Plenarvorträge - DPG-Tagungen

Magnetismus Dienstag
MA 13.113 Di 15:00 Bereich A
Untersuchung der martensitischen Umwandlung in den
Heusler-Systemen Ni-Mn-Ga-Al und Ni-Co-Al durch Magnetisierungsmessungen
und Abbildung der Mikrostruktur
— •T. Krenke, E. Duman, M. Acet und E.F. Wassermann —
Experimentalphysik AG Farle, Universität Duisburg-Essen, Standort
Duisburg, Lotharstrasse 1, 47048 Duisburg
Es werden Ergebnisse von mikrostrukturellen und magnetischen Messungen
an den Heusler-Sytemen Ni-Mn-Ga-Al und Ni-Co-Al präsentiert.
Es wurden Legierungen Ni50Mn25(Ga1−xAlx)25 0.2 < x < 0.8 hergestellt,
um den Einfluss von Aluminium (Gallium) auf die bekannte Formgedächtnis-Legierung
Ni2MnGa (Ni2MnAl) zu studieren. Die Fragestellung
lautet hier, inwieweit sich die magnetische Ordnung und die Mikrostruktur
durch Zulegieren entwickeln und ob der magnetische Formgedächtnis-Effekt
(MFGE) dabei erhalten bleibt. Als Indiz für den MFGE
werden Magnetisierungskurven in kleinen Feldern (50 Oe) herangezogen,
die mit früheren Messungen an Ni-Mn-Ga und Ni-Mn-Al verglichen werden
können. Ni-Co-Al weist, im Gegensatz zu den Mangan-Legierungen,
durch die Existenz einer A1-Phase in Korngrenzen duktile Eigenschaften
auf. Es wurden drei verschieden Konzentrationsreihen im ternären
Phasendiagramm zwischen 7.4 < e/a < 8.0 gewählt und sowohl die Magnetisierungskurven
auf das Vorhandensein eines MFG-Effektes als auch
die Mikrostruktur untersucht.
MA 13.114 Di 15:00 Bereich A
Characterization of doped Heusler alloys: Co2Cr1−xFexAl — •S.
Wurmehl 1 , G. H. Fecher 1 , H. C. Kandpal 1 , K. Kroth 1 , H.-J.
Elmers 1 , G. Schönhense 1 , C. Felser 1 , J. Morais 2 , Y. Hwu 3 , and
R. Klauser 4 — 1 Johannes Gutenberg - Universität, D-55099 Mainz,
Germany — 2 Universidade Federal do Rio Grande do Sul, 91501-970
Porto Alegre, Brazil — 3 Academia Sinica, Taipei, 11529, Taiwan —
4 National Synchrotron Radiation Research Center, Hsinchu, 30077, Tai-
wan
Doped Heusler alloys and compounds of the Co2Cr1−xFexAl with varying
Cr to Fe ratio x were produced and experimentally investigated. Most
of the samples showed pronounced disorder instead of the expected L21
structure. The sample surfaces exhibited a strong selective oxidation of
chromium. The bulk samples were characterized by standard methods
such as X-ray diffraction, SQUID magnetometry, and depth profiling.
X-ray absorption was used together with XMCD to examine different
oxidation stages in bulk and powder samples and to determine element
specific magnetic moments. Surface sensitive methods, like low energy ion
scattering, photoemission, PEEM, and scanning XPS microscopy, were
used to examine the oxidation process. The nature of the selective oxidation
of chromium in these compounds will be discussed on hand of the
experimental findings.
MA 13.115 Di 15:00 Bereich A
Photogenerierte Raumladungen im ladungsgeordneten Zustand
von Pr0.63Ca0.37MnO3 Einkristallen — •V. Morchshakov 1 , M.
Boshta 1 , K. Bärner 1 , I. Medvedeva 2 , C. Martin 3 und B. Raveau
3 — 1 Fachbereich Physik, 4. Phys. Institut, Universität Göttingen,
Tammannstr.1, D-37077 Göttingen, FRG — 2 Institut für Metallphysik,
Kovalevskaya Str. 18, 620219 Ekaterinburg, Russland — 3 ISMRA, Caen,
Frankreich
Zeitaufgelöste Thermokraftsmessungen (TTE) an Volumeinkristallen
nahe der Zusammensetzung Pr2/3Ca1/3MnO3 zeigen einen zweiten Spoannungstransienten
V2(t) unterhalb der Ladungsordnungstemperatur
Tco. Dieser Transient ist schneller als der Transient der quasistatische
Thermokraft, V3(t), der üblicherweise bei Manganitkeramik als einziger
beobachtet wird, und kann daher leicht separiert werden. Aus V3(t)
erhalten wir die Wärmeleitfaehigkeit und die Thermokraft im Temperaturbereich
30K¡ T ¡ 350K. Der ausserordentliche Transient wird dem
Abbau einer ambipolaren Raumladung zugeschrieben die sich anschliessend
an eine Photoerzeugung von Ladungstraegern ausbildet, aber nur
dann wenn die thermische Reemission aus den Haftstellen gering bleibt.
MA 13.116 Di 15:00 Bereich A
Doping and lattice effects in the half-metallic double perovskite
Sr2CrWO6 — •Stephan Geprägs, Petra Majewski, Jan
B. Philipp, Andreas Erb, Matthias Opel, Lambert Alff, and
Rudolf Gross — Walther-Meißner-Institut, Bayerische Akademie der
Wissenschaften, Walther-Meißner-Str. 8, 85748 Garching
The double perovskite Sr2CrWO6 and related compounds combine
Curie-temperatures TC well above room temperature and a half-metallic
ferromagnetic behavior, that is, they are expected to have a full spin
polarization of the charge carriers at the Fermi level. This fact makes
them very attractive for spintronic devices. In the compound Sr2FeMoO6
electron doping achieved by the substitution of Sr 2+ by trivalent La 3+
has been reported to lead to an increase of TC [1]. On the other hand,
in Sr2CrWO6 the same substitution leads to a clear decrease of TC [2].
We have fabricated a series of high-quality polycrystalline samples of the
two compounds with different La doping in order to compare carefully
the effect on TC. All samples were characterized by x-ray powder diffractometry,
magnetization and transport measurements. The results are of
particular interest with respect to the physical mechanism stabilizing the
high TC values: Within a double-exchange model one would rather expect
an increase of TC with carrier doping, whereas a kinetic energy driven
exchange mechanism is expected to result in a decrease of TC.
[1] J. Navarro et al., Phys. Rev. B 64, 092411 (2001).
[2] J. B. Philipp et al., Phys. Rev. B 68, 144431 (2003).
MA 13.117 Di 15:00 Bereich A
The effect of Zn and Cr doping on structural and magnetic
properties of Sr2FeReO6 — •A. Jung, I. Bonn, J. Ensling, V.
Ksenofontov, C. Felser, and W. Tremel — Johannes Gutenberg -
Universität, Institut für Anorganische Chemie und Analytische Chemie,
55099 Mainz
Magnetic properties of doped double perovskites with formula
Sr2Fe1−xMxReO6 (M = Zn, Cr) strongly correlate with their structure.
The relation of ionic radii of iron and substituting transitional metals
is a decisive factor which determines the cubic or tetragonal modifications
of doped Sr2FeReO6. We report on the magnetic and structural
properties of solid solutions Sr2Fe1−xMxReO6 in a wide range of concentrations
0 ≤ x ≤ 1. 57 Fe Mössbauer spectroscopic studies revealed that
the relation of amount of Fe 2+ and Fe 3+ ions is another important factor
influencing the structure of these compounds. Magnetic properties
of doped double perovskites are interpreted by means of FLAPW band
structure calculations using the WIEN97 software package.
MA 13.118 Di 15:00 Bereich A
X-ray resonance exchange scattering from 4d transition metal
compounds — •Ioannis Zegkinoglou 1 , Joerg Strempfer 1 ,
Christie S. Nelson 2 , John P. Hill 2 , Jonathan Lang 3 , George
Srajer 3 , and Bernhard Keimer 1 — 1 Max-Planck-Institut fuer
Festkoerperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany
— 2 Department of Physics, Brookhaven National Laboratory, Upton,
NY 11973-5000, USA — 3 Advanced Photon Source, Argonne National
Laboratory, Argonne, IL 60439, USA
We present results of x-ray resonance exchange scattering (XRES)
from single crystal Ca2RuO4, performed at the Ru L2 and L3 absorption
edges at photon energies between 2.8 keV and 3 keV. A large resonance
enhancement of the magnetic signal is observed. Its origin lies
in the 2p → 4d dipolar electronic transitions that probe directly the 4d
orbitals, which are responsible for magnetism. This provides new possibilities
of studying the magnetic properties, as well as possible chargeand
orbital- ordering in 4d transition metal compounds. The enhancement
at K-edges, where the d-orbitals are only probed indirectly via the
p-orbitals, is generally much weaker.
MA 13.119 Di 15:00 Bereich A
Design for a low temperature spin-polarized scanning tunnelling
microscope — •Nathaniel Janke-Gilman, Wulf Wulfhekel,
and Jürgen Kirschner — Max Planck Institut für Mikrostrukturphysik
We present the design of a new ultrahigh vacuum (UHV) spin polarized
scanning tunnelling microscope (SP-STM). The apparatus is designed to
operate inside a closed 4K chamber, allowing for excellent vacuum conditions
and exceedingly long sample lifetimes. Magnetic fields of up to 0.5
Tesla may be applied at the sample position in the parallel or perpendicular
direction either before or during SP-STM operation. Sample and tip
exchange is possible with visual control. Both in-plane and out-of-plane
spin polarization is measurable by using either a ring or sharp tip at the
tunnelling junction, respectively.