Plenarvorträge - DPG-Tagungen

Magnetismus Dienstag
len: einen mit einer linearen Zunahme von K·d für Schichtdicken kleiner
15 Monolagen (ML) und einen mit einer linearen Abnahme von K·d für
Schichtdicken größer 15 ML. Zwei lineare Bereiche der Anisotropie, durch
einen Knick voneinander getrennt, werden in verspannten Systemen mit
senkrechter Magnetisierung gefunden [1]. Das Auftreten zweier linearer
Bereiche ist auch bei uniaxialer Anisotropie und Magnetisierung in der
Ebene ein starkes Indiz für eine Verspannung bzw. Spannungsabbau. Die
Existenz derartiger Verspannungen steht im Widerspruch zu jüngsten
Messungen von Chandesris et al. [2]. Im Gegensatz zu diesen Messungen
zeigen wir, daß die Dickenabhängigkeit der Anisotropie sowie eine
von Null verschiedene z-Komponente der Magnetisierung [3] durch Verspannungen
und eine dem gestuften System angepaßte Formanisotropie
beschrieben werden kann.
[1] M. T. Johnson Rep. Prog. Phys. 59, 1409 (1996)
[2] D. Chandesris et al., J. Phys. C 15, S657 (2003)
[3] N. Mikuszeit et al., J. Mag. Mag. Mater. 268, 340 (2003)
MA 13.16 Di 15:00 Bereich A
Spin-wave excitation in Co/Cu(001), Fe/Co/Cu(001), and
Fe/W(110) by spin-polarized electron energy loss spectroscopy
— •R. Vollmer 1 , M. Etzkorn 1 , P. S. Anil Kumar 1 , W.
Tang 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
Recently, we have shown for the case of fcc Co films on Cu(001) that
surface spin-wave excitations can be observed up to the surface Brillouin
zone boundary by spin-polarized electron energy loss spectroscopy
(SPEELS) [1]. In this contribution we compare these results with new
SPEELS measurements on a 3 ML Fe film on 1 ML Co/Cu(001) and on 5
ML Fe on W(110). The SPEEL spectra of 3 ML Fe on 1 ML Co/Cu(001)
show a clear spin-wave signal as well. Similar to Co, only one spin-wave
loss peak is visible in the spectrum. Its width is significantly broader than
that of the spin-wave peak in Co films. For 5 ML Fe on W(110) we find
merely an extremely broad feature without clear maximum. It extends
up to about 400 meV independent of the wavevectors. While a strong
spin-wave signal can be observed only for low energies E0 of the incident
electrons below about 10 eV in the case of Co/Cu(001) and 3 ML Fe
on 1 ML Co/Cu(001), it is almost independent of E0 for Fe on W(110).
We compare our experimental results to new theoretical calculations of
spin-wave spectra of Co and Fe films [2].
[1] R. Vollmer, M. Etzkorn, P. S. Anil Kumar, H. Ibach, and J. Kirschner,
Phys. Rev. Lett. 91, 147201 (2003).
[2] R. B. Muniz and D. L. Mills, Phys. Rev. B 66, 174417 (2002).
MA 13.17 Di 15:00 Bereich A
Influence of substrates on hard magnetic Nd-Fe-B films — •Ah-
Ram Kwon, Ullrich Hannemann, Sebastian Fähler, Volker
Neu, Bernhard Holzapfel, and Ludwig Schultz — IFW Dresden,
Helmholzstr. 20, 01069 Dresden and SFB 463
Nd2Fe14B has excellent intrinsic properties such as high anisotropy
and high saturation magnetization. This material is already used in high
performance bulk magnets. Thin films are interesting for application in
microelectromechanical systems. In both cases, high remanence is required
which can be achieved in the film by textured growth. This can
be controlled by suitable choice of buffer and substrate materials as well
as deposition conditions.
Nd-Fe-B films were prepared by pulsed laser deposition on Ta or Ir
buffer layers using Al2O3(0001), MgO(100) or amorphous SiN substrates.
They possess an out-of-plane c-axis texture and therefore show a high
remanence to saturation magnetization ratio around 0.95. The films exhibit
coercivities between 1.3 T and 2.0 T when measured perpendicular
to the film plane, i.e. parallel to the c-axis (easy magnetization axis). We
compare results of phase formation, microstructure and magnetic properties
of hard magnetic Nd-Fe-B films deposited on different buffers and
substrates.
MA 13.18 Di 15:00 Bereich A
Textured and highly coercive SmCo5 PLD thin films on
amorphous substrates — •S. Leinert, V. Neu, S. Fähler, B.
Holzapfel, and L. Schultz — IFW Dresden, Helmholtzstr. 20,
01069 Dresden and SFB 463
Hardmagnetic SmCo5 thin films have been prepared by pulsed laser
deposition from elemental targets under UHV conditions on amorphous
SiN substrates. The films have Cr buffer and cover layers to prevent oxidation.
Experiments have been carried out that show the dependence of
the phase formation on pulse frequency and pulse alternation. These deposition
parameters, apart from the intended film thickness, determine
the deposition time and thus decide the available time for atomic diffusion.
For the best developed phases room temperature coercivities as
high as 3.5T for a film thickness around 100nm were achieved. Beyond
that, efforts have been made to improve the texture of the films with
the aim to orient the easy magnetisation axis along one axis within the
film plane. Investigations concentrated on substrate tilting (inclined substrate
deposition) and ion beam assisted deposition with low energy Ar
ions (different ion energies, beam currents and angles of incidence). Ion
beam etching experiments on SmCo have been performed to find possible
texturing mechanisms in the IBAD process.
MA 13.19 Di 15:00 Bereich A
Magnetic and Magneto-Optical Properties of UH3 Thin Films
— •D. Kolberg 1 , A. Navratil 1 , T. Gouder 2 , F. Wastin 2 , and J.
Schoenes 1 — 1 Institut für Halbleiterphysik und Optik und Magnetfeldanlage,
TU Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig,
Germany — 2 European Commission, Joint Research Centre, Institute for
Transuranium Elements, Postfach 2340, D-76125 Karlsruhe, Germany
The first ever observed ferromagnet among actinide compounds was
UH3 with its astonishingly high TC of 180 K [1]. Bulk uranium metal
powderises during hydrogen loading, which makes it impossible to investigate
optical and magneto-optical properties of UH3. This drawback can
be circumvented when growing thin films of uranium with a cap layer of
Pd. However, subsequent H loading leads to inhomogeneous films and
possible destruction [2]. Our new approach is to grow uranium thin films
and to hydrogenate them at the same time. The protection against oxydation,
i.e., the Pd cap layer, is grown on top of the UH3 films. The
cleanliness of the films will be checked via in-situ XPS. Secondly, we
monitor the hydrogen uptake using SQUID magnetometry. Finally, we
investigate the magneto-optical properties of UH3 films in magnetic fields
up to 11.5 Tesla and in the range 0.2 eV < ¯h ω < 5 eV.
Financial support of the DFG (Scho 642/2-2) is acknowledged.
[1] R. Troć and W. Suski, J. Alloys Comp. 219 (1995) 1 [2] D. Kolberg
et al., Verhandl. DPG (VI) 38 (2003) 324
MA 13.20 Di 15:00 Bereich A
Temperature-dependent surface state and magnetization of thin
ferromagnetic EuS films — •W. Nolting and W. Müller —
Lehrstuhl Festkörpertheorie, Institut für Physik, Humboldt-Universität
zu Berlin
We present results for the temperature- and layer-dependent electronic
structure of thin ferromagnetic EuS (100) films. A combination
of a many-body evaluation of a multiband Kondo-lattice model and a
first-principles band structure calculation (TB-LMTO) is used to get as
realistically as possible temperature- and correlation effects. The calculated
layer dependent magnetization shows a good agreement with the
experiment. The results are discussed in terms of spectral densities, densities
of states and band-structures. Special attention is devoted to the
appearance of magnetic 5d-surface states.
MA 13.21 Di 15:00 Bereich A
Strukturelle und magnetische Eigenschaften epitaktischer Manganat
Multilagen — •Oswaldo Moran 1 , Thorsten Schwarz 1,2 ,
Dirk Fuchs 1 und Rudolf Schneider 1 — 1 Karlsruhe, Institut für
Festkörperphysik, 76021 Karlsruhe — 2 Universität Karlsruhe, Fakultät
für Physik, 76128 Karlsruhe
Zur Untersuchung der Austauschwechselwirkung und des Exchange Bias
zwischen ferromagnetischen und antiferromagnetischen Systemen sind
Multilagenstrukturen besonders geeignet. Ein hervorragendes Beispiel
hierfür sind Schichtsysteme bestehend aus La2/3Ca1/3MnO3, ferromagnetisch
(FM) mit TC = 250 K, und La1/3Ca2/3MnO3, antiferromagnetisch
(AFM) mit TN = 170 K. Aufgrund der chemischen und strukturellen
Ähnlichkeit beider Komponenten können sehr hohe Wachstumsgüten bei
epitaktischen Übergitterstrukturen erzielt werden. Die Präparation epitaktischer
Manganat Multilagen mittels Laserdeposition sowie die Charakterisierung
der strukturellen und magnetischen Eigenschaften durch
Röntgendiffraktion und DC-SQUID-Magnetometrie werden vorgestellt
und diskutiert.