Plenarvorträge - DPG-Tagungen

Magnetismus Montag
MA 4 Oberflächenmagnetismus
Zeit: Montag 10:15–13:15 Raum: H22
MA 4.1 Mo 10:15 H22
Structure and magnetism in the MnCu/Cu(001)-c(2x2) and
p2gg-(4x2) surface alloys — •Popescu R. 1 , Meyerheim H.L. 1 ,
Sander D. 1 , Ernst A. 1 , Robach O. 2 , Ferrer S. 2 , Kirschner J. 1 ,
and Bruno P. 1 — 1 MPI für Mikrostrukturphysik, Weinberg 2, 06120
Halle, Germany — 2 ESRF, BP 220, F-38043 Grenoble, France
The two-dimensional magnetic alloy phases formed by deposition of Mn
on Cu(001) in the coverage range between about 0.5 and 1 monolayer
(ML, 1ML=1.53×10 15 atoms/cm 2 ) were investigated by surface x-ray
diffraction (SXRD) and calculations based on the full potential layered
Korringa-Kohn-Rostoker-method implemented within the DFT. While
for the c(2x2) phase formed at 0.5 ML the basic two dimensional alloy
structure model with Mn-atoms replacing every other Cu surface atom
is confirmed (Mn at 0.20± 0.05 ˚Aabove the Cu-layer), in detail an accurate
fit to the 228 reflections (unweighted R≈ 0.07) is only obtained by
taking into account 0.5 ML of Cu atoms above the alloy layer and a fraction
of 20 percent anti-phase domains. The calculations show, that the
magnetic moment of the Mn-atoms is in the 3.8 µB-range independent
of the presence or absence of the Cu-adlayer. The p2gg-(4x2) superstructure
is also characterized by a single MnxCu1−x alloy layer, where the
Mn-concentration (x) is in the range of about 0.8 and the corrugation in
the 0.4˚Arange. The implications of the increased Mn-concentration (x)
on the magnetic moment is discussed.
MA 4.2 Mo 10:30 H22
Magnetism-induced symmetry breaking in photoelectron
diffraction patterns — A. Chassé 1 , •W. Kuch 2 , M. Kotsugi 3 ,
Xingyu Gao 2 , F. Offi 2 , S. Imada 3 , S. Suga 3 , H. Daimon 4 , and
J. Kirschner 2 — 1 Fachbereich Physik, Martin-Luther-Universität
Halle-Wittenberg, D-06099 Halle — 2 Max-Planck-Institut für
Mikrostrukturphysik, Weinberg 2, D-06120 Halle — 3 Graduate School of
Engineering Science, Osaka University, 1–3 Machikaneyama, Toyonaka
560-8531, Japan — 4 Graduate School of Materials Science, Nara
Institute of Science and Technology, Ikoma, Nara 630-0192, Japan, and
CREST-JST, Japan
The influence of spin-polarized photoelectron scattering on the circular
dichroism of 2p3/2 photoelectrons from a ferromagnetic Fe(001)
single crystal surface is studied experimentally and theoretically. Twodimensional
photoelectron intensity angular distribution patterns are
presented. They were recorded at 482.5 eV kinetic energy for both helicities
and for opposite magnetization directions by a display-type spherical
mirror analyzer, which allows simultaneous energy and momentum analysis
of emitted photoelectrons. Multiple-scattering cluster photoelectron
diffraction calculations agree well with the experiment, and reproduce
even fine details of the observed photoelectron diffraction features. A
breaking of symmetry in the circular dichroism of the Fe 2p3/2 photoelectron
angular distribution patterns, related to the presence of magnetic
moments on the Fe atoms, is observed both in the experiment and the
calculation. Backed by theoretical calculations, such measurements bear
the potential to study also systems with complicated spin structures.
MA 4.3 Mo 10:45 H22
Search for structural relaxations upon spin reorientation of
Ni-monolayers by x-ray diffraction and MOKE — •Meyerheim
H. L. 1 , Sander D. 1 , Popescu R. 1 , Robach O. 2 , Ferrer S. 2 , and
Kirschner J. 1 — 1 MPI für Mikrostrukturphysik, Weinberg 2, 06120
Halle, Germany — 2 ESRF, BP 220, F-38043 Grenoble, France
Using surface x-ray diffraction (SXRD) and magneto-optical Kerr effect
(MOKE) measurements, the correlation between adlayer induced
spin reorientation and structure relaxation was investigated in the trilayer
system Ni/Fe/Ni on W(110). While the deposition of one Fe layer
on the Ni film [Ni-thickness: 5, 8 and 10 layers on W(110)] induces a
switching of the easy axis from in-plane to out-of-plane, subsequent deposition
of another Ni layer reverts the easy axis back to in-plane. SXRD
measurements along the reciprocal lattice rods before and after ad-layer
deposition indicate that changes of peak positions, if present at all, must
be below about 5×10 −4 reciprocal lattice units. This corresponds to a
change in the interlayer spacing in the order of 0.002 ˚Aand represents
the limit of the experimental accuracy. The analysis of the peak shapes
does not provide any evidence for film structure in-homogeneities. The
results are discussed in terms of strain and surface contributions to the
magnetic anisotropy.
MA 4.4 Mo 11:00 H22
Convergence in ab initio nonlinar surface magneto-optics of a
Ni/Cu (001) bilayer — •Torsten Andersen 1,2 , Rajeev Ahuja 1 ,
and Wolfgang Hübner 2 — 1 CMT, Dept. of Physics, Uppsala Universitet
(S) — 2 CMT, FB Physik, TU Kaiserslautern (D)
We discuss some aspects of creating a well-converged optical secondharmonic
spectrum from a Ni/Cu bilayer in an ab initio calculation. It
turns out that the convergence of the nonlinear optical susceptibility
requires much more than a simple convergence of the band structure to
1/1000th of the optical energy. Whereas both magnetic and nonmagnetic
symmetries can be resolved using the nonlinear optical intensity at this
level convergence, the symmetries of the individual tensor elements in the
nonlinear optical susceptibility can not. We show by varying the magnetization
direction in the Ni/Cu bilayer that even nonmagnetic tensor
elements require a much higher level of convergence than what is needed
for a simple recognition of the crystal magnetic symmetry. Convergence
is reached by increasing the demands on the band structure, and by a
dense sampling in reciprocal space.
MA 4.5 Mo 11:15 H22
Ab initio study of the magnetic structure of fcc Fe grown on a
Cu(001) substrate — •Bogdan Yavorsky and Ingrid Mertig —
Fachbereich Physik, Martin-Luther-Universität Halle, Deutschland
First-principle total energy studies of Fe films on a Cu (001) substrate
are presented. The films are modeled by symmetric 6Fe/8Cu/6Fe slabs.
Both collinear and noncollinear magnetic configurations are considered.
The effect of the surface relaxation on the total energy of the system is
discussed. It was found that the energy difference between the favored
collinear double-layered antiferromagnetic state and the noncollinear configurations
is reduced by contraction of the Fe sublayer but still remains
positive. The possibility of stable noncollinear magnetic configurations in
the system is discussed.
MA 4.6 Mo 11:30 H22
Spin waves in ultrathin Fe films on Co/Cu(001) — •Markus Etzkorn
1 , P. S. Anil Kumar 1 , W. Tang 1 , R. Vollmer 1 , H. Ibach 2 ,
and J. Kirschner 1 — 1 Max-Planck Institut für Mikrostrukturphysik,
Weinberg 2, 06120 Halle — 2 Institut für Schichten und Grenzflächen
ISG3, Forschungszentrum Jülich, 52425 Jülich
By using spin polarized electron energy loss spectroscopy (SPEELS)
we measured spin-wave excitations in 3 ML Fe on 1 ML Co/Cu(001).
Similar to Co on Cu(001) [1], the spin waves appear as a single peak in
the SPEEL-spectra that can be followed up to the surface Brillouin zone
(SBZ) boundary. The spin-wave energy at the SBZ boundary is about
140 meV. The ratio of spin-wave width to spin-wave energy is about one
for large wave vector spin waves. This suggests that these spin waves
strongly interact with Stoner excitations. In contrast to the case of Co
the dispersion in Fe can not be fitted by a nearest neighbor (NN) Heisenberg
model with a single exchange coupling constant J. In theoretical
calculations [2] the exchange coupling constant of the surface layer of Fe
on Cu(001) is enhanced by about a factor of 2 compared to its bulk value.
Using a NN Heisenberg model with an enhanced surface exchange coupling
constant J1 = 2J a spin-wave stiffness of about 150 meV˚A 2 results
from the fit to the data. This is significantly lower than the value of bulk
bcc Fe (280 meV˚A 2 ).
[1] R. Vollmer, M. Etzkorn, P. S. Anil Kumar, H. Ibach, and J. Kirschner,
Phys. Rev. Lett. 91 (2003) 147201.
[2] M. Pajda, J. Kudrnovsk´y, I. Turek, V. Drchal, and P. Bruno, Phys.
Rev. Lett. 85 (2000) 5424.
MA 4.7 Mo 11:45 H22
Spin-Polarized Scanning Tunneling Spectroscopy of Co Islands
on Cu(111) — •Oswald Pietzsch, Stefan Heinze, André Kubetzka,
Matthias Bode, and Roland Wiesendanger — Institute
of Applied Physics, University of Hamburg, Jungiusstrasse 11, 20355
Hamburg, Germany
Spin-averaged and spin-polarized scanning tunneling spectroscopy at
low temperature was performed on nanometer-scale triangular Co islands
grown epitaxially on Cu(111) in the sub-monolayer coverage regime. Two