Plenarvorträge - DPG-Tagungen

Magnetismus Montag
MA 8 Elektronentheorie
Zeit: Montag 15:45–18:00 Raum: H22
MA 8.1 Mo 15:45 H22
High-field magnetic susceptibility of ferromagnetic metals —
•Sergiy Mankovskyy and Hubert Ebert — Dept. Chemie/Phys.
Chemie, Universität München, Butenandtstr. 5-13, D-81377 München,
Germany
The results of a theoretical study of the high-field magnetic susceptibility
of ferromagnetic metals and alloys are presented. The theoretical
description of the magnetic susceptibility is based on a combination of
a linear response approach and the fully relativistic Green’s function
formalism. Additional contributions to the magnetic susceptibility (e.g.
spin-charge response function, contribution related to a Fermi level shift)
become important in the ferromagnetic state if compared with the paramagnetic
one. Illustrating examples are presented for various 3d transition
metal systems. In particular, the magnetic susceptibilities have been calculated
for weak magnetic systems such as ZrZn2 and TiBe2. As is known
from experiment [1,2] they are ferromagnetic under substitution of some
Zn and Be atoms by vacancies and Cu atoms, respectively. The calculations
have been performed both for the ferromagnetic and paramagnetic
states of ZrZnx and TiBexCu1−x alloys. The XMCD spectra of ferromagnetic
solids in the presence of an external magnetic fields are calculated
and compared to those obtained for the case without any fields. [1] B. T.
Mattias and R. M. Bozorth, Phys. Rev., 109 (1958) 604. [2] G. S. Knapp,
F. Y. Fradin and H. L. Culbert, J. Appl. Phys., 42 (1971) 1041.
MA 8.2 Mo 16:00 H22
Structural and magnetic PD–pMOKE of low symmetric Co
— •Helmut Rathgen 1,2 , Mikhail I. Katsnelson 1 , Olle Eriksson
1 , and G. Zwicknagl 2 — 1 Condensed Matter Theory, Department
of Physics, Uppsala University, Box 530, S-75121 Uppsala, Sweden
— 2 Institut für Mathematische Physik, Technische Universität Braunschweig,
Mendelssohnstraße 3, 38106 Braunschweig, Germany
We calculate for the first time the polar MOKE of low symmetric systems
from ab initio methods with a full electrodynamics approach. By
that, a new anisotropy effect is investigated, which is the dependence of
the polar MOKE on the direction of the polarization vector relative to
the crystal lattice. We call this effect PD–pMOKE.
The problem of reflection and transmission from multilayer systems is
solved accurately with a numerical approach. With this, we present a new
computational tool that covers a broad range of magneto–optical effects
of bulk materials as well as multilayer systems. E.g. MOKE, the Farraday
effect, x–ray standing wave effects and the optical response from a Bloch
wall. Results are presented for the Kerr effect of fcc 110 Co as well as
mono–, poly–, and bi–crystalline hcp 11¯20 Co. Pronounced PD–pMOKE
is found, with magnitudes resolvable by experiments. Exceptional frequencies
are predicted at which anisotropies vanish. The special role of
bi–crystals is explained. A short glance is taken on the possibilities of
the new approach. Expectations are presented concerning experiments
on the PD–pMOKE of Co as well as low symmetric FePt.
MA 8.3 Mo 16:15 H22
Influence of dynamical correlation effects on magneto-optical
properties of 3d metals — •Stanislav Chadov 1 , Alexander
Perlov 1 , Hubert Ebert 1 , Livio Chioncel 2 , Alexander Lichtenstein
2 , and Michael Katsnelson 3,4 — 1 Phys. Chemistry Institute,
University of Munich, D-80333 Munich, Germany — 2 University of Nijmegen,
NL-6525 ED Nijmegen, The Netherlands — 3 Institute of Metal
Physics, 620219 Ekaterinburg, Russia — 4 Department of Physics, Uppsala
University, Box 530, SE-75121 Uppsala, Sweden
The optical and magnetooptical properties of 3d metals are theoretically
investigated taking into account dynamical correlation effects. Various
schemes to approximate the effective self-energy, including Second
Order Perturbation Theory and Dynamic Mean Theory+Fluctuation Exchange
methods have been implemented and investigated. It is shown
that accounting for many-body correlation effects is very important for
3d metals to get a quantitative agreement with experimental results.
While LSDA-based calculations of optical and magnetooptical properties
in general lead to a satisfying qualitative agreement with experimental
spectra, inclusion of correlation effects in particular give the correct
position of prominent spectral features.
MA 8.4 Mo 16:30 H22
Bestimmung magnetischer 5d-Momente seltener Erden mittels
XMCD — •H. Wende 1 , A. Scherz 1 , C. Sorg 1 , K. Baberschke 1 ,
A.L. Ankudinov 2 und J.J. Rehr 2 — 1 Institut für Experimentalphysik,
Freie Universität Berlin, Arnimallee 14, D-14195 Berlin — 2 Department
of Physics, Box 351560, University of Washington, Seattle, WA 98195
Die Bestimmung der 5d-Momente seltener Erden mittels magnetischem
Röntgenzirkulardichroismus (XMCD) an den L2,3-Kanten wird nicht nur
durch elektrische Quadrupol-Beiträge (E2: 2p → 4f) erschwert, die
zusätzlich zu den elektrischen Dipol-Beiträgen (E1: 2p → 5d) auftreten
[1]. Vielmehr zeigt es sich, dass die Dipol-Übergangsmatrixelemente eine
starke Spin- und Energieabhängigkeit aufweisen. Daher ist das dichroische
Signal an den L2,3-Kanten seltener Erden selbst nach Trennung
der E1- und E2-Beiträge nicht proportional zur Spin-Zustandsdichte.
Wird die Spin- und Energieabhängigkeit der Matrixelemente bei der Anwendung
der integralen Summenregeln nicht berücksichtigt, so wird sogar
fälschlicherweise eine antiparallele Orientierung der 5d- zu den 4f-
Momenten bestimmt. Wir zeigen hier, wie die Ergebnisse der Anwendung
der Summenregeln am Beispiel experimenteller Tb-Spektren mit
Hilfe von ab initio Rechnungen korrigiert werden können. Gefördert durch
BMBF (05 KS1 KEB4).
[1] H. Wende et al., J. Appl. Phys. 91 (2002) 7361
MA 8.5 Mo 16:45 H22
The electronic structure of ordered and disordered, doped
Heusler alloys: Co2Cr1−xFexAl — •H. C. Kandpal, G. H.
Fecher, S. Wurmehl, H.-J. Elmers, G. Schönhense, and C.
Felser — Johannes Gutenberg - Universität, 55099 Mainz
Doped Heusler alloys and compounds of the type Co2Cr1−xFexAl with
varying Cr to Fe ratio x were investigated experimentally and theoretically.
The electronic structure of the doped Heusler compound was calculated
using different types of band structure calculations (FLAPW,
LMTO, SPRKKR). The ordered compounds turned out to be ferrimagnetic
with the Al magnetic moment being aligned anti-parallel to the 3d
transition metal moments. Only the pure compound Co2CrAl shows - in
non-relativistic calculations - clearly a gap in the minority bands around
the Fermi-energy. The ordered pure compounds exhibit an indirect minority
gap, whereas the ordered, doped compounds exhibit a direct gap.
Magnetic circular dichroism (MCD) in X-ray absorption was measured at
the LII,III edges of Co, Fe, and Cr of the pure compounds and the x = 0.4
alloy in order to determine element specific magnetic moments. Calculations
and measurements show an increase of the magnetic moments
with increasing iron content. The experimentally observed reduction of
the magnetic moment of Cr can be explained by Co-Cr site-disorder. The
presence of the gap in the minority bands of Co2CrAl can be attributed
to the occurrence of pure Co2 and mixed CrAl (001)-planes in the L21
structure. It is retained in structures with different order of the CrAl
planes but vanishes in the X-structure with alternating CoCr and CoAl
planes.
MA 8.6 Mo 17:00 H22
Über die Spinpolarisation von ”halbmetallischen Ferromagneten”
— •Gerhard H. Fecher und Claudia Felser — Institut
für Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-
Universität, 55099 Mainz
Materialien mit einer vollständigen Spinpolarisation an der Fermienergie
werden als halbmetallische Ferromagnete bezeichnet. Diese Definition
setzt eine vollständige Trennung der Spin und Bahn Freiheitsgrade
der Elektronen im Festkörper voraus. Anhand von Symmetrieargumenten
wird aufgezeigt, daß Materialien mit vollständiger Spinpolarisation
prinzipiell unmöglich sind. Dies ist eine direkte Konsequenz der Dirac
Gleichung. Spin-Bahn Wechselwirkung alleine - z.B. als Störung in nichtrelativistischen
Theorien - löst diesen Konflikt nicht, solange nicht voll
relativistische Wellenfunktionen betrachtet werden. Voll-relativistische
Berechnungen der elektronischen Struktur zeigen, daß halbmetallische
Ferromagnete nur als Näherung im nicht-relativistischen Grenzfall existieren.
Letzteres schliesst das Auftreten von Materialien mit extrem hoher
Spinpolarisation in der Nähe der Fermie-energie nicht aus.