Currently, we are aiming to combine the different models in one unified

picture.


48 FNA Annual Report 2010 6.Results

6.3 Ultrafast Spin Dynamics

6.3.1 Theory of femtosecond laser-induced magnetization dynamics

A.J. Schellekens, T. Roth b , G. Malinowski, F. Dalla Longa, K.C. Kuiper, D. Steiauf a , M. Fähnle a , M. Cinchetti b ,

and M. Aeschlimann b and B. Koopmans

a

Max-Planck-Institut für Metallforschung Stuttgart, Germany

b

Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, Germany

General Introduction

All-optical techniques exploiting femtosecond laser pulses have opened the way towards the exploration of the

ultimate limits of magnetization dynamics, providing means to manipulate magnetic systems at down to

femtosecond time scales. Apart from addressing fundamental issues in the field of (nano)magnetism the

approach is also considered to be of extreme relevance for future progress in (high data rate) magnetic

recording and spintronic applications. In 1996, Beaurepaire and coworkers found that magnetic order in

ferromagnetic transition metals can be quenched within a few hundred femtoseconds after laser heating. In

contrast, earlier work by Vaterlaus et al. on gadolinium reported a much slower response of 100 ± 80 ps, i.e. a

factor of thousand slower! The apparent incompatibility of the two results, combined with the large uncertainty

in the earlier measurements on gadolinium, has fuelled intense scientific discussion about its origin, and even

whether results for gadolinium could be trusted at all.

Results 2010

Recently, in a joint effort with researchers from the University of Kaiserslautern and the MPI Stuttgart we

succeeded in providing a coherent explanation for the contrasting results for the different materials. We

introduced a theoretical framework based on Elliott-Yafet type of spin-flip scattering that successfully explains

all phenomena and timescales on equal footing. Calculations were based on a simple model Hamiltonian

describing (spinless) free electrons, representing phonons within the Einstein or Debye model, and treating spin

excitations using a mean-field Weiss model. Thus we derived a microscopic version of the three-temperature

model (M3TM, Fig. 1c & 1d), describing the magnetization dynamics by a simple differential equation. Fig. 1a &

1b show calculated profiles of the electron (red) and lattice (blue) temperature, as well as the magnetization

(green) after pulsed laser heating for Ni and Gd, resp., using spin scattering probabilities of ~ 10% in both cases.

Despite these similar values, the calculated transients match well with experimentally observed

demagnetization time scales of ~ 100 fs and ~ 50 ps, resp. The values of the spin-flip probability were shown to

agree well with ab initio calculations of the spin-mixing in these materials. More recently, we performed detailed

temperature- and laser-fluence-dependent experiments on Ni and Co, and refined our M3TM. We find that in

all aspects the model's predictions agree well with experiment, including the appearance of a ‘two-step’

demagnetization –such as prominently visible for Gd (Fig. 1b)– when performing measurement on Ni at

elevated temperature.


FNA Annual Report 2010 6.Results 49

These results can be understood by further inspection of the theory. It is found that the demagnetization rate is

governed by the ratio of the Curie temperature to the atomic magnetic moment. Comparing Ni and Co, this

ratio is almost identical, explaining the similar dynamics for the two transition metals. However, Gadolinium

has twice a lower Curie temperature than Ni, while its atomic moment is a factor of 13 larger (Fig. 1f & 1g).

Thus, the demagnetization process gets so slow, that the demagnetization is not yet completed once the electron

and lattice systems have reached mutual thermal equilibrium. This explains both the much longer time scale, as

well as the occurrence of a two-step process. More generally, we constructed a generic view on laser-induced

demagnetization, introducing a phase diagram separating two classes of dynamics. After having made this

significant step forward in our understanding of laser-induced ultrafast demagnetization, next challenges will

be the implementation of more realistic spin excitation spectra, and applying the models to more complicated

magnetic materials (including ferrimagnetics), and new laser-based scenarios (including switching by circularly

polarized light).

Output

Explaining the paradoxical diversity of ultrafast laser-induced demagnetization

B. Koopmans, G. Malinowski, F. Dalla Longa, D. Steiauf, M. Fähnle, T. Roth, M. Cinchetti and M. Aeschlimann

Nature Materials 9, 259 (2010)

Temperature dependence of laser-induced demagnetization in Ni: A key for identifying the underlying

mechanism

T. Roth, A. J. Schellekens, S. Alebrand, O. Schmitt, D. Steil, M. Cinchetti, B. Koopmans and M. Aeschlimann.

(Submitted)

Fig. 1 Calculated dynamics of laser-induced

demagnetization for Ni and Gd. (a), Ultrafast

demagnetization m(t) (green), as well as Te(t)

(red) and Tp(t) (blue) profiles, simulating

experimental results for Ni. (b), Similar for

the two-step process, as observed for Gd. (c),

Schematic representation of the threetemperature

model for Ni, as a

representative for the 3d transition metals.

Energy equilibration is indicated by twosided

arrows; angular momentum flow is

controlled by interaction with the lattice

(dashed arrow). (d), Similar for Gd, with the

extra 4f system. (e), Elliott–Yafet spin-flip

scattering on emission of a phonon, taking

over angular momentum. (f), Spin-flip

scattering in the 3d4sp band of Ni. The

orange shading represents the number of

uncompensated spins. (g), Similar diagram

for Gd; scattering is occurring only in the

5d6sp band with small magnetic moment,

whereas localized 4f states predominantly

contribute to the magnetic moment.


50 FNA Annual Report 2010 6.Results

6.3.2 Magnetism and dynamics of Pt / Co / for domain wall devices

A.J. Schellekens, F. Elich and B. Koopmans

General Introduction

Magnetization dynamics in perpendicularly magnetized materials have received much interest from the

scientific community, as they are important candidates for new magnetic storage media and spintronic devices.

The typically large perpendicular anisotropy results in narrow and simple Bloch domain walls, which facilitates

current driven domain wall motion as well as high density data storage. A problem of many materials with a

perpendicular anisotropy is the hindered domain wall motion due to pinning. However, recently large domain

wall velocities (> 100 m/s) have been measured in a new type of perpendicular magnetized material, namely

trilayers, making them extremely interesting for future spintronic devices.

Magnetization dynamics are governed by the Landau-Lifschitz-Gilbert equation. An important parameter in

this equation is the Gilbert damping parameter α, as it determines the magnetization relaxation rate and thereby

also the domain wall velocity, the current to trigger domain wall propagation and the timescales of switching a

magnetic memory element. The goal of this project is to determine α for

trilayers, as it is a crucial

parameter for the spin dynamics and could elucidate the origin of the large domain wall velocities in these

materials.

Results 2010

The samples used in this project are Pt / Co / Al cross wedges (Fig. 1), where on the same sample both the cobalt

and aluminum thickness is varied. The sample is oxidized by plasma oxidation resulting in an Al2O3 top layer.

To see for which Co and Al thicknesses the sample is perpendicularly magnetized, the magnetization

component perpendicular to the sample surface is measured by means of the magneto-optical Kerr effect

(MOKE). The results are depicted in Fig. 2. Three distinct regions are visible in this contour plot, namely (i) a

region where the aluminum is under-oxidized and the magnetization lies in plane, (ii) a region where the

sample is optimally oxidized and (iii) the magnetization is out of plane, and a region where not only the

aluminum but also the cobalt is oxidized, resulting in a quenched magnetization.

Fig. 1 Pt / Co / Al wedge structure, where both the Al as

the Co thickness are varied.

Fig. 2 Out-of-plane normalized remanent magnetization of

a

cross wedge.


FNA Annual Report 2010 6.Results 51

The Gilbert damping parameter is determined by measuring laser-induced magnetization dynamics exploiting

the time-resolved magneto-optical Kerr effect (TR-MOKE). An external magnetic field is applied at an angle of ~

10˚ with the sample surface. Typical TR-MOKE traces are depicted in Fig. 3. After ~ 10 ps a damped precession

of the magnetization occurs due to a change in anisotropy during laser pulse excitation. From the LLG-equation

it is derived that , where is the precession frequency and is the damping time, hence can be

determined by fitting this damped precessional motion.

In Fig. 4 the fitted values for are plotted as a function of the applied magnetic field for different thicknesses of

the aluminum layer. A magnetic field dependence of is observed, which becomes more pronounced for small

aluminum thicknesses. This magnetic field dependence has in literature been suggested to be caused by

dephasing of the precession by a spread in the local anisotropies. The data in Fig. 4 is fitted by a macro-spin

model including such a spread. What can be observed is that for a large Al thickness, so a moderate magnetic

field dependence, the experimental data and fits correspond well. However, for a thinner Al top layer with a

stronger field dependence, the data cannot be fitted by the simple model. This raises the question whether the

magnetic field dependence of the damping parameter is really caused by a dispersion in the anisotropy or that a

different mechanism is at play.

A conclusion that can be drawn from the experimental data is that seems to converge to values between 0.1

and 0.2 on increasing the applied field, which is comparable to the values found for Pt / Co / Pt and

Pt / CoFeB / Pt films. Further experiments and calculations are required to pinpoint the origin of the magnetic

field dependence of the Gilbert damping parameter in these ferromagnetic thin films.

2.0

1.8 nm Co

0.8

1.8 nm Co

1.5

0.6

d al

= 6.6 Å

d al

= 6.1 Å

M z

(arb. units)

1.0

0.5

d al

= 6.1 Å

d al

= 6.6 Å

d al

= 7.1 Å

d al

= 7.6 Å


0.4

0.2

d al

= 7.1 Å

d al

= 7.6 Å

d al

= 8.1 Å

0.0

d al

= 8.1Å

0 50 100 150 200

Delay (ps)

0.0

0.25 0.50 0.75 1.00

B applied

(T)

Fig. 3 Measurements and fits of the precessional dynamics

of Pt / Co / Al2O3 as a function of Al thickness for an

applied field of 1 Tesla.

Fig. 4 Obtained values for the Gilbert damping parameter

as a function of the applied field. The lines are fits assuming

a dispersion in the perpendicular anisotropy.


52 FNA Annual Report 2010 6.Results

6.3.3 Experiments and simulations on femtosecond laser-induced magnetization dynamics

K.C. Kuiper, A.J. Schellekens, T. Roth a , M. Cinchetti a , M. Aeschlimann a and B. Koopmans

a

Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, Germany

General Introduction

For more than 10 years, it is known that the magnetization of a ferromagnetic transition metal can be quenched

within a few hundred femtoseconds (fs) by an intense fs-laser pulse. Since the measurements of Beaurepaire in

1996, several models have been introduced, which try to unravel the demagnetizing processes. One of these

models is called the 3 temperature model (3TM), which divides the ferromagnet into 3 subsystems, being the

electron-, lattice- and spin system. All these subsystems have their individual temperature. The quenching of

the magnetization is assumed to be caused by the increase of the spin temperature.

Although the 3TM can successfully reproduce experimental observations, its main shortcoming is that only the

energy flow among the subsystems is modeled, whereas the conservation of angular momentum is not

considered. This problem was recently solved by the introduction of a microscopic extension of the Three

Temperature Model (M3TM), in which the transfer of angular momentum between the electron-, lattice- and

spin system is mediated by Elliott-Yafet type of spin scattering.

Results 2010

M3TM simulations on nickel films were carried out to demonstrate that utmost care has to be taken in deriving

quantitative information from experiments when using high laser fluences. More specifically, modeling the

dynamics including a finite penetration of the laser light and heat diffusion, it is shown that extrinsic

parameters, such as the thickness of the ferromagnetic layer or the sample structure, can cause a change in the

observed demagnetization time by up to a factor of three.

The influence of the sample thickness on the quenching of the magnetization is shown in Fig. 1. From this

figure, it is clear that for a thin isolated film, the quenching rapidly increases for increasing fluence and the film

is completely demagnetized abruptly. However for a relative thick film, the quenching seems to saturate for

large laser fluences. This saturating behavior was observed in many earlier experiments.

In Fig. 2, the effect of the sample structure on the demagnetization time is shown. In this case, two different

nickel films were simulated. Both samples consisted of a nickel film with variable thickness. However, one film

was assumed to be on thermally isolated substrate, whereas the other was on a conductive substrate. The

overall higher demagnetization times of the isolated structure can be explained by slower heat dissipation,

leading to overall higher ambient temperature and thereby slower magnetization dynamics.

An important parameter governing the magnetization dynamics within the M3TM is the Elliott-Yafet spin-flip

probability . In search for materials with deviating , and to explore the influence of multi-layer structures

with many interfaces, we performed experiments on especially engineered Co/Pt composite multilayers over a

wide fluence range (cooperation with the University of Kaiserslautern). The demagnetization traces can be well

fitted in a global fitting routine to the M3TM model as shown in Fig. 3. In these fits, only the laser fluence is

allowed to vary among the separate curves. The extracted demagnetization times (Fig. 4) are smaller than those

of normal cobalt (typically around 250 fs) and the spin-flip parameter is 2 times as large. This is probably caused

by the increased spin-orbit coupling at the cobalt-platinum interface. The individual data points in Fig. 4

represent the demagnetization times as determined by fitting the individual demagnetization traces with the

ordinary 3TM.


FNA Annual Report 2010 6.Results 53

Output

Nonlocal Ultrafast Demagnetization Dynamics in the High Fluence Limit

K.C. Kuiper, G. Malinowski, F. Dalla Longa, and B. Koopmans

Journal of Applied Physics

(Accepted)

M max

/M 0

1.0

0.8

0.6

0.4

0.2

d = 5 nm

d = 30 nm

0.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0 5 10 15 20 25

T pump

/T thickness (nm)

C

M

* (fs)

240

220

200

180

160

isolated

conductive

Fig. 1 The maximum quenching of the magnetization

( ) as a function of laser fluence .

Open symbols: optically thin Ni film compared to the laser

penetration depth, i.e. 15 nm. Closed symbols: optically

thick Ni film

Fig. 2 The demagnetization time ( ) as a function of the

sample thickness on a thermally isolated and conductive

substrate

M/M 0

1.0

0.8

0.6

0.4

0.2

M

* (fs)

120

100

80

60

40

data 3TM

M3TM fit

0.0 0.2 0.4 1 2 3 4

delay (ps)

0 20 40 60 80

M max

/M 0

(%)

Fig. 3 Demagnetization traces of a Co/Pt-multilayer for

several laser fluences. The full lines represent the results of a

global M3TM fitting routine.

Fig. 4 The demagnetization times as determined by the

M3TM compared to the fits of the individual traces of

Figure 3 using the ordinary 3TM.


54 FNA Annual Report 2010 6.Results

6.3.4 Towards ultrafast studies of current induced domain wall motion

R. Paesen, E. Murè, K.C. Kuiper and Bert Koopmans.

General Introduction

With the advent of spintronics, in 1996, the realization of a storage device based on current induced DW motion

became an attractive challenge for the scientific community. Recently, shift register devices, based on the

nucleation and displacement of several domains in the same structure, have been proposed and studied. For the

development of such complex systems a better understanding and characterization of the domain wall

dynamics is needed. This project is aimed at probing the DW dynamics on a picoseconds timescale. This is

made possible thanks to the combination of electric and optical experimental schemes, based on the use of an

ultrafast laser set-up (laser pulse duration 70 fs). Thanks to the pulsed excitation of DW dynamics, we expect to

be able to exit the creep motion regime and study the DW motion in flow regime, deducing a clearer picture of

the spin torque induced dynamics.

Results 2010

The results achieved in the past year mainly concern the generation, propagation and detection of ultrafast

current pulses. A SEM image of the test device used for it is shows in Fig. 1. The pulse generation is based on an

Auston switch device: a pulsed laser is shone on a GaAs substrate, connected to two biased gold pads. The

incident photons excite the electrons from the valence to the conduction band and make the GaAs

instantaneously conductive. This allows us to convert the femtosecond laser pulse into an electric pulse, suitable

for DW motion experiments. The Auston switch is connected to a gold wire with magnetic island patterned on

it. Under the effect of an applied voltage the current pulse propagates through the gold wire, inducing an

Oersted field around it. The study of the time resolved magneto optic Kerr effect (TRMOKE) on the Co islands

permits information on the field pulse to be inferred. In Fig. 2 is shown the Kerr signal measured on a Co island

for different values of the bias voltage applied to the waveguide. From the LLG analysis of those data we

deduce a current pulse with typical rise time

and a decay time

The next step will be to nucleate a domain wall in a magnetic wire and use the ultrashort current pulses to

excite its dynamics. Our magnetic wire consists of a Pt(4nm) / Co(0.5nm) / Pt(2nm) stack, characterized by a

strong perpendicular magnetic anisotropy. A Ga focused ion beam is used to irradiate and damage a region of

the wire, in such a way to create an anisotropy gradient and therefore a pinning site for DWs. The nucleation of

a DW is done by applying a static magnetic field, whose value is comprised between the coercive field of the

irradiated and non irradiated regions. The DW is then pushed by the current produced in an Auston switch.

The use of extremely short pulses allows us to increase the current amplitude without risking a damage of the

magnetic wire. Typically, a current pulse with a peak current density of about

would induce a

heating of about 30K (equivalent to the effect of a continuous current density of

). A 1D model

calculation shows that such a high current density ensures a DW displacement of the order of 100 nm, in the

span of time of our measurement. The high temporal resolution and signal to noise ratio of the experiment is

based on the use of a stroboscopic technique. Therefore the DW has to be resettled to its initial position after

each measurement shot. This can be done by applying a second current pulse of opposite polarity of by using

an offset magnetic field. In Fig. 3 is sketched the expected response of the DW position ( ) and canting angle

( ) to a train of current pulses having the typical , and repetition rate achievable with our

TRMOKE set-up. In Fig. 4 is shown a possible scheme of the final measurement set-up, suitable for both an

optical detection (Kerr signal from the Pt / Co / Pt wire) and an electrical detection. Indeed we suggest the

possibility to use a second Auston switch to perform a ‚pulsed‛ detection of the extraordinary Hall effect (EHE)

at a Hall Cross patterned in the magnetic structure.


FNA Annual Report 2010 6.Results 55

Output

Toward ultra fast spin transfer detection

R. Paesen

Master project

Fig. 1 Test sample for ultrafast current pulse generation and

detection. The pump beam is shone on the GaAs region and

produces a current pulse which propagates through the gold

wire and induces a magnetic field on the Co islands. The

probe beam is used to detect the Kerr signal from the Co.

Fig. 2 Kerr signal measured on a Co island. The external

field and the laser fluence are maintained constant and the

voltage across the waveguide is varied, varying the intensity

of the Oersted field on the Co. The inset shows the signal

dependence on the voltage amplitude (e.g. on the amplitude

of the current pulse).

Fig. 3 Qualitative response of DW position and canting

angle to a train of high density current pulses (extracted

from 1D model calculations).

Fig. 4 Schematic sample design for ultrafast CIDM

experiment. This measurement scheme would permit us to

do both the optical and electrical detection (double Auston

switches scheme).


56 FNA Annual Report 2010 6.Results

6.3.5 Resolving the genuine laser-induced ultrafast dynamics of exchange interaction in

ferromagnet/antiferromagnet bilayers

F. Dalla Longa, J.T. Kohlhepp and B. Koopmans

General Introduction

The so-called exchange bias effect which describes the magnetic exchange coupling between a ferromagnet (FM)

and an antiferromagnet (AFM) is an issue of practical interest because of its importance for the realization and

the functionality of present and future magneto and spintronics devices, since it creates the possibility to

manipulate the switching behavior of magnetic layers. As a result of the exchange interaction at the interface the

FM displays a unidirectional anisotropy and its hysteresis loop is shifted along the field axis by a quantity HEB

called exchange bias field. Although this effect was already discovered more than 50 years ago and also

extensively exploited in magneto-electronic devices since about 20 years, a comprehensive microscopic

understanding of the mechanisms involved is still far from being reached. A particularly exciting new challenge

in the field is to control the spin dynamics by modifying the exchange interaction between FM/AFM bilayers.

The aim of this project is gaining fundamental understanding of the phenomenon of exchange coupling and in

particular the physics of the dynamics of the exchange interaction on the sub-picosecond time scale in carefully

engineered samples. The experiments are carried out by driving exchange coupled bilayers out of equilibrium

through laser excitation and following the dynamics of the ferromagnetic spins thereof by means of timeresolved

magneto-optics.

Results 2010

We chose for our study polycrystalline bilayers consisting of ferromagnetic Co and antiferromagnetic IrMn (see

Fig. 1). The experimental geometry is sketched in Fig. 1: the sample lies in the x-y plane, the exchange bias field

acts along the negative x direction, and an external field is applied in the sample plane along the y

axis. The effective field acting on the FM magnetization is given by the vectorial sum of and

When the laser hits the sample the exchange interaction is quenched, leading to a change in the orientation and

magnitude of the (from point 1 to point 2) and therefore a torque acts on the magnetization and a

precession according to the Landau-Lifshitz-Gilbert (LLG) equation is triggered. Precessional transients were

measured with TRMOKE for a range of applied fields; one example is shown in Fig. 2.

The precession is determined at each time by the value and orientation of the effective field acting on the

magnetization at that particular time, and, in turn, these depend on the value of The idea was to retrieve

the

by carefully analyzing the precessional transients. To do this we wrote the LLG

equation in the approximation of a small perturbation and invert it, expressing as a function of

(the measured data), its derivative and its integral. The calculation was performed for all the measured

transients, and the resulting field pulses have been averaged and smoothed by adjacent averaging over a period

of 7 ps, yielding the plot shown in Fig. 3 (dots). The data could be fitted (dashed line) revealing that after laser

excitation the exchange bias field is quenched to a minimum with a characteristic decay time

, and its recovery can be described by two exponentials with time scales and

The fitting function was then deconvoluted to get rid of the effect of the adjacent

averaging, yielding the genuine temporal evolution of the exchange bias field (solid line).

We conjecture that the quenching of the interface exchange interaction is caused by laser-induced disordering of

the spins at the FM/AFM interface. A loss of spin ordering in the AFM which has a significant lower ordering

temperature then the FM could trigger the fast quenching of . Therefore, we anticipate that our method

could prove useful not only for investigating the dynamics of exchange interaction but also to indirectly probe

the loss and recovery of magnetic ordering in AFM’s in the femtosecond regime.


FNA Annual Report 2010 6.Results 57

Output

Resolving the genuine laser-induced ultrafast dynamics of exchange interaction in

ferromagnet/antiferromagnet bilayers

F. Dalla Longa, J.T. Kohlhepp, W.J.M. de Jonge, and B. Koopmans

Physical Review B 81, 094435 (2010)

Fig. 1 Left: sketch of the experimental

configuration showing the quenching of

by an intense laser pulse and the

consequent change of the equilibrium

direction of the magnetization from A to B.

Right: sample structure and normalized

magnetization loops parallel and

perpendicular to the exchange bias

direction.

hot e

thermalized e-p-s

Fig. 2 Precessional transient obtained at ,

the changing slope during the first 5 ps is highlighted by the

dashed line.

Fig. 3 Average of the reconstructed pulses on the short and

long time scales (symbols); the solid line describes the

exchange bias field pulse after deconvolution.


58 FNA Annual Report 2010 6.Results

6.3.6 Magnetization dynamics in racetrack memory

B. Bergman a , R. Moriya b , L. Thomas b , M. Hayashi b , X. Jiang b , and S.S.P. Parkin b and B. Koopmans

a

carrying out Ph.D. work at IBM

b

IBM Almaden Research Center, San Jose, California, US

General Introduction

Several interesting concepts have been proposed recently for memory and logic devices based on the

manipulation of magnetic domain walls (DWs). This has stimulated research into DW dynamics, particularly

those resulting from interactions with current passing through the DW via the phenomenon of spin momentum

transfer (SMT). This interaction can result in the motion of domain walls along magnetic nanowires and is a

basic concept of the Magnetic Racetrack Memory. A better understanding of DW dynamics is of vital

importance for the further development of such devices.

In this project we probe the DW magnetization dynamics through an optical pump-probe technique, in which

the magneto-optical Kerr effect (MOKE) is used to probe changes in a nanowire’s magnetization M when it is

‘pumped’ with short current pulses.

Results

At IBM’s Almaden research laboratory a pump-probe Kerr magnetooptical scanning microscope has been

developed. In order to control DW injection, motion and reset, magnetic fields have to be applied locally on the

nanowire. For this a special Damascene CMOS chip has been fabricated at the 200 mm wafer facility at IBM

Microelectronics Research Laboratory (MRL). Probing of the local magnetization is done with a focused pulsed

laser spot of 400 nm diameter where the polarization rotation caused by the Kerr effect is measured after

reflection. In order to achieve optimal focusing a perpendicular incident laser beam is focused with a high

numerical aperture objective. Synchronized ‘pumping’ in this scheme is achieved by successively: 1. injecting a

DW; 2. propagate the DW down the nanowire with either current through or an applied field pulse over the

nanowire; and 3. resetting the whole nanowire to its original magnetization by applying a large field together

with the injection of an opposite magnetic domain. With this setup field and current induced DW motion is

studied in permalloy nanowires ranging in width from 200 to 700 nm and thickness of 20 nm.

For control of DWs in Racetrack memory it is important to understand the different mechanism for driving a

DW already in motion (dynamic) and driving a DW that is currently at rest (static). The propagation field, the

minimum field below which no DW motion takes place, is measured for both dynamic DWs and static DWs

(Fig. 1). It is found that Static DWs require a much higher field than DWs already in motion. A model is build

where this effect is related to the wire roughness, successfully describing the existence of a propagation field,

the difference between both propagation fields and a specific effect related to the method of injection.

One important property affecting DW velocity and possibly also the critical current is Gilbert damping. Gilbert

damping in permalloy can be tuned by doping the nanowires with osmium. This is used to prepare a sample

series with increasing Gilbert damping. Measurement of both magnetic field and current-induced DW velocity

revealed a profile well known that includes the Walker breakdown (a maximum field where further increasing

field strength does not further increase the DW velocity). From these profiles the dependence of the Walker

breakdown, DW mobility and maximum DW velocity, as well as the spin torque efficiency (β/α) on Gilbert

damping has been determined.


FNA Annual Report 2010 6.Results 59

Output

An investigation of the static and dynamic domain wall propagation fields in permalloy nanowires using

pump-probe Kerr microscopy

Bastiaan Bergman, Rai Moriya, Masamitsu Hayashi, Luc Thomas, Bert Koopmans and Stuart S.P. Parkin

(Submitted)

M X

/ M S

Probability

1

0

-1

1

0

a

b

random

Dynamic

Static

2.3 m

2.7 m

3.3 m

4.3 m

5.2 m

0 Oe

2 Oe

4 Oe

Fig. 1 Example of experiments and simulations

distinguishing static and dynamic domain wall

motion. (a) Experimental results of

propagation field for dynamic DWs (open

symbols) and static DWs (closed symbols) for

five different starting positions. Incomplete

switching (from –MS to +MS) should be seen as

a limited probability for DW propagation. (b)

Propagation probability for static DWs

obtained from a 1D model using a wash board

energy landscape. For DWs at a random initial

position (half open symbols) and for DWs

positioned using a bias field (closed symbols).

For details see PhD thesis.

0 5 10 15

H (Oe)


60 FNA Annual Report 2010 6.Results


FNA Annual Report 2010 7.Output 61

7 Output

7.1 Publications

Explaining the paradoxical diversity of ultrafast laser-induced demagnetization

B. Koopmans, G. Malinowski, F. Dalla Longa, D. Steiauf, M. Faehnle, T. Roth, M. Cinchetti, and M.

Nature Mat. 9, 259 (2010)

Reduced DW pinning in ultrathin Pt-CoB-Pt with perpendicular magnetic anisotropy

R. Lavrijsen, G. Malinowski, J.H. Franken, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans

Appl. Phys. Lett. 96, 022501 (2010)

Current-induced domain wall motion in Co/Pt nanowires: Separating spin torque and Oersted-field effects

J. Heinen, O. Boulle, K. Rousseau, G. Malinowski, M. Klaeui, H.J.M. Swagten, B. Koopmans, C. Ulysse, G. Faini

Appl. Phys. Lett. 96, (2010)

Controlled domain-wall injection in perpendicularly magnetized strips

R. Lavrijsen, J.H. Franken, J.T. Kohlhepp, H.J.M. Swagten en B. Koopmans

Appl. Phys. Lett. 96, 222502 (2010)

Frequency dependence of organic magnetoresistance

W. Wagemans, P. Janssen, E.H.M. van der Heijden, M. Kemerink, B. Koopmans

Appl. Phys. Lett. 97, 123301 (2010)

Fe:O:C: grown by focused-electron-beam-induced deposition; magnetic and electric properties

R. Lavrijsen, R. Cordoba, F.J. Schoenaker, T.H. Ellis, B. Barcones, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans,

J.M. De Teresa, C. Magen, M.R. Ibarra, P. Trompenaars, J.J.L. Mulders

Nanotech 22, 025302 (2010)

Resolving the genuine laser-induced ultrafast dynamics of exchange interaction in

ferromagnet/antiferromagnet bilayers

F. Dalla Longa, J.T. Kohlhepp, W.J.M. de Jonge, and B. Koopmans

Phys. Rev. B 81, 094435 (2010)

Spin transport and magnetoresistance in organic semiconductors

W. Wagemans and B. Koopmans

Phys. Stat. Sol. (b), Published on-line 3 November 2010

Point-defect interactions in electron-irradiated titanomagnetites - as analyzed by magnetic after-effect

spectroscopy on annealing within 80 K < T < 1200 K

F. Walz, V.A.M. Brabers, H. Kronmueller

J. Phys. Cond. Mat. 22, 046007 (2010)

Separating photocurrent and injected current contributions to the organic magnetoresistance

W. Wagemans; W.J. Engelen, F.L. Bloom, B. Koopmans

Synth. Met. 160, 266 (2010)

Spin relaxation and magnetoresistance in disordered organic semiconductors

P.A. Bobbert, T.D. Nguyen, W. Wagemans, F.W.A. van Oost, B. Koopmans, M. Wohlgenannt

Synth. Met. 160, 223 (2010)


62 FNA Annual Report 2010 7.Output

7.2 Presentations

Femtosecond laser-induced demagnitzation from a thermodynamic perspective

B. Koopmans

Magnetism & Magnetic Materials; Washington, DC, USA (18 jan - 22 jan 2010)

Sub-surface nanoclusters and near-surface quantum wells in anisotropic metals (Invited)

O. Kurnosikov

Invited talk at Georg-August-Universitaet Gottingen, Germany (29 jan 2010)

Laser-induced femtosecond magnetization dynamics - Reconciling a paradoxical history

B. Koopmans

Kolloquium Forschungszentrum Juelich, Deutschland (29 jan 2010)

Spin in organics, a new route to spintronics (Invited)

B. Koopmans

Fruhjahrstagung 2010; Molecular Spintronics; Regensburg, Germany (21 jan - 29 jan 2010)

Molecular Spintronics - Currrent status and Challenges (Focused Session)

Spin electronica - geleiding en toepassing op de nanometer schaal

H.J.M. Swagten

Annual DocNdag; Eindhoven, Netherlands, the (17 mrt 2010)

Spinning dynamics! - New trends in spintronics (Invited)

B. Koopmans

Fysica 2010; Utrecht, Netherlands, the (23 apr 2010)

Focussessie Nanofysica

Spinnende elektronica - Spannende technologie

B. Koopmans

Voordracht voor alumnivereniging Veni, Trafalgar Pub, Eindhoven (2 mei 2010)

Recent progress on domain-wall physics in perpendicular systems (Invited)

H.J.M. Swagten

Trends in Spintronics, Korean-Dutch Workshop; Eindhoven, Netherlands, the (23 jun - 27 jun 2010)

Influencing the exchange interactions at ferromagnet/antiferromagnet interfaces (Invited)

J.T. Kohlhepp

Trends in Spintronics, Korean-Dutch Workshop; Eindhoven, Netherlands, the (23 jun - 27 jun 2010)

Spin in organics, a new route to spintronics (Invited)

B. Koopmans

Trends in Spintronics, Korean-Dutch Workshop; Eindhoven, Netherlands, the (23 jun - 27 jun 2010)

Dutch-Korean Workshop Trends in Spintronics

Tunable domain wall injection in perpendicularly magnetized strips

J.H. Franken, M. Hoeijmakers, R. Lavrijsen, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans

Joint European Events Office; Krakow, Poland (22 aug - 28 aug 2010)

Spin-spin interactions in organic magnetoresistance

W. Wagemans, A.J. Schellekens, M. Kemper, F.L. Bloom, B. Koopmans

SPINOS III - Conference on Spins in Organic Semiconductors; Amsterdam, Netherlands, the (30 aug - 3 sep

2010)

Frequency dependence of organic magnetoresistance

P. Janssen, W. Wagemans, E.H.M. van der Heijden, M. Kemerink, B. Koopmans

SPINOS III - Conference on Spins in Organic Semiconductors; Amsterdam, Netherlands, the (30 aug - 3 sep

2010)

Investigating OMAR device models experimentally

F.L. Bloom, H. Moons, J.M. Veerhoek, E. Goovaerts, B. Koopmans

SPINOS III - Conference on Spins in Organic Semiconductors; Amsterdam, Netherlands, the (30 aug - 3 sep

2010)


FNA Annual Report 2010 7.Output 63

Subsurface nanoclusters in metallic substrate: localized quantum wells, electron interference and inner

electron

O. Kurnosikov, C.O. Avci, H.J.M. Swagten, B. Koopmans

European Conference Surface Science; Groningen, Netherlands, the (29 aug - 3 okt 2010)

Femtosecond laser-induced magnetization dynamics (invited)

B. Koopmans

Workshop 2010 Strongly correlated transition metal compounds III; Bergisch Gladbach, Germany (8 sep - 10 sep

2010)

Domain walls in perpendicularly magnetized stripes violating spin-transfer torque? (Invited)

R. Lavrijsen, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans

International symposium on metallic multilayers; Berkeley, California, U.S. (13 sep - 17 sep 2010)

De diverse gezichten van nanotechnologie (Popular scientific)

B. Koopmans

Workshop met docenten middelbare school; Venice, Italy (27 sep - 27 sep 2010)

Femtosecond magnetization dynamics - From demagnitization to spin transfer (Invited)

G. Malinowski, B. Koopmans

Workshop Laser-induced magnetization in nanostructures; Stoos, Switzerland (6 okt - 8 okt 2010)

Controlling domain wall motion in perpendicularly magnetized materials

J.H. Franken

Joint FOM Spin/NanoNed Spintronics Meeting; Nijmegen, Netherlands, the (14 okt - 15 okt 2010)

Domain wall motion in perpendicularly magnetized strips. Violating spin transfer torque?

R. Lavrijsen

Joint FOM Spin/NanoNed Spintronics Meeting; Nijmegen, Netherlands, the (14 okt - 15 okt 2010)

EBID of magnetic nanostructures

T.H. Ellis

Joint FOM Spin/NanoNed Spintronics Meeting; Nijmegen, Netherlands, the (14 okt - 15 okt 2010)

Magnetism and dynamics of Pt/Co/ for domain wall devices

A.J. Schellekens

Joint FOM Spin/NanoNed Spintronics Meeting; Nijmegen, Netherlands, the (14 okt - 15 okt 2010)

Ultrafast spin-transfer torque in nanomagnets

B. Koopmans

Joint FOM Spin/NanoNed Spintronics Meeting; Nijmegen, Netherlands, the (14 okt - 15 okt 2010)

Tunable domain wall injection and pinning in perpendicularly magnetized strips

J.H. Franken, M. Hoeijmakers, R. Lavrijsen, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans

Conf. Magnetism & Magnetic materials; Atlanta, Georgia, U.S. (14 nov - 18 nov 2010)

Domain wall in perpendicularly magnetized stripes violating spin-transfer torque?

R. Lavrijsen, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans

Conf. Magnetism & Magnetic materials; Atlanta, Georgia, U.S. (14 nov - 18 nov 2010)

Frequency dependence of organic magnetoresistance

P. Jansssen, W. Wagemans, E.H.M. van der Heijden, M. Kemerink, B. Koopmans

Conf. Magnetism & Magnetic materials; Atlanta, Georgia, U.S. (14 nov - 18 nov 2010)

Nonlocal ultrafast magnitization dynamics in the high fluence limit

K.C. Kuiper, G. Malinowski, A.J. Schellekens, B. Koopmans, T. Roth, S. Alebrand, D. Steil, M. Cinchetti, M. Aeschlimann

Conf. Magnetism & Magnetic materials; Atlanta, Georgia, U.S. (14 nov - 18 nov 2010)

Nano-engineering for magnetic domain wall devices (Invited)

J.T. Kohlhepp

MicroNano conference; Twente, Netherlands (17 nov - 18 nov 2010)

Spin in organics, a new route to spintronics (Colloquium)

B. Koopmans

Kolloquium Universiteit Duisburg-Essen Germany (1 dec 2010)


64 FNA Annual Report 2010 7.Output

Laser induced magnetization dynamics in exchange coupled FM/AFM Co/Ir/Mn bilayers (Invited)

J.T. Kohlhepp

Internation conference of AUMS, 2010; Jeju Island, Korea (5 dec - 8 dec 2010)

Local formation of a Heusler type structure in CoFeAl current perpendicular to the plane GMR spin valves

J.T. Kohlhepp

Internation conference of AUMS, 2010; Jeju Island, Korea (5 dec - 8 dec 2010)

Tunable domain-wall injection and propagation in ferromagnetic nanowires (Invited)

H.J.M. Swagten

Internation conference of AUMS, 2010; Jeju Island, Korea (5 dec - 8 dec 2010)


FNA Annual Report 2010 7.Output 65

7.3 Chapters

Magnetoresistance and spin transport in organic semiconductor devices

M. Wohlgenannt, P. A. Bobbert, B. Koopmans, and F. L. Bloom

In: Organic Spintronics

Ed. by: Z.V. Vardeny (Taylor & Francis, 2010), pp. 67-136

Magnetic tunnel junctions

H.J.M. Swagten, P.V. Paluskar

In: Encyclopedia of Material Science & Technology

Ed. by: (Elsevier Ltd., ISBN: 978-0-0804-3152-9, p. 1-7, 2010), pp. 1-7

7.4 Guest Lectures

Towards nanomagnetic probing in SEM-STM

Serhiy Vasnev (Radboud University, Nijmegen)

FNA seminar, 6 apr 2010

Influence of local laser heating on domain wall propagation

Philipp Möhrke (University of Konstanz)

FNA seminar, 16 apr 2010

Electron scattering and Kondo resonances at Co subsurface impurities in copper

M. Wenderoth (Uni. Goettingen, Germany)

FNA seminar. 17 mei 2010


66 FNA Annual Report 2010 7.Output

7.5 Posters

Laser-induced magnetization dynamics in Co/Pt based multilayers

K. Kuiper, G. Malinowski, R. Lavrijsen, B. Koopmans

FOM -dagen jan. 2010; Veldhoven, Netherlands, the (19 jan - 20 jan 2010)

Magnetic nanowires by EBID

R. Lavrijsen, F. Schoenaker, T. Ellis, B. Barcones, H. Swagten, B. Koopmans, J. de Teresa, R. Cordoba, H. Mulders, P.

FOM -dagen jan. 2010; Veldhoven, Netherlands, the (19 jan - 20 jan 2010)

Organic magnetoresistance: unravelling the origin of sign changes

P. Janssen, W. Wagemans, F.L. Bloom, B. Koopmans

FOM -dagen jan. 2010; Veldhoven, Netherlands, the (19 jan - 20 jan 2010)

Multiple quantum wells states induced by subsurface nano-reflectors

C.O. Avci, O. Kurnosikov, H.J.M. Swagten, B. Koopmans

Dutch SPM symposium; Eindhoven, Netherlands, the (5 feb 2010)

Organic magnetoresistance: unravelling the origin of sign changes

P. Janssen, W. Wagemans, F.L. Bloom, B. Koopmans

cNM Research Day 2010; Eindhoven, Netherlands, the (30 jun 2010)

Magnetic effects in FeNi structures codeposited with atom nanolithography

T. Meijer, J. Beardmore, E. Vredenbregt, M. Hoeijmakers, J.H. Franken, B. Koopmans, T. van Leeuwen

NNV-AMO; Lunteren, Netherlands (12 aug 2010)

Modeling spin interactions in disordered organic semiconductors

A.J. Schellekens, M.J.M. van Schijndel, W. Wagemans, B. Koopmans

SPINOS III - Conference on Spins in Organic Semiconductors; Amsterdam, Netherlands, the (30 aug - 3 sep

2010)

Magnetic properties of Fe nanowires grown by focused-electron-beam-induced deposition

R. Lavrijsen, R. Cordoba, F. Schoenaker, T. Ellis, B. Barcones Campo, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans, J.M.

de Teresa, C. Magen, M.R. Ibarra, P. Trompenaars, J.J.L. Mulders

International symposium on metallic multilayers; Berkeley, California, U.S. (13 sep - 17 sep 2010)

Tunable domain wall injection PMA stripes by focused He and Ga beams

J.H. Franken, M. Hoeijmakers, R. Lavrijsen, J.T. Kohlhepp, E. van Veldhoven, D.J. Maas, H.J.M. Swagten, B. Koopmans

International symposium on metallic multilayers; Berkeley, California, U.S. (13 sep - 17 sep 2010)


FNA Annual Report 2010 7.Output 67

7.6 PhD Theses

Plastic Spintronics; spin transport and intrinsic magnetoresistance in organic semiconductors

W. Wagemans

June 2010

Organic Magnetoresistance; An investigation of microscopic and device properties

Francisco Bloom

November 2010

7.7 Master Theses

Exploring spin interactions in organic semiconductors

A.J. Schellekens

February 2010

Domain wall motion in perpendicularly magnetized ultrathin Pt/CoFeB/Pt films

J.H. Franken

February 2010

Exploring the fabrication of ferromagnetic nanostructures by electron beam induced deposition

F.J. Schoenaker

February 2010

Near surface quantum wells induced by buried nanoparticles in Copper

C.O. Avci

Augustus 2010

Novel experimental and modeling approaches to organic spin-valves (Intern)

M.J.M. van Schijndel

October 2010

Toward ultra fast spin transfer detection

R. Paesen

October 2010

Nano-stencil fabrication process for spin-torque devices

P.E.D. Soto Rodriquez

November 2010

Racing domain walls - A micromagnetic study

G.C.F.L. Kruis

December 2010


68 FNA Annual Report 2010 7.Output

7.8 Internship Reports

The Gilbert damping constant in Ga irradiated Pt/CoFeB/Pt

M. Herps

January 2010

Innovative materials and characterization techniques for organic spin valve devices

M. Hoeijmakers

ISMN-CNR Bologna, Italy

March 2010

Resistive behavior of Fe-rich amorphous

M.M. Haverhals

University of Uppsala, Sweden

Augustus 2010

Gilbert damping of perpendicularly magnetized Pt/Co/

F.H.A. Elich

October 2010


FNA Annual Report 2010 7.Output 69

7.9 Publicity

(See also pages 72-76 for a selection of items shown below)

Bronzen schaatsmedaille voor professor Koopmans

N.a.v. behalen van derde plaats in het gewestelijk kampioenschap marathonschaatsen voor Brabant, Limburg

en Zeeland in de categorie Masters

Cursor 16 (21 jan 2010)

En ik vind… BKO op herhaling

Blog Henk Swagten

Cursor, (4 feb 2010)

VICI-hoogleraar Henk Swagten, interview

Reprint in first trial edition of N!

N! periodic of SVTN "J.D. van der Waals", STOOR and VENI, (spring 2010)

Nieuws op onderzoeksgebied; Snellere harde schijven door laserlichtflitsen

Interview met B. Koopmans

Matrix (spring 2010)

Spins in organische materialen

Interview met A.J. Schellekens

N! periodic of SVTN "J.D. van der Waals", STOOR and VENI, (autumn 2010)

OMAR: over plastic en koelkast magneetjes

Interview met Wiebe Wagemans

Cursor, (17 jun 2010)

OMAR: over plastic en magneetvelden

Interview met Wiebe Wagemans

MATRIX, (autumn 2010)

Vraag het vier vaders

Paul Janssen (board member VENI), interview met Bram van Gessel, Maikel Goosen, Jurgen Schoonus and Henk Swagten

N! periodic of SVTN "J.D. van der Waals", STOOR and VENI, (summer 2010)

Hoe?zo! Radio (RTN radio)

Interview met Bert Koopmans

Hilversum, (30 sep 2010)

Nominatie voor Huijbregtsenprijs ‘Wetenschap & Maatschappi’

B. Koopmans

Ridderzaal, Den Haag, (1 nov 2010)


70 FNA Annual Report 2010 7.Output

70

En ik vind...

BKO op herhaling

Prof. Fred Steutel, emeritus hoogleraar Wiskunde, vraagt zich in de rubriek ‘En ik vind


FNA Annual Report 2010 7.Output 71

71

Bronzen schaatsmedaille voor professor Koopmans

21 januari 2010 - Prof.dr. Bert Koopmans van de faculteit Technische Natuurkunde is tijdens het

gewestelijk kampioenschap marathonschaatsen voor Brabant, Limburg en Zeeland derde

geworden in de categorie Masters. Op een natuurijsbaan in Wijk en Aalburg moest hij woensdag

13 januari slechts twee andere concurrenten voor laten gaan.

Koopmans bereikte de finish na 45 ronden, omgerekend ruim twintig kilometer. Over de piste niets

dan lof. ‚Geweldig ijs. Geen scheurtje te zien. Ze hadden de baan goed geveegd en de bochten lagen

er prachtig bij. Ach, goed of slecht, natuurijs is altijd een genoegen.‛

Het gebeurt niet iedere dag dat een TU/e-hoogleraar een medaille haalt. En zeker niet tijdens een

kampioenschap op natuurijs, dat door de zachte winters nog maar sporadisch mogelijk is. Maar dat

Koopmans in de prijzen rijdt, is voor insiders niet echt een verrassing. Hij mag dan wel hoogleraar

Fysica van Nanostructuren zijn, maar is tevens een begenadigd schaatser. Geboren in Norg, in de

strenge winter van 1963. Het legendarische schaatsjaar waarin Reinier Paping de Elfstedentocht

won.

In zijn jeugdjaren zat hij in de schaatsselectie van Jong Oranje en op zijn 23ste kreeg hij een plaats

binnen de Nederlandse kernploeg. Baantjes draaien met Hein Vergeer en Leo Visser, met een

voorliefde voor de lange afstand. Op NK’s veroverde hij een reeks medailles en in 1987 was hij

reserve voor het EK en WK. ‚De mate van getraindheid is tegenwoordig wat minder‛, zegt hij. ‚Ik

oefen nog maar twee keer een uurtje per week op de ijsbaan in Eindhoven.‛ (FvO)/.


72 FNA Annual Report 2010 7.Output

72

Reprint


FNA Annual Report 2010 7.Output 73

73

OMAR: over plastic en koelkast magneetjes

17 juni 2010 - Stroomgeleidende plastics vinden razendsnel

toepassing in displays en lichtgevende folies (op basis van

zogeheten OLED’s) en zonnecellen. Maar daarmee zijn de

grenzen van deze plastics, die in potentie snel, goedkoop en

milieuvriendelijk kunnen worden geproduceerd, nog niet

bereikt. Dr.ir. Wiebe Wagemans onderzocht in de groep

Physics of Nanostructures van prof.dr. Bert Koopmans de

effecten van magneetvelden op geleidende plastics.

Wiebe Wagemans bij de gloveboxopstelling.

Foto: Bart van Overbeeke

Wiebe Wagemans (29) vertelt enthousiast over zijn promotieonderzoek: ‚Zo rond de tijd dat ik bij Koopmans afstudeerde, werd duidelijk

dat de weerstand voor stroomgeleiding van deze plastics verandert als je ze blootstelt aan een magneetveld.‛ Een intrigerend fenomeen,

vindt hij. ‚De weerstand kan wel tot wel tientallen procenten veranderen bij magneetvelden kleiner dan een koelkastmagneet. En dat ook

nog eens gewoon bij kamertemperatuur. Van vrijwel alle materialen verandert de elektrische weerstand onder invloed van magneetvelden,

maar dat is vaak pas bij heel sterke magneetvelden of extreem lage temperaturen.‛

Dat dit magnetische effect, OMAR gedoopt (van ‘organic magnetoresistance’), optreedt onder zulke milde omstandigheden maakt het

volgens Wagemans zeer bruikbaar voor toepassing in magneetsensoren. Het effect lijkt bovendien te bestaan bij alle geleidende plastics.

‚OMAR is zelfs gewoon zichtbaar in de bestaande OLED’s. Het lijkt erop dat je magnetische pennen kunt ontwikkelen waarmee je op

OLED’s kunt schrijven, daarvoor hoef je de OLED niet eens van een extra laag te voorzien.‛ Je hebt ook geen heel bijzonder materiaal nodig,

zegt Wagemans. Vrijwel elk plastic dat stroom geleidt, vertoont het OMAR-effect. ‚Het maakt niet uit of je plastics maakt uit lange

polymeren, of dat je organische materialen maakt uit kleine moleculen. Alleen voor de sterkte van het effect maakt het wat uit of je bij wijze

van spreken spaghetti of macaroni gebruikt.‛

Magnetoresistance is een term die ook bekend is uit de reguliere elektronica. Zo werken afleeskoppen van moderne harddisks op basis van

giant magnetoresistance (GMR), dat de ontdekkers in 2007 nog de Nobelprijs opleverde. Toch werkt OMAR heel anders dan zijn

tegenhanger in metalen. Dat hangt samen met de manier waarop lading wordt getransporteerd in plastics, zegt Wagemans. ‚De elektronen

zijn er minder vrij dan in metalen. Ze hoppen echt van een plek naar de volgende, waarna ze weer een tijdje stilzitten. Ze gaan 101 stappen

naar voren en 100 terug. Daarbij bewegen ze zich als het ware door een soort berglandschap, waar ze over bergpaadjes moeten wandelen.

Hierdoor komen de elektronen elkaar ook regelmatig tegen. Stel nu dat er een elektron in een kuil is beland en een ander elektron er langs

wil, dan lukt dat alleen als de twee elektronen tegengestelde spin hebben.‛

Spin is een eigenschap van elektronen die kan worden opgevat als een inwendig magneetje dat ontstaat doordat het elektron snel om zijn as

‘spint’. De richting van deze elektronspin bepaalt of het ‘vrije’ elektron het elektron in de kuil gemakkelijk kan passeren. ‚Twee elektronen

die elkaar ontmoeten, hebben in de helft van de gevallen tegengestelde spin. Dan hebben ze geen last van elkaar. Zolang de spins echter in

dezelfde richting staan, zitten ze vast.‛ Gelukkig gaan de spins draaien als ze worden blootgesteld aan kleine lokale magneetvelden,

opgewekt door naburige waterstofatomen. Dan is het een kwestie van wachten totdat de spins een verschillende richting krijgen en het vrije

elektron kan doorstromen. En die wachttijd bepaalt de elektrische weerstand van het materiaal.

Wagemans: ‚Omdat de magneetvelden van de omliggende waterstofatomen in willekeurige richting wijzen, komen de spins op een

gegeven moment altijd in tegengestelde richting te staan. Behalve wanneer je een sterker extern magneetveld aanlegt; dan gaan alle

elektronen daar op dezelfde manier omheen draaien en verandert de relatieve oriëntatie niet.‛ Met andere woorden: met een extern

magneetveld blijven de elektronen in een impasse zitten, met een grotere elektrische weerstand als gevolg.

Wagemans stelde een eenvoudig model op waarin slechts het elektron in de kuil en de wachtende voorbijganger figureren. ‚Heel simpel,

maar het bevat alle essentiële ingrediënten.‛ Daarnaast bracht hij veel tijd in het lab door, waar hij een gloveboxopstelling -een luchtdichte

ruimte waar je van buitenaf met handschoenen bij kunt- bouwde voor de organische samples die hij maakte met hulp van de groep van

prof.dr.ir. René Janssen, expert op organische zonnecellen en andere geleidende plastics. ‚Die samples zijn heel gevoelig voor water en

zuurstof. Daarom heb ik de experimenten in de glovebox onder beschermende stikstofatmosfeer uitgevoerd.‛

In de glovebox positioneerde Wagemans de samples tussen twee forse elektromagneten, waarmee hij de sterkte en frequentie van het

magneetveld kon variëren. Hij ontwikkelde een extra gevoelige techniek om de invloed van de oriëntatie en frequentie van het externe

magneetveld op de weerstand van het sample te kunnen meten. Hierdoor ontdekte Wagemans dat de spins niet alleen afhankelijk zijn de

magneetvelden uit de omgeving, maar dat de elektronspins ook met elkaar ‘praten’, zoals hij het noemt.

Wagemans onderzocht ook of je geleidende plastics kunt gebruiken in GMR-sensoren. ‚In principe werkt dit zelfs beter met organisch

materiaal dan met koper, omdat de spin van de elektronen beter behouden blijft in organische materialen. Dat blijkt te kloppen, maar je

hebt nog wel last van de wisselwerking tussen de elektronspins onderling.‛

Toch concludeert Wagemans dat het zeker mogelijk is om afleeskoppen en andere magnetische elementen van organische materialen te

maken. ‚Je moet dat niet zien als concurrentie voor de huidige metalen GMR-systemen, maar het is voor de organische elektronica wel een

belangrijke stap vooruit als je sensoren en ook geheugenelementen van plastic kunt maken.‛ (TJ)/.


74 FNA Annual Report 2010 7.Output 75

74


FNA Annual Report 2010 8.Social Events 75

8 Social Events

Publication parties

Within FNA it is common that great academic success

is followed up by even greater parties, where the

group comes together with food, beer and sometime

even champagne. This year’s scientific output ensured

that many afternoons ended with a toast.

Garden party

Also this year the whole group moved to the far east

of Nuenen to enjoy the hospitability of Bert during the

yearly garden party. Unless the blistering cold, most

group members stayed outside until the bitter end,

while enjoying the great food, wine, beer and other

drinks.

Spin ‘m d’r in – Championship

The final season of the indoor soccer

championship was one not to forget. The

talented squad of Spin ‘m d’r in won all

of their matches with relative ease and

therefore ranked 1 st in their competition!

Unfortunately team coach Koen forgot to

thank his team with delicious ‘vlaai’, but

star players Jeroen and Sjors saved the

day so that FNA could celebrate their

unbeaten championship.

TU/e Experience

Since we are always eager to share our dedication

to fundamental physics with a broad audience,

FNA highly contributed to the successful TU/e

Experience (formerly known as the

‘publieksdag’). With for example a challenging

Nanoquiz, a tour through the decorated ‘kopzaal’

and a demonstration of the magnetic coil gun,

FNA managed to give many people some more

insight in the work that we do.

Table tennis tournament

When someone enters the e-wing of our n-laag building, there is one object in particular that will immediately

draw your attention. One of the two student rooms is equipped with a professional table tennis setup to prevent

the hard working students to suffer from RSI. The combination of the setup and a group of ambitious FNA

physicists of course resulted in a challenging tournament. This year Koen and Jef made it towards the final, but

after a true epic battle, Koen turned out to be the deserved winner of the tournament!


76 FNA Annual Report 2010 8.Social Events

Group outing

By Sjors Schellekens

Every year there is one day in particular where physics is not the most debated topic between the group

members. This day is of course the legendary annual FNA group outing. This year the bus with all the members

set course for Oirschot, where some typically Dutch activities were prepared waiting. After arriving at a

beautiful small farm, and drinking a fast coffee to make sure that everybody was awake, the group was split in

two. The first group started ‚Skiking‛, which can best be described as cross-country skiing on wheels. After a

hesitating start everybody got the hang of it, and a wonderful tour through the countryside was made.

Soaked in sweat the group finally skiked back to the farm, where the other group was playing the typically

Dutch game ‚Ganzebord‛. However, this version of Ganzebord was not played on a board but on a small

pasture. The players had to get to the end of the game by rolling an enormous dice and answering difficult

questions about the Dutch countryside. As if this was not hard enough, players had to cooperate in small

groups of two, were the legs of the players were tied together. After a tough game of Ganzebord everybody was

hungry, indicating that it was time for typically Dutch farmer’s lunch.

When all group members were stuffed, both groups switched activities. The day at the farm ended by another

typically Dutch game, namely ‚klootschieten‛. The idea of the game is simple; each team has to throw a ball

along a route over the countryside. The team that requires the smallest amount of throws to get to the finish

wins. However, in practice the game turned out to be far more difficult than expected. Sharp turns, shallow

waters and large trees were all over the route, making the journey to the finish far from trivial. Luckily all teams

made it to the end without losing the precious kloot.

They day ended back in Eindhoven, were Can organized a dinner at a delightful Turkish restaurant. The food

was terrific, the Turkish beer was surprisingly good, and the atmosphere was even better. A perfect way to end

a successful group outing, let’s hope next year’s outing will be as good as this one!

Sinterklaas

In december 2010, Sinterklaas and his

Black Pete (Mathijs and Matthijs) came

all the way from Boekel to Eindhoven to

visit the group of FNA, where they

surprised the members with spicenut,

marzipan, chocolate, drinks and other

traditional snacks. On top of that, each

member was given a small gift and a

short but brilliant poem, mostly

containing a wholehearted hint for the

future.


FNA Annual Report 2010 8.Social Events 77

Oudejaarsbraspartij

If you ask one of the FNA members about the best way to celebrate Christmas and New Year’s Eve in one night,

they will definitely tell you about their yearly ‘Oudejaarsbraspartij’. At this evening, all members came together

in the decorated presentation room and contributed to the dinner with starters, main courses and deserts. This

year, the menu (which was coordinated by Sonja, girlfriend of Matthijs Cox) consisted for example among other

things of Freshly Fried Turkish Feta Rolls, Traditional Italian Tiramisu and Extremely Heavy Meat Loaf. Despite

the surprising absence of ‘Oliebollen’ on the menu, we all can

say that it just was delicious! The evening was ended up in

style by the traditional ‘piekschieten’ (tree-peak-shooting) and

the election of the Blunderbokaal election.

Blunderbokaal

The person who is responsible for the greatest work

related blunder within FNA, is every year rewarded with

the famous Blunderbokaal. This year, the competition

was fierce, but at the end it was clear that the nomination

of Sjors Schellekens was most voted for. In spite of his not

so strong pledge for innocence, he won the trophy for his

attempt to ruin both of the laser setups and thereby

bringing himself (and others) in danger. Sjors did not

want to comment on his blunder, but claimed that he

received the ‘Blunderbokaal’ on a personal note

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