Plenarvorträge - DPG-Tagungen

Magnetismus Mittwoch
MA 15.5 Mi 16:15 H10
Thickness and temperature driven Spin Reorientation Transition
in ultrathin Co films on Pd(110) — •L. Yan, Y. Lu,
H. Meyerheim, M. Przybylski, W. Wang, J. Barthel, and J.
Kirschner — Max-Planck-Institut für Mikrostrukturphysik, Weinberg
2, 06120 Halle
Morphology, structure and magnetism of ultrathin Co films (0.1 to
3ML) grown epitaxially on Pd(110) at room temperature (RT) were studied.
The elongated Co stripes resulting from the anisotropy of Pd(110)
surface are observed by STM. These Co stripes become ferromagnetic
above 0.2ML at low temperature (LT=80K), and are magnetized outof-plane.
Between 0.5ML and 1.1ML, the polar MOKE signal (measured
in remanence) decreases with increasing Co coverage. The magnetization
rotates towards the film plane showing an increasing longitudinal
component to the total Kerr signal. With further increasing coverage,
another spin reorientation transition occurs. At the thickness of about
2ML the film magnetization is again oriented out-of-plane at LT. However,
with increasing temperature the magnetization rotates towards the
film plane and no polar MOKE signal is detected at RT. The films are
magnetized in-plane independently of the temperature above a thickness
of about 2.7ML. We suggest that such magnetic behaviors are attributed
to the morphology and structural reconstruction which the film experiences
with increasing thickness.
MA 15.6 Mi 16:30 H10
Prinzipielle Unterschiede bei Ni/Pd-Grenzflächen — •Y.
Manzhur 1 , K. Potzger 1 , M. J. Prandolini 1 , H. H. Bertschat
1 und Mark Dietrich 2 — 1 Hahn-Meitner-Institut, Bereich
Festkörperphysik, Glienicker Strasse 100, 14109 Berlin — 2 Mark
Dietrich, ISOLDE-Collaboration, EP Division, CERN, CH 1211 Genf
Ni/Pd-Grenzflächen können u. a. durch Aufdampfen von Pd auf Ni-
Einkristallen erzeugt werden oder durch Aufdampfen von Ni auf Pd-
Einkristallen. Pd auf Ni ist ein häufiger studiertes System und wurde
auch von uns mit der PAC-Spektroskopie unter Verwendung von radioaktiven
Proben untersucht [1]. Das mehrfach bestätigte Ergebnis ist, dass
wenige Monolagen Pd auf Ni eine ferromagnetische Ordnung annehmen.
Dagegen gibt es neben der PAC so gut wie keine Experimentiermethode,
um das magnetische Verhalten von Ni bedecktem Pd zu erforschen. In
einen früheren Experiment [2] fanden wir, dass in Pd einige Atomlagen
von der Grenzfläche entfernt induzierte magnetische Wechselwirkungen
nachzuweisen sind, Pd aber paramagnetisch bleibt. Nun konnten wir zeigen,
dass auch die oberste Pd-Atomlage des Einkristalls, die sich in direktem
Kontakt zu Ni befindet, keine ferromagnetische Ordnung annimmt,
im Kontrast zu dünnem Pd auf Ni.
Die Messungen wurden am Massenseparator ISOLDE / CERN durchgeführt.
[1] Phys. Rev. Lett. 80, 2721 (1998).
[2] Phys. Rev. Lett. 78, 342 (1997).
MA 15.7 Mi 16:45 H10
Spin-Polarisation von EuS oberhalb 77 K: Temperaturabhängige
MOKE- und XMCD-Messungen an dünnen
Co/EuS-Vielfachlagen-Filmen — •C. Müller, H. Lippitz, J.J.
Paggel und P. Fumagalli — Institut für Experimentalphysik, Freie
Universität Berlin, Arnimallee 14, 14195 Berlin
Sein großes Spin-Moment (mS = 7/2) macht den Halbleiter EuS interessant
für Spintronics-Anwendungen. Demgegenüber steht die niedrige
Curie-Temperatur (TC=16.5 K), die eine kommerzielle Nutzung
ausschließt. Messungen des magnetischen Röntgen-Circulardichroismus
(X-MCD) an Vielfachlagen von Co und EuS zeigen eine Spin-Polarisation
von EuS bei einer Temperatur größer als 77 K. Als Ursache wird eine
antiferromagnetische Kopplung zwischen Co und EuS festgestellt, d.h.
die Polarisation des EuS wird durch das Co induziert. Tieftemperatur-
Messungen (10 K) des magnetooptischen Kerr-Effektes (MOKE) können
dies bestätigen: Nichtlinearitäten in Hysterese-Kurven werden durch
einen solchen Kopplungs-Effekt erklärt. Bei Raumtemperatur liefern
MOKE-Messungen keinen eindeutigen Nachweis für die Existenz einer
antiferromagnetischen Kopplung.
Diese Arbeit wurde unterstützt durch die Deutsche Forschungsgemeinschaft
im Rahmen des SFB 290.
MA 15.8 Mi 17:00 H10
Pulsed Laser Deposition of Epitaxial SmCo Thin films — •A.
Singh 1 , R. Tamm 2 , V. Neu 1 , S. Leinert 1 , S. Fähler 1 , C.-G. Oertel
2 , W. Skrotzki 2 , L. Schultz 1 , and B. Holzapfel 1 — 1 IFW
Dresden, Helmholtzstr. 20, 01069 Dresden — 2 Inst. f. Strukturphysik,
TU Dresden, 01062 Dresden
To explore the potential of a permanent magnetic material with uniaxial
anisotropy, a well aligned easy magnetisation axis is required. One way
to achieve this is by growing epitaxial films on single crystal substrates.
With the same aim and motivation of achieving high coercivity and remanance
along a crystallographic direction, epitaxial SmCo on Cr buffered
(100) and (110) MgO single crystal substrates was deposited. These films
were prepared using PLD from elemental targets, and showed the epitaxial
relation: MgO(001)[100]-Cr(001)[110]-SmCo(110)[001], which indicates
an in plane direction of the easy axis. But besides this the easy axis
also shows a 60 ◦ out of plane component coming from a (1 1 16) texture.
These results have been reached at by careful analysis of several pole figures
measured in the texture goniometer and magnetic measurements in
the SQUID. The volume fractions of these two texture components show
a strong dependence on the deposition temperature. The exact contributions
of these two components will be discussed in relation to the results
from magnetic properties.
MA 15.9 Mi 17:15 H10
Magnetic microstructure of granular epitaxial NdFeB films —
•V. Neu, S. Melcher, U. Hannemann, and S. Fähler — IFW
Dresden, Helmholtzstr. 20, 01069 Dresden
Epitaxial NdFeB films combine a perfect alignment of the easy magnetic
axis perpendicular to the substrate with a high coercivity up to 2
T [1]. They possess high nucleation fields and the coercivity is mainly
limited by local stray fields. Furthermore they grow in isolated, rectangular
grains, which makes these films an ideal system to locally study
the magnetic domains of a self-organized microstructure.
In this paper we used Magnetic Force Microscopy (MFM) and global
magnetization measurements to examine a series of Nd-rich NdFeB films
deposited at different temperatures. At deposition temperatures around
545 ◦ C the films neither have a perfect degree of texture nor do they switch
homogeneously. The microstructure consists of a small grained film of Nd-
FeB with the Nd-rich phase segregated as large spherical grains. At the
optimum temperature of 630 ◦ C uniform switching occurs and a coercivity
of 2 T is obtained. These films consist of rectangular, µm-sized NdFeB
grains, which show typical stripe domains with an average width of 150
nm. At higher temperatures (670 ◦ C) the grain size distribution broadens
significantly. This is reflected in a less uniform switching behavior, which
is observed with global and local measurement techniques.
[1] U. Hannemann, S. Fähler, V. Neu, B. Holzapfel, L. Schultz, Appl.
Phys. Lett. 82, 3710 (2003)
MA 15.10 Mi 17:30 H10
Highly coercive and highly remanent Nd-Fe-B films —
•Sebastian Fähler, Jörg Buschbeck, Ah-Ram Kwon, Ullrich
Hannemann, Volker Neu, Bernhard Holzapfel, and Ludwig
Schultz — IFW Dresden, Helmholtzstr. 20, 01069 Dresden and SFB
463
Miniaturised magnets with high remanences are needed for microelectromechanical
systems (MEMS). A possible solution is epitaxially
grown Nd2Fe14B films which exhibit due to the perfect alignment of the
c-axis (easy magnetisation direction) very high remanence to saturation
magnetisation ratios close to unity. Furthermore these films reach coercivities
up to 2 T [1]. Films, which were deposited at lower temperatures
and subsequently annealed at higher temperatures, show similar results
despite the different temperature treatment [2]. One result of the different
temperature treatment is a change in the morphology of the films,
which consists of isolated grains for the epitaxial films or appears continuous
for the post-deposition annealed films. We present a study of the
effect of the 2 different deposition conditions on the microstructure and
consequently on the magnetic properties of the films.
[1] U.Hannemann, S. Fähler, V. Neu, B. Holzapfel, L.Schultz, Appl.
Phys. Lett. 82 (2003) 3710
[2] L.K.E.B Serrona, A. Sugimura, N. Adachi et al., Appl. Phys. Lett.
82 (2003) 1751