Plenarvorträge - DPG-Tagungen

Magnetismus Dienstag
in der Schichtebene führt. Bei den Fe-Schichten tritt die Austauschanisotropie
überraschenderweise nicht auf. Die Effekte werden an Hand von
detaillierten Magnetisierungsmessungen diskutiert
[1] B. Sass et al., submitted to J. Phys. C
MA 13.55 Di 15:00 Bereich A
Magneto-Transport Properties of Multilayer Antidot Arrays
— •Kornelius Nielsch, Fernando J. Castaño, and Caroline
A. Ross — Department of Materials Science and Engineering, Massachusetts
Institute of Technology, Cambridge, MA, USA
The magneto-transport properties of Pseudo-spin-valve (PSV) thin
film structures grown onto sub-200 nm porous alumina templates have
been investigated. Hexagonal and quadratic pore arrangements were
achieved by defining a grid in silicon substrates using a combination
of interference lithography and reactive ion etching. An aluminum layer
was then evaporated and anodized, resulting in large area porous structures
with interpore distance of 180 nm (quadratic lattice) and 208 nm
(hexagonal lattice) and a pore diameter ranging from 80 to 150 nm. The
PSV antidot arrays were fabricated by depositing NiFe/Cu/CoFe thin
film structures onto the alumina templates. The room temperature hysteresis
loops of the antidot structures show the distinct switching of both
soft and hard magnetic layers, as well as a significant shearing of the
loop, as compared with that of the PSV thin film structure on a Si substrate.
Magnetic anisotropies induced by the different arrangements will
be discussed. Surprisingly the giant magnetoresistance ratio of the PSV
antidot arrays is only slightly smaller than that of the PSV thin film
structure.
MA 13.56 Di 15:00 Bereich A
Interlayer exchange coupling of magnetic rare-earth thin films
— •J.E. Prieto, K. Döbrich, O. Krupin, F. Heigl, G. Kaindl,
and K. Starke — Institut für Experimentalphysik, Freie Universität
Berlin, Arnimallee 14, 14195 Berlin
Exchange coupling between magnetic layers determines the properties
of magnetic heterostructures. Magnetic moments in rare-earth metals
are strongly localized and couple only indirectly through the conduction
electrons. In the case of magnetic films separated by nonmagnetic spacer
layers, the thickness and electronic structure of the latter determine the
sign and magnitude of the coupling. We have studied epitaxial Gd/Y/Tb
trilayers on W(110) element-specifically by means of x-ray magnetooptical
Kerr effect (XMOKE) performed at the M4,5 absorption thresholds
of Gd and Tb. The two magnetic rare-earth films have different coercivities.
For applied magnetic fields smaller than the Tb coercivity, shifted
Gd hysteresis curves are observed. A strong dependence of this exchange
bias on the thickness of the Y spacer layer as well as on the temperature
is found.
MA 13.57 Di 15:00 Bereich A
Transition rates via Bethe ansatz for the XXZ and the XX
chain — •Daniel Biegel 1 , Michael Karbach 1,2 , Gerhard
Müller 2 , and Klaus Wiele 1 — 1 Bergische Universität Wuppertal,
Fachbereich Mathematik und Naturwissenschaften, Physik, 42097
Wuppertal — 2 Department of Physics, University of Rhode Island,
Kingston, RI 02881-0817, USA
As part of a study that investigates the dynamics of the s = 1
2 XXZ
model in the planar regime |∆| < 1, we discuss the singular nature of
the Bethe ansatz equations for the case ∆ = 0, i.e. the XX model. We
identify the general structure of the Bethe ansatz solutions for the entire
XX spectrum, which include states with real and complex magnon momentas.
We present determinantal expressions for transition rates of spin
fluctuation operators between Bethe wave functions. As an application,
we calculate lineshapes for dynamic structure factors relevant for experiments.
The predominant excitations are identified within the framework
of quasiparticles excited from a corresponding vacuum.
MA 13.58 Di 15:00 Bereich A
Generation of ultrashort current pulses with photoconductive
switches — •S. Bretschneider 1 , A. Parge 1 , J. Seeger 2 , T.
Salditt 2 , A. Förster 3 , and M. Münzenberg 1 — 1 IV. Phys. Inst.
Universität Göttingen — 2 Inst. für Röntgenphysik Universität Göttingen
— 3 Inst. für Schichten und Grenzflächen (ISG1) Forschungszentrum
Jülich
Ultra short current pulses can be generated by excitation of a photoconductive
switch with fs-laser pulses from a TiSa-laser system. As material
for the switches we use low temperature grown GaAs (LT-GaAs).
Due to the short carrier life time in LT-GaAs current pulses of the optical
switches are in the ps-time scale. It is shown how to prepare the
switches with ebeam- and optical lithography and how to characterize
the current pulses with auto correlation and photoconductive sampling
techniques. In the future these current pulses will be used to study the
magnetization dynamics of magnetic micro- and nanostructures which is
for example important for magnetic switching in MRAM-elements.
MA 13.59 Di 15:00 Bereich A
Simulation of ultrafast dynamics in all optical pump probe experiments
on nanoscale Permalloy elements — •G. Eilers and
M. Münzenberg — IV. Phys. Inst., Universität Göttingen
Nano-size ferromagnetic structures are of strong interest for future high
speed magnetic random access memory and ultra high density magnetic
storage. Magnetic precession Eigen modes depending on the length/width
ratio of rectangular nanostructures of Permalloy are studied using the
Landau-Lifshitz-Gilbert equation. Aim is to investigate the limits of precessional
switching speed in the limit of smallest sized elements.
Demagnetization effects observed in all optical pump probe experiments
with ultrafast Ti:Sapphire lasers still lacks the theoretical background:
electrons are excitated by a pump laser pulse and a subsequent
demagnetization of the spin system in the order of 5% is observed. We
used a demagnetization effect of a laser spot in between 5 to 100 %
in lateral ferromagnetic thin films and nanostructured rectangular ferromagnetic
elements in our simulations. Magnetization dynamics in the
ps to ns range is compared to precession modes observed in all optical
pump probe experiments. For the simulation the oommf micromagnetic
simulation tool is used.
MA 13.60 Di 15:00 Bereich A
Multiples Schaltverhalten von Nanoleiterbahnen komplexer
Geometrie — •Mario Brands 1 , Britta Hausmanns 2 und
Günter Dumpich 1 — 1 Experimentalphysik, AG Farle, Institut für
Physik, Fakultät IV, Universität Duisburg-Essen, Standort Duisburg,
Lotharstrasse 1, 47048 Duisburg — 2 AG Magnetismus, Fachbereich
Physik, Technische Universität Kaiserslautern, Erwin-Schrödinger-Str.
56, 67663 Kaiserslautern
Es wurden Widerstandsmessungen an dünnen Kobalt-Leiterbahnen bei
einer Temperatur von T = 4, 2K und in einem äußeren Magnetfeld von
bis zu B = 4, 5T durchgeführt. Die Leiterbahnen wurden mittels eines
Zwei-Schritt-Elektronenstrahllithografie (EBL)-Prozesses auf GaAs Substraten
hergestellt und haben eine konstante Länge von l = 400µm wohingegen
ihre Breite (w = 100nm bis w = 2000nm) und ihre Schichtdicke
(t = 7nm bis t = 32nm) systematisch variiert wurde. Zum Schutz vor
Oxidation wurden die Leiterbahnen in-situ mit einer t = 2nm dicken Platinschicht
abgedeckt. Der Magnetowiderstand (senkrecht, out-of-plane)
ist negativ und kann im wesentlichen auf den Anisotropen Magnetowiderstand
(AMR) zurückgeführt werden. In longitudinaler Geometrie werden
bei geringen Magnetfeldern (Bsw ≈ 100mT) charakteristische Peaks
im Magnetowiderstand beobachtet, die den Ummagnetisierungsprozess
einzelner Leiterbahnabschnitte charakterisieren. Auf der Basis dieser Ergebnisse
wird das Auftreten von Domänenwandwiderstandsbeiträgen diskutiert.
Diese Arbeit wird gefördert von der DFG im Rahmen des SFB 491.
MA 13.61 Di 15:00 Bereich A
Inductive time-domain measurement of magnetization dynamics
in epitaxial Fe1−xCox-films — •Tobias Martin, B. Becker, S.
Ganzer, T. Hagler, W. Kipferl, M. Sperl, and G. Bayreuther
— Institut für Experimentelle und Angewandte Physik, Universität Regensburg,
93040 Regensburg
For the development of faster memory-technologies, it is important to
study the magnetization dynamics in thin magnetic films.
Here, the precessional motion of the magnetization is excited by a
fast magnetic field step or pulse with a risetime of about 70ps, which is
produced by a current guided through a coplanar waveguide. The precession
of the magnetization induces a signal in the same or in a second
waveguide, which is detected by a digital sampling oscilloscope with a
bandwidth of 20 Ghz.
This setup is used to investigate the magnetization precession in epitaxially
grown Fe1−xCox-films. The film and the waveguide are brought into
close proximity thus allowing for simple changing of samples and pulsefield
angle. The magnetization dynamics, i.e. precessional frequency and
damping coefficient, are determined for different values of the uniaxial
and fourfold magnetic anisotropies, which can be controlled by the thick-