Plenarvorträge - DPG-Tagungen

Magnetismus Montag
structurally different island types can clearly be distinguished by their
spin-averaged electronic structure. Spin-polarized measurements allow a
separation of spectral contributions arising from different island stacking
or from opposite magnetization states, respectively. We compare these
results with ab initio electronic band structure calculations.
In an applied magnetic field both island types are found to be magnetized
perpendicular to the surface, with large values of saturation field,
remanence, and coercivity. It is found that for one island type the saturation
field is significantly higher than for the other.
As was shown in spin-averaged measurements [1], bias-voltage dependent
oscillation patterns of the local density of states (LDOS) due to
electron confinement can be observed on the Co islands. Here we present
spin-polarized data revealing the oscillation amplitude to depend on the
spin direction.
[1] L. Diekhöner et al., Phys. Rev. Lett. 90, 236801 (2003).
MA 4.8 Mo 12:00 H22
Magnetism of Fe on W(001) — •Kirsten von Bergmann,
Matthias Bode, and Roland Wiesendanger — University of
Hamburg, Institute of Applied Physics, Jungiusstr. 11, 20355 Hamburg,
Germany
There has been a lot of interest in the magnetic properties of the highly
strained pseudomorphic layers of Fe on W(001). Using spatially averaging
methods contrary results have been reported concerning the anisotropy
axis [1-3]. To overcome this discrepancy we used spin-polarized scanning
tunneling microscopy (SP-STM) to investigate this system: the method
is magnetically sensitive and has a high spatial resolution which allows
not only the investigation of magnetic properties but also a direct correlation
to electronic structure measurements and detailed topographic
studies.
The domain structure of Fe on W(001) in the low coverage regime is
imaged and the four fold anisotropy can directly be deduced from maps
of differential conductance (dI/dU). Quantitative analysis of the intensity
of the dI/dU signal reveals a layer-dependent anisotropy axis in the
pseudomorphic regime: the magnetic easy axis rotates by 45 ◦ from 〈110〉
in layers with a local coverage of 2 and 3ML to 〈100〉 at a local coverage
of 4ML.
[1] G. A. Mulhollan et al., Phys. Rev. B 43, 13645 (1991).
[2] T. L. Jones and D. Venus, Surf. Sci. 302, 126 (1994).
[3] W. Wulfhekel et al., Europhys. Lett. 49, 651 (2000).
MA 4.9 Mo 12:15 H22
Spin-orbit interaction at the Gd(0001) and O/Gd(0001) surfaces
— •O. Krupin 1 , G. Bihlmayer 2 , S. Gorovikov 3 , J. E. Prieto
1 , K. Döbrich 1 , G. Kaindl 1 , S. Blügel 2 , and K. Starke 1 —
1 Institut für Experimentalphysik, Freie Universität Berlin — 2 Institut
für Festkörperforschung (IFF), Forschungszentrum Jülich — 3 MAX-Lab,
Lund University, Sweden
We compare experimental angle-resolved photoemission investigations
with first-principles DFT calculations, including spin-orbit interaction,
of Gd(0001) and O/Gd(0001) surface states. For both systems, experimental
and theoretical results reveal that the energy dispersion of the
surface state depends on the relative orientation of the propagation direction
of the electrons and the spin-quantisation axis. While for the clean
Gd surface only the majority state of the exchange split surface states is
occupied, both the minority and majority states appear below the Fermi
level upon oxygen adsorption. Angle-resolved photoemission experiments
on these two surface states directly demonstrate that the observed effect
on the energy dispersion is caused by spin-orbit interaction. In addition,
oxygen adsorption leads to a modified near-surface electronic structure
that results in an increased spin-orbit interaction.
MA 4.10 Mo 12:30 H22
Rashba splitting of surface states at Gd(0001) surfaces — •G.
Bihlmayer 1 , O. Krupin 2 , J.E. Prieto 2 , S. Blügel 1 , G. Kaindl 2 ,
and K. Starke 2 — 1 Institut für Festkörperforschung, Forschungszentrum
Jülich, Germany — 2 Fachbereich Physik, Freie Universität Berlin,
Germany
By angle resolved photoemission measurements on clean and O-covered
Gd(0001) surfaces a dependence of the dispersion of the surface state on
the magnetization direction of the sample was found. We present results
of first-principles calculations showing that this behavior is a direct
manifestation of a relativistic effect, known as Rashba effect in the field
of semiconductor heterostructures. Normally, this tiny effect is difficult
to observe directly or to access with ab-inito calculations, but on some
metal surfaces it is large; we discuss, why the strength of this ’spin-orbit
splitting’ is big for some states (O/Gd(0001)) and rather small for others
(clean Gd(0001)). Our results are in good quantitative agreement with
measured data. The differences to the nonmagnetic surfaces, on which
this effect also has been observed (e.g. Au(111)) are addressed. Other
systems, on which this effect could be observed experimentally are suggested.
MA 4.11 Mo 12:45 H22
Theoretical study of magnetic and spectroscopic properties of
supported transition metal clusters — •J. Minar 1 , H. Ebert 1 ,
V. Popescu 1 , I. Cabria 2 , R. Zeller 2 , and P.H. Dederichs 2 —
1 Dep. Chemie/Phys. Chemie, Universty of Munich, Muenchen, Germany
— 2 Forschungszentrum Juelich, Juelich
Magnetic clusters receive recently a lot of attention in academic but
also in technological research. On the one hand side, the interest is caused
by the fact that clusters provide a bridge between atoms and bulk material
often showing quite peculiar properties. On the other hand, the
ongoing need for miniaturizing, in particular in data storage technology,
leads to smaller and smaller functional units leading finally to small clusters.
In this work the fully relativistic spin-polarized KKR method has
been used to study the magnetic and spectroscopic properties for supported
clusters. For clusters supported on a transition metal substrate it
is shown that the magnetic properties depend on many different parameters
as substrate type, cluster size and shape and so on. This applies especially
if one considers properties that are caused by spin-orbit coupling
as the X-ray circular dichroism. In line with recent experimental findings
a very pronounced magnetic circular dichroism in X-ray absorption is
found for Co-clusters on Pt(111). The results for the MCXD spectra and
their connection with the spin, orbital and spin dipolar moments will be
discussed on the basis of the so-called sum rules.
MA 4.12 Mo 13:00 H22
Electronic structure versus spin polarization in NiMnSb —
•Jürgen Braun, Hristo Kolev, Georgi Rangelov, and Markus
Donath — Physikalisches Institut, Westfälische Wilhelms-Universität
Münster, Wilhelm-Klemm Str. 10, 48149 Münster
Half-metallic semi Heusler alloys like NiMnSb have been ascribed
promising materials for spintronic devices, because of a theoretically
predicted high spin polarization at the Fermi level. Unfortunately, from
our spin-resolved Appearance Potential Spectroscopy (SRAPS) measurements
an unexpected low spin polarization was observed. This result is
in agreement with experimental findings from other groups but yet not
understood. To investigate the physical origin of this discrepancy we
performed a quantitative theoretical analysis of the experimental data.
Using a fully relativistic theory of SRAPS, which describes the measured
spectra as the self-convolution of the matrix-element weighted, orbitallyresolved
unoccupied density of states, we are able to present a quantitative
explanation for the small spin polarization experimentally found in
(semi) Heusler alloys.