Plenarvorträge - DPG-Tagungen

Magnetismus Donnerstag
Fe/MnF2. The exchange bias results from an unidirectional anisotropy
which is established below the Néel temperature of the antiferromagnetic
layer and can be identified as a shift of the magnetic hysteresis loop. In
time-resolved Kerr rotation, an ultrashort laser pump pulse generates an
unidirectional anisotropy field pulse which triggers coherent precession
of the magnetization in the ferromagnetic layer. This coherent precession
is monitored by a time-delayed laser probe pulse yielding a quantitative
method to study magnetic anisotropy, ultrafast switching and damping
phenomena.
Work supported by DFG through SPP 1133, grant no. BE 2441/2-1
MA 28.12 Do 18:00 H23
Evidence of spin-pumping effect in the FMR of coupled trilayers
— •T. Toliński 1,2 , K. Lenz 1 , J. Lindner 1,3 , E. Kosubek 1 , and
K. Baberschke 1 — 1 Institut für Experimentalphysik, Freie Universität
Berlin, Arnimallee 14, 14195 Berlin, Germany — 2 Permanent address: Institute
of Molecular Physics, PAS, Smoluchowskiego 17, 60-179 Poznań,
Poland — 3 New address: Institut für Physik, Universität Duisburg-Essen,
Lotharstr. 1, 47048 Duisburg, Germany
The spin dynamics between two ultrathin ferromagnetic layers (Ni, Co)
exchange coupled via a nonmagnetic Cu spacer of a few monolayers is
studied using in situ ferromagnetic resonance. The angular and the temperature
dependence of the linewidth shows pronounced changes that can
be understood assuming the presence of an additional Gilbert damping
resulting from a spin-current pumped through the interface between the
ferromagnetic and nonmagnetic layers and by considering the dynamical
part of the interlayer exchange coupling. In contrast to investigations on
Fe films decoupled by thick Au spacers [1] the effect of the interlayer
exchange coupling on this phenomenon is presented here. The competition
between the Gilbert damping inherent in the ferromagnetic film
and the Gilbert damping developed by the interface leads to an irregular
behavior of the temperature dependence of the resonance linewidth for
both the in-phase and out-of-phase precessing modes. It is shown that
the oscillatory nature of the coupling is also revealed in the dependence
of the resonance linewidth ∆H on the spacer thickness. Supported by
the DFG (Sfb 290, TP A2).
[1] B. Heinrich et al., Phys. Rev. Lett. 90, 187601 (2003).
MA 28.13 Do 18:15 H23
Comparison of Network Analyzer Ferromagnetic Resonance
(NA-FMR) with pulsed inductive Measurements — •Ingo
Neudecker 1 , Georg Woltersdorfer 2 , Matthias Buess 1 , Christian
Back 1 , and Bret Heinrich 2 — 1 Fakultaet fuer experimentelle
und angewandte Physik, Universitaet Regensburg, Universitaetstrasse
31, 93044 Regensburg — 2 Simon Fraser University, 8888 University
Drive, Burnaby, British Columbia V5A-1S6, Canada
MA 29 Hauptvortrag Nielsch
We placed a 20 ML thick Fe film on top of a lithographically fabricated
coplanar waveguide. In a 20 GHz high frequency setup the ferromagnetic
system was excited first with a continuous sinusoidal excitation using a
Vector Network Analyzer and second without changing the setup with a
field pulse (pulse generator with 70 ps rise time) using a 20 GHz sampling
Oscilloscope to detect the signal.
By means of the resonance line one obtains from the NA-FMR both
the resonance frequency from its position and the damping constant from
its width. From the pulsed inductive measurement one extracts the resonance
frequency using Fourier transformation and the damping constant
from the decay of the observed oscillations.
Varying the magnetic bias field one can consequently derive the resonance
behavior for the two methods.
Our purpose was to compare the fairly new NA-FMR method with its
excellent signal to noise ratio which yields data in the frequency domain
with the pulsed inductive measurement providing a time domain access
to the resonance properties of the system.
MA 28.14 Do 18:30 H23
Coherent magnetization dynamics on Gd(0001) at 3 THz and its
temperature dependence — •Uwe Bovensiepen, Alexey Melnikov,
Ilie Radu, Oleg Krupin, Kai Starke, Eckart Matthias,
and Martin Wolf — Freie Universität Berlin, Fachbereich Physik,
Arnimallee 14, 14195 Berlin
Resonant absorption of femtosecond laser pulses at the exchange split
surface state of Gd(0001) excites coherent optical phonons, which lead
to a periodic modulation of the distance between surface and subsurface
layer with a frequency of 3 THz [1]. This vibration has been investigated
in situ by time-resolved second harmonic (SH) generation on Gd(0001)
films grown in ultrahigh vacuum on W(110). The magnetic SH contribution
has been extracted by analysis of the SH yield for opposite magnetization
directions. As a function of pump-probe delay a modulation of the
surface magnetization at 3 THz is detected, which is attributed to a modulation
of the exchange coupling due to the lattice vibration. We suggest,
that this quasi-instantaneous phonon-magnon coupling is observed near
the surface due to a crossing of phonon and magnon dispersion relations.
As a function of temperature the lattice and the magnetic oscillation
amplitude decrease and vanish near the Curie temperature.
Coherent bulk dynamics is observed by transient reflectivity at 10%
higher frequency. These modes are excited due to coupling of the subsurface
region to the surface vibration.
[1] A. Melnikov et al.; Phys. Rev. Lett., (2003) in press.
Zeit: Freitag 10:15–10:45 Raum: H10
Hauptvortrag MA 29.1 Fr 10:15 H10
Ferromagnetische Nanostäbe und Nanoröhren in selbstgeordneten
Aluminiumoxid-Membranen — •Kornelius Nielsch
— BMBF-Nachwuchsgruppe für Multifunktionale Nanostäbe und
Nanoröhren, Max-Planck-Institut für Mikrostrukturphysik, Halle
Die potentiellen Anwendungsgebiete für magnetische Nanoröhren und
Nanostäbe reichen von Biotechnologie zu MRAMs. Aufgrund der potentiellen
Anwendung als Datenspeicherung haben magnetische Nanostabensembles
eine besondere wissenschaftliche Aufmerksamkeit in jüngster
Zeit erfahren. Als Formstruktur wurden bienenwabenförmige Al2O3-
Membranstrukturen zur Herstellung der Ni-, Fe- oder Co-Stabensembles
mit periodischen Abständen von 65 nm und 100 nm und Durchmessern
von 25 bis 55 nm verwendet. Aufgrund von Selbstorganisation sind
die Nanostäbe hexagonal im 2D-Gittern quasi polykristallin angeordnet.
Vor allem Nickel-Nanostabensemles zeigten eine geringe Dipolwechselwirkung
im Ensemble und wurden intensiv mit SQUID-Magnetometer
und MFM charakterisiert. Mittels Imprint-Lithographie konnten großflächig
perfekt geordnete Stabensembles erzeugt werden. Ferner wurden
mittels Brillouin-Lichstreuung quantisierte Energiezustände in den
Magnetstäben nachgewiesen. Durch Polymerbeschichtung der Membranwände
konnten Kobalt-Nanoröhren mit einer Wandstärke von 5 bis
20 nm und einem Durchmesser von 200 bis 500 nm hergestellt werden.
Diese Arbeit fand in Zusammenarbeit mit den Arbeitsgruppen von Prof.
C.A. Ross (MIT, Boston, USA), Prof. H. Kronmüller (MPI-Stuttgart),
Prof D. Weiss (Uni Regensburg) und Prof. M.H. Kuok (National Universität
Singapur) statt.