Plenarvorträge - DPG-Tagungen

Magnetismus Donnerstag
MA 24.4 Do 12:30 H23
MAGNETIC-FIELD-INDUCED STRAINS IN FERROMAG-
NETIC SHAPE-MEMORY ALLOYS — •U.K. Rößler 1 ,
A.N. Bogdanov 1,2 , A. DeSimone 3 , S. Müller 2 , and F. Otto 4
— 1 Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden,
Postfach 270116, 01171 Dresden — 2 Max-Planck-Institut für Mathematik
in den Naturwissenschaften, Inselstr. 22 - 26, 04103 Leipzig
— 3 International School for Advanced Studies, via Beirut 2-4, 34014
Trieste — 4 Institut für Angewandte Mathematik, Universität Bonn,
Wegelerstr. 10, 53115 Bonn
Phenomenological equations for multivariant states in a ferromagnetic
martensitic materials have been derived [1]. We apply the theory to describe
the evolution of a tetragonal twinned martensitic microstructure
consisting of two variants. Analytical solutions for the components of the
strain tensor can be obtained as functions of external stresses and the
volume fractions of the twin variants. Equilibrium magnetic parameters
have been calculated within phase theory approximation and for a model
of stripe domains. Critical lines between multivariant (twinned) states
and the two types of homogeneous states and corresponding field-stress
phase diagrams have been calculated. Sufficiently high stresses can block
multivariant states. Then, the transition between the two homogeneous
states occurs as a first-order process. Time-dependent processes can be
simulated based on a kinetic equation within this phenomenological theory.
[1] A.N. Bogdanov, A. DeSimone, S. Müller, U.K. Rößler, J. Magn.
Magn. Mater. 261, 204 (2003).
MA 24.5 Do 12:45 H23
Modulated martensitic structures in MBE-grown NiMnAl
magnetic shape memory alloys — •Sigurd Thienhaus 1 , Ralf
Hassdorf 1 , Jürgen Feydt 1 , Rene Borowski 1 , Markus Boese 2 ,
and Michael Moske 1 — 1 Forschungszentrum caesar, 53175 Bonn —
2 Universität Bonn, Institut für Anorganische Chemie, 53117 Bonn
Composition spreads close to the Heusler alloy Ni2MnAl were grown
onto four-inch wafer substrates by molecular beam epitaxy. Compositional
variations of around 10 at.% relative to each constituent enable a
direct comparison of the chemical-structural relationship with respect to
martensitic transformation and to magnetic ordering as well as an efficient
identification of the emerging phase stability regions. In our study,
MA 25 Hauptvorträge Thiaville / Kläui
we set the primary focus on the structural aspects of the transformation
behavior as confirmed by X-ray microdiffraction in combination with a
specially designed heating stage. Notably, high-resolution cross-sectional
TEM imaging of the respective composition areas reveals a laminated
two-phase martensitic structure within the single grains, identified as
a sequence of 2M and 14M variants. Stress relief upon transformation
is observed during thermal processing in a bending-beam stress apparatus
and ranges from 50 to 200 MPa depending on the composition.
Vibrating-sample magnetometry so far suggests magnetic ordering to occur
well below room temperature. In this context, the compositional and
microstructural aspects of the phase stabilities will be discussed.
MA 24.6 Do 13:00 H23
Ab-initio Struktur-Simulationsrechnungen zur magnetischen
Formgedächtnislegierung Ni-Mn-Al — •Thomas Büsgen 1 ,
Jürgen Feydt 1 , Ralf Hassdorf 1 , Sigurd Thienhaus 1 , Markus
Boese 2 , Alexey Zayak 3 , Peter Entel 3 und Michael Moske 1
— 1 Center of Advanced European Studies and Research (caesar),
Ludwig-Erhard-Allee 2, 53175 Bonn, Germany — 2 Universität Bonn,
Institut für Anorganische Chemie, 53117 Bonn, Germany — 3 Gerhard-
Mercator-Universität Duisburg-Essen, Lotharstr. 1, 47048 Duisburg,
Germany
Es werden die Ergebnisse von ab-initio Berechnungen, basierend auf
DFT und PAW Basissätzen, zur magnetischen Formgedächtnislegierung
Ni-Mn-Al vorgestellt. Dabei wurde die Abhängigkeit der Stabilität und
physikalischer Eigenschaften von der Zusammensetzung verfolgt. Es kann
gezeigt werden, dass der Grundzustand der Legierung bei einer festen
Ni-Konzentration von 50 at.% im Bereich von 14 bis 31 at.% Mn ferromagnetisch
ist. Die magnetische Eigenschaften werden u.a. anhand der
Zustandsdichten erklärt.
Desweiteren wurden die martensitischen Phasen 2M, 10M und 14M
mit langperiodischen Modulationen für Ni2MnAl simuliert. Alle untersuchten
Martensitphasen sind metastabil für die exakt stöchiometrische
Zusammensetzung. Ihre Stabilität wird in Bezug zu Zustandsdichten und
Ladungsverteilung zwischen den Atomen diskutiert. Aus den relaxierten
Strukturdaten wurden Röntgendiffraktogramme berechnet, die zusammen
mit TEM-Untersuchungen die Existenz der 2M- und 14M-Phase in
Proben belegen, die mittels Molekular-Strahl-Epitaxie hergestellt wurden.
Zeit: Donnerstag 14:00–15:00 Raum: H10
Hauptvortrag MA 25.1 Do 14:00 H10
The physics of magnetic domain wall motion in nanostructures
— •André Thiaville, Yoshinobu Nakatani, and Jacques Miltat
— CNRS-Uni. Paris-sud, Lab. physique des solides, 91405 Orsay, France
This talk will present our work on the domain wall motion in lithographically
defined flat nanowires (thickness 5-10 nm, width 100-200 nm).
These structures show domain walls and a dynamic behaviour that are
slightly more complex than a one-dimensionnal (1d) solution, whose prototype
is the Walker theory for a 1d Bloch wall. First I will discuss field
driven propagation (field along the wire axis), which is characterized by
the magnetization precession around the applied field. As the wire is too
wide to be 1d the so-called Walker breakdown, where a continuous precession
of the magnetic moment of the domain wall sets in, is shown to
be replaced by the periodic injection and disappearance of an antivortex.
This phenomenon is very sensitive to the edges perfection : a wire having
rough edges will not undergo this mechanism and hence support fast wall
propagation at high fields. Then I will explain our study of the domain
wall motion induced by a current in the nanowire, due to the spin-transfer
torque. As the form of this torque is not yet fully agreed on, especially
in the wire geometry, I will present calculations incorporating different
formulations of this torque.
Hauptvortrag MA 25.2 Do 14:30 H10
Magnetotransport and current induced domain wall propagation
in ring structures with constrictions — •Mathias Kläui 1,2 ,
C. A. F. Vaz 2 , J. A. C. Bland 2 , L. J. Heyderman 3 , W. Wernsdorfer
4 , and U. Rüdiger 1 — 1 Universität Konstanz, Fachbereich
Physik, 78457 Konstanz, Deutschland — 2 Cavendish Laboratory, Madingley
Road, Cambridge, CB3 0HE, UK — 3 LMN, Paul Scherrer Institut,
5232 Villigen-PSI, Schweiz — 4 Laboratoire Louis Neel-CNRS, BP 138,
38274 Grenoble, Frankreich
The key to using magnetic nano-elements in applications is the presence
of well-defined remanent states and reproducible switching. A possible
geometry that fulfills these criteria is the ring geometry where due to
the high symmetry particularly simple states and switching occur [1]. In
addition to the flux-closure vortex state, the high-moment ’onion state’
was observed, which is characterized by two transverse or vortex headto-head
domain walls [1].
Owing to the constant curvature, rings have been found to be an ideal
geometry for probing domain wall properties. Using magnetoresistance
measurements, the behaviour of the different types of domain walls at
constrictions has been investigated (attraction or repulsion). Furthermore
the change of resistance as a domain wall moves into a constriction
can be accounted for by the change in spin structure. Finally, high current
densities are used to displace head-to-head domain walls. Such currentinduced
domain wall propagation is explained by a spin-torque effect. [2]
[1] M. Kläui et al., Topical Review in J. Phys. Condens. Matter 15, 985
(2003) [2] M. Kläui et al., Appl. Phys. Lett. 83, 105 (2003)