Plenarvorträge - DPG-Tagungen

Magnetismus Mittwoch
MA 16 Magn. Kopplungsphänomene
Zeit: Mittwoch 15:15–17:30 Raum: H22
MA 16.1 Mi 15:15 H22
Temperature Dependence of Interlayer Exchange Coupling
— •Stephan Schwieger and Wolfgang Nolting — Humboldt-
Universität zu Berlin, Institut für Physik, Newtonstr.15, 12489
Berlin
The coupling of two magnetic layers (e.g. Co,Ni) seperated by a nonmagnetic
spacer (e.g.Cu) is well understood in terms of the quantum
interference model as e.g. proposed by P. Bruno.
The dominant contribution to it’s temperature dependence is less certain.
Two mechanisms that are discussed are the softening of the Fermi
surface within the spacer and the interaction of spin wave excitations
within the magnetic material.
Here another mechanism is proposed, that is based on the strong temperature
dependence of thermodynamic potentials as the free energy.
This reflects the fact, that expectation values like 〈 ¯ Sia ¯ Sib〉 , where ¯ Sia
and Sib indicate spins in different magnetic layers, are strongly temperature
dependent compared to excitations like spin waves or the softening
of the Fermi surface.
This approach is evaluated within the Heisenberg model and compared
to FMR experiments.
MA 16.2 Mi 15:30 H22
Untersuchung der antiferromagnetische Kopplung in CoFeB-
Systemen — •Nils Wiese1,2 , Theodoros Dimopoulos1 , Manfred
Rührig1 , Joachim Wecker1 , Hubert Brückl2 und Günter
Reiss2 — 1Siemens AG, Corporate Technology, Erlangen — 2Universität Bielefeld, Nano Device Group, Bielefeld
In Systemen aus zwei ferromagnetischen Schichten, die durch ein nichtmagnetisches
Material voneinander getrennt sind, kommt es bei dünnen
Zwischenschichtlagen zu einer Kopplung der ferromagnetischen Schichten.
Es wird gezeigt, daß in Systemen, bestehend aus zwei Lagen aus
amorphen CoFeB und einer dünnen Ru-Zwischenschicht, eine oszillierende
Kopplung bei Variation der Zwischenschichtdicke beobachtet wird.
Die Größe der antiferromagnetischen Kopplung J beträgt im ersten Maximum
der Kopplung J = −0.8 J
m2 und ist damit um einen Faktor 10
kleiner als in synthetischen Antiferromagneten aus CoFe/Ru/CoFe.
Die Koerzitivfeldstärke Hc kann über Q = m1+m2 eingestellt werden,
m1−m2
wobei m1, m2 die magnetischen Momente der Einzelschichten sind,[1] und
ändert sich für CoFeB linear mit Q zwischen 0.3 und 1.4 kA
m .
[1] v.d. Berg et al., IEEE Trans. Magn. 32, 4624 (1996)
MA 16.3 Mi 15:45 H22
Evidence for three-dimensional non-collinear antiferromagnetic
order in single-crystalline FeMn ultrathin films — •W. Kuch,
L. I. Chelaru, F. Offi, J. Wang, M. Kotsugi, and J. Kirschner
— Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120
Halle
Experimental evidence for a three-dimensional non-collinear antiferromagnetic
spin structure in ultrathin single-crystalline fcc Fe50Mn50 layers
is obtained from magnetic circular dichroism photoelectron emission
microscopy (XMCD-PEEM) and x-ray magnetic linear dichroism. Layerresolved
as-grown domain images of epitaxial trilayers grown on Cu(001)
are presented in which FeMn is sandwiched between ferromagnetic layers
with different easy axes. Measurements of trilayers using Co or Ni
as the bottom layer, and combinations of Co and Ni as top layer reveal
the simultaneous presence of antiferromagnetic spin components in the
film plane and normal to the film plane. The absence of x-ray magnetic
linear dichroism at the Fe L3 absorption edge in FeMn/Co bilayers points
towards an FeMn spin structure with no collinear order in the film plane
even when in contact with a saturated ferromagnetic layer. This result
has important implications on theoretical models of the magnetic interface
coupling between ferromagnetic and antiferromagnetic layers and
for understanding the exchange bias effect along different magnetization
components.
MA 16.4 Mi 16:00 H22
Magnetically frustrated regions in ultrathin antiferromagnetic
Mn films on Fe(001) — •U. Schlickum, N. Janke-Gilman, B.
Slowik, W. Wulfhekel, and J. Kirschner — Max-Planck-Institut
für Mikrostrukturphysik, Weinberg 2, 06120 Halle
By using spin-polarized scanning tunneling microscopy [1], we studied
the magnetic behavior of ultra-thin antiferromagnetic Mn films grown on
a ferromagnetic Fe(001) substrate. Mn on Fe(001) is a topological antiferromagnet,
so that adjacent Mn layers couple antiferromagnetically to
each other. Where Mn overgrows a step of the underlying Fe(001) substrate,
the thickness of Mn on different sides of the step edge differs by
one monolayer (ML). This results in a magnetically frustrated region at
the step edges. As theoretically predicted [2], this frustration resembles
a 180 ◦ domain wall in the Mn film. We investigated the width of these
topologically enforced walls in the antiferromagnet as a function of Mn
film thickness (between 2 and 20 monolayers). A linear broadening of the
wall width with increasing Mn thickness was found. The width of the
wall is twice the Mn thickness. The experimental results are compared
to theoretical calculations.
[1] U. Schlickum et al., Appl. Phys. Lett. 83, 2016 (2003). [2] D. Stoeffler
et al., J. Magn. Magn. Mater.147, 260 (1995).
MA 16.5 Mi 16:15 H22
Reduction of Néel ”orange peel”coupling in magnetic tunnel
junctions due to interface smoothing using low energy ion beams
— •P.A. Beck, B.F.P. Roos, S.O. Demokritov und B. Hillebrands
— Fachbereich Physik und Forschungsschwerpunkt MINAS,
Technische Universität Kaiserslautern, Erwin-Schrödinger-Str. 56, 67663
Kaiserslautern
The most decisive and sensitive characteristic of magnetic tunnel junctions
(MTJ), which are used in magnetic sensors and memory, is a small
and reproducible switching field. Due to the magnetic dipole coupling
(Neel ”orange peel”coupling) between the free and the pinned magnetic
layers this field is strongly affected by the morphology of the interfaces
in MTJ.
In this work we have used normal incident argon ions with their energy
of 20 − 90eV to smooth the interfaces of MTJs. First, to study the
influence of the ion bombardment a single layer 15nm Ni80Fe20 /SiO2/Si,
identical to a free magnetic layer of a typical MTJ has been used. We
have studied the influence of the ion energy on the smoothing and have
determined the optimum conditions for this process. The morphology of
the layer surface has been imaged using an in-situ scanning tunneling
microscope (STM). The obtained images document the reduction of the
roughness of over 40% after the bombardment. Second, MTJs were prepared
based on the smoothed layers, and their remagnetization curves
have been recorded. In agreement with the STM results, the ”orange
peel”coupling determined from the curves is significantly reduced due to
bombardment.
MA 16.6 Mi 16:30 H22
Two dimensional recrystallization, the key towards temperature
stable Co/Cu multilayers — •Sonja Heitmann 1 , Andreas
Hütten 1 , Guido Schmitz 2 , and Günter Reiss 1 — 1 Universität
Bielefeld, Fakultät für Physik, Universitätsstraße 25, 33615 Bielefeld —
2 Westfälische Wilhelms-Universität Münster, Institut für Materialphysik,
Wilhelm-Klemm-Straße 10, 48149 Münster
One of the major concerns for automotive sensor application is the temperature
stability at elevated temperatures. The breakdown of the GMReffect
amplitude of Co/Cu multilayers beyond 250 ◦ C [1] and 350 ◦ C [2] for
Co layer thickness of 1nm and 1.5nm, respectively, reveals the weakness
of conventionally prepared multilayer systems.
However, the objective of this talk is to demonstrate that alternatively
prepared Cu/Co multilayers are very much able to withstand high temperature
without a degradation of the GMR-effect amplitude. The key
is to utilize a two dimensional recrystallization process at sufficient high
temperatures around 500 ◦ C. It will be shown that these multilayers are
stable upon subsequent annealing at temperatrures between 400 ◦ C and
500 ◦ C. The microstructural evolution will be linked to results concerning
transport, magnetic and microstructure measurements and will reveal
the interplay of layer quality, grain size and orientation with crystalline
anisotropy, antiferromagnetic coupling and GMR.
MA 16.7 Mi 16:45 H22
90 ◦ coupling in (Fe/Cr/Fe)AFM/Cr/Fe system epitaxially
grown on GaAs(001) — •M. Przybylski 1 , J. Grabowski 1 , W.
Wulfhekel 1 , M. Rams 2 , and J. Kirschner 1 — 1 Max-Planck-
Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle — 2 Institute
of Physics, Jagiellonian University, Reymonta 4, 30-059 Krakow, Poland