Plenarvorträge - DPG-Tagungen

Magnetismus Dienstag
spintronic devices. A class of such materials are the half-metallic Heusler
alloys. Here we present a study, based on density functional theory, of the
electronic and magnetic properties of the half-Heusler alloy NiMnSb. Using
the FLEUR code, an implementation of the Full-potential Linearized
Augmented Plane Wave (FLAPW) method, we investigate the influence
of the spin-orbit interaction and find that it reduces the spin-polarization
at the Fermi level by less then 1%. Performing spin-spiral calculations
to obtain the spin-wave dispersion in the frozen magnon approximation
we estimate the Curie-temperature in the RPA. The obtained value of
780K is in close agreement with the experimental results (∼ 730K). The
applicability of the force theorem in these calculations is investigated.
MA 11.5 Di 11:15 H22
Investigation of intra- and inter-atomic mechanisms in
La1−xBaxMnO3: A combined x-ray spectroscopic study —
•Karsten Kuepper 1 , Manfred Neumann 1 , Vadim Galakhov 2 ,
Kevin Prince 3 , Maurizio Matteucci 4 , and Mihaela Falub 5 —
1 Universität Osnabrück, FB Physik, Barbarastr. 7, 49069 Osnabrück —
2 Institute of Metal Physics,Russian Academy of Sciences - Ural Division,
620219 Yekaterinburg, Russia — 3 Sincrotrone Trieste, in Area Science
Park, Basovizza (Trieste), Italy — 4 Institute of Condensed Matter,
National Research Council c/o Sincrotrone Trieste, Italy — 5 Paul
Scherrer Inst., Swiss Light Source, WSLA-R 226, CH-5232 Villingen,
Switzerland
The electronic properties of the colossal magneto resistance (CMR)
compounds La1−xBaxMnO3 (0 ≤ x ≤ 0.55) are investigated by a detailed
spectroscopic study. A combination of different techniques, namely x-ray
photoelectron spectroscopy (XPS), x-ray emission spectroscopy (XES),
x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering
(RIXS), is used to reveal a detailed picture of the electronic structure
in the presence of Ba doping in different concentrations. The results are
compared with available theory. The valence band is dominated by La 4d,
Mn 3d and O 2p states, and strong hybridization between Mn 3d and O
2p states is present over the whole range of Ba concentration. XAS spectra
indicate that the hole doping leads to mixed Mn 3d - O 2p states.
Furthermore, RIXS at the Mn L edge probed d − d transitions between
the Mn d bands as well as transitions between strongly hybridized Mn
3d - O 2p states.
MA 11.6 Di 11:30 H22
Influence of nitrogen on the magnetic properties of LaFe11.4Si1.6
— •Aru Yan, Kalyan Mandal, Axel Handstein, Dieter
Eckert, Konstantin Nenkov, Oliver Gutfleisch, and Karl-
Hartmut Müller — Leibniz Institute of Solid State and Materials
Research Dresden, POB 270116, 01171 Dresden
Recently, it was found that the intrinsic magnetic properties of La(Fe,
M)13 compounds (M = Al, Si) can be modified with interstitial H, C
and N atoms, but the reported values of magnetic properties of the
La(Fe, M)13 interstitially modified compounds are much lower than those
of the 2:17 and 1:12 interstitially modified compounds. In this work,
LaFe11.4Si1.6Nx compounds have been prepared by nitrogen absorption
and their magnetic properties have been investigated. It was found that
the cubic NaZn13-type structure of the Si containing compounds remains
unchanged, while the lattice constant at room temperature is increased
from 11.47 to 11.56 ˚A after nitrogen absorption at 973 K, leading to
a large volume expansion of 2.4%. A drastic improvement of magnetic
properties has been observed after the introduction of interstitial nitrogen.
The Curie temperature is significantly increased from 200 K to 995
K and a high saturation magnetization of 93 emu/g is obtained. Further
analysis revealed that the volume expansion plays a predominant role
in improving the magnetic properties of La(FexSi1−x)13Nx. In particular,
the significant increase of Curie temperature in LaFe11.4Si1.6Nx can be attributed
to the enhancement of the exchange interaction by the nitrogen
absorption.
MA 11.7 Di 11:45 H22
Hartmagnetische Eigenschaften von amorphen Nd60FexCo30xAl10
Legierungen — •Olivier Perroud 1 , Elvira Garcia-
Matres 1 , Albrecht Wiedenmann 1 und Golden Kumar 2 —
1 Hahn-Meitner Institut Berlin, Glienickerstr.100 D-14109 Berlin —
2 Institut fuer Festkoerper- und Werkstoffforschung, PFf 270116, D-01171
Dresden
Wir berichten über strukturelle und magnetische Untersuchungen an
Nd60FexCo30-xAl10 (x=20, 7.5) und Nd80Fe20, das durch langsames
Abkühlen im Gussverfahren hergestellt wurde.
Magnetische Messungen belegen hartmagnetisches Verhalten für
Nd60FexCo30-xAl10 mit zwei charakteristischen Curietemperaturen.
(Tc1= 50 K und Tc2= 500 K)
Durch Kleinwinkelstreuung mit polarisierten Neutronen (SANSPOL)
konnten magnetischen Nanostrukturen nachgewiesen werden. Die Untersuchungen
erfolgen im Temperaturbereich zwischen 2K und 550K und
im außern magnetischen Feld bis 6 Tesla. In Übereinstimmung mit Magnetisierungsmessungen
konnten zwei Nd-reich Nanokristallite detektiert
werden mit mitleren Radien von 1.8nm und 5.7nm. Oberhalb Tc1 werden
die Nd-Nanopartikeln paramagnetisch und oberhalb Tc2 wird die Matrix
paramagnetisch.
Diese Arbeit wird im Rahmen des DFG Schwerpunktprogramms 1120
gefördert.
MA 11.8 Di 12:00 H22
Interaction domains in NdFeB magnets in dependence on the
degree of texture — •Kirill Khlopkov, Oliver Gutfleisch, Dietrich
Hinz, Axel Handstein, Karl-Hartmut Müller, and Ludwig
Schultz — IFW Dresden, Postfach 270016, 01171 Dresden
The magnetic domain structure of die-upset NdFeB magnets with different
degrees of texture has been investigated using the magneto-optical
Kerr effect and magnetic force microscopy (MFM). For the non-textured
i.e. isotropic hot pressed starting material interaction domains could not
yet be observed by optical Kerr-microscopy. Interaction domains could
be observed by Kerr-microscopy in die-upset samples with a deformation
degree of more than 52% (medium degree of texture). In contrast
MFM reveals the presence of interaction domains already for considerably
lower degrees of texture. Magnetostatic interaction is assumed to cause
the interaction domains since the grains are separated by thin amorphous
Nd-rich grain boundaries of about 1 nm thickness which are expected to
suppress exchange coupling between the grains. The interaction domains
are cooperative phenomena on a length scale much larger than the grain
size of typically 100nm × 400nm for highly textured sample with a deformation
degree of about 76%.
This work was supported by SFB463.
MA 11.9 Di 12:15 H22
Corrosion behaviour of polymer-bonded Nd-Fe-B-based
nanocrystalline magnets — •Mihaela Rada, Anke Kirchner,
Annett Gebert, Oliver Gutfleisch, and Ludwig Schultz —
IFW Dresden, Helmholtzstr. 20, 01069, Dresden
The corrosion resistance of polymer bonded nanocrystalline NdFeB
magnets with Nd-rich, stoichiometric and Fe-rich compositions and that
of their main single phases is assessed in aqueous environments. For the
evaluation of corrosion properties, gravimetric tests as well as potentiodynamic
polarization measurements were performed in different nitrogenpurged
electrolytes: 0.1 M sulfuric acid (pH 0.7), 0.5 M phthalate buffer
(pH 5) and 0.1 M sodium hydroxide (pH 13). Before and after corrosion
testing, the sample surfaces were investigated by means of high-resolution
scanning electron microscopy. The single phases show anodic dissolution
in strong acidic environment and stable passivation in alkaline solution. In
weakly acidic phthalate buffer Nd-rich and Fe phases exhibit limited passive
regions, whereas Fe3B and Nd2Fe14B do not show stable passivation.
Polymer-bonded magnets of different compositions have a significantly
improved resistance towards strongly acidic environments as compared
to hot pressed magnets. Enrichment in Fe content and additions like Co
and Nb improve their corrosion behavior.
MA 11.10 Di 12:30 H22
Microstucture of bulk SmCo2.5Cu2.5 and magnetisation in high
pulse fields — •Peter Kerschl 1 , Kirill Khlopkov 1 , Axel
Handstein 1 , Dieter Eckert 1 , Juan-Carlos Tellez-Blanco 2 ,
Roland Grössinger 2 , Karl-Hartmut Müller 1 , and Ludwig
Schultz 1 — 1 Leibniz-Institut für Festkörper- und Werkstoffforschung
Dresden, Postfach 270116, D-01171 Dresden, Germany — 2 Institut für
Festkörperphysik, Technische Universität Wien, Wiedner Hauptstraße 8-
10, A-1040 Wien, Austria
SmCo5 has a very good performance as permanent magnet material.
It is known that a strong influence of Cu substitution on the magnetic
properties exists: An unusual jump from low to high magnetisation state
in an increasing external field is observed. To obtain a better insight,
a SmCo2.5Cu2.5 compound was prepared. Phase composition and microstructure
have been investigated by X-ray diffraction, scanning electron
microscopy and EDX analysis. The results of high field magnetisation
measurements in pulse fields up to 48 T at temperatures from