Plenarvorträge - DPG-Tagungen

Magnetismus Dienstag
microscopy and SQUID magnetometry. The evaluation is based on the
preparation of different low temperature magnetic states of the samples
and measuring its magnetization depending on temperature.
MA 13.81 Di 15:00 Bereich A
Magnetization of fcc FexPt1−x nanoparticles: from Langevin to
Stoner-Wohlfarth behaviour — •Frank Wiekhorst and Jürgen
Kötzler — Institut f. Angewandte Physik, Univ. Hamburg, D-20355
Hamburg
We present field-dependent magnetizations of chemically disordered
nanocrystals, d = 4 mm, imbedded in an organic matrix, recorded between
350 K and 5 K. Taking into account an effective uniaxial anisotropy,
the temperature dependence of which has been determined by FMR
[1] and by low-frequency absorption in zero field, we are able to describe
quantitatively the crossover from isotropic Langevin behaviour,
via anisotropic superparamagnetism [2], to the hysteretic regime below
the blocking temperature. Our model considers the effects of anisotropy
on both the equilibrium and non-equilibrium magnetization in a consistent
way and involves the averaging upon randomly distributed axes.
Possible origins of the temperature dependent anisotropy are discussed.
[1] F. Wiekhorst et al., Proc. ICM 2003, Rome, J.Magn.Magn. Mat.,
accepted
[2] F. Wiekhorst et al., Phys. Rev. B 67, 224416 (2003)
MA 13.82 Di 15:00 Bereich A
Oberflächeneffekte und magnetische Nahordnung in FePt Nanoteilchen
— •Jens Ellrich 1 , Branko Stahl 1,2 , Gerhard Miehe
1 und Horst Hahn 1 — 1 Fachbereich Material- und Geowissenschaften,
Petersenstr. 23, TU Darmstadt, 64287 Darmstadt — 2 Institut für
Nanotechnologie, Forschungszentrum Karlsruhe, 76021 Karlsruhe
FePt Nanoteilchen wurden nasschemisch auf unterschiedlichen Syntheserouten
hergestellt. Durch die Verwendung verschiedener Liganden und
Precursoren konnte der Einfluss der Oberfläche auf die magnetischen Eigenschaften
der Nanoteilchen mit Hilfe der Mössbauerspektroskopie demonstriert
werden.
Zur weiteren strukturellen Charakterisierung wurde sowohl
Röntgendif-fraktometrie als auch hochauflösende Transmissionselektronenmikroskopie
eingesetzt. Trotz unterschiedlicher Syntheserouten
und Precursoren besitzen die Teilchen die kubisch flächenzentrierte
Struktur der ungeordneten Hochtemperaturphase im System Fe-
Pt. Unterschiede in der Nahordnung und deren Korrelation mit
magnetischen Eigenschaften konnten durch Untersuchungen mittels
Röntgenabsorptionsspektroskopie (EXAFS) näher beleuchtet werden.
MA 13.83 Di 15:00 Bereich A
Magnetic ordering transition in a dense frozen ferrofluid — •S.
Bedanta 1 , S. Sahoo 1 , Xi Chen 1 , W. Kleemann 1 , D. Sudfeld 2 ,
K. Wojczykowski 2 , and A. Hütten 2 — 1 Angewandte Physik, Universität
Duisburg-Essen, D-47048 Duisburg — 2 Fachbereich Physik, Universität
Bielefeld, Postfach 100131, 33501 Bielefeld
Magnetic properties of a dense frozen ferrofluid [Fe55Co45/hexane] are
measured with SQUID magnetometry and susceptometry. Above the
blocking temperature, Tb ≈ 25 K, of the particles with size 4.6 nm,
Langevin-type M(H) curves reveal small moments per particle, µ ≈ 50
µB, and biased minor hysteresis loops (non-saturating for µ0H ≤ 5 T) after
field-cooling to below Tb. Sharp peaks of χ ´ (T) and a sizeable decrease
of the coercive field at Tc ≈ 9 K seem to indicate a transition with critical
behavior of both susceptibility and spontaneous magnetization. Two
heterogeneous models are discussed to explain these features. Either the
nanoparticles contain a ferromagnetic core and a disordered shell [1], possibly
stabilized by an oxidic surface layer, or they are non-collinearly antiferromagnetic
with uncompensated net moments [2]. Within the frame of
the above models the observed transition might then be either exchangedriven
into an intra-shell frozen cluster arrangement [1] or dipolar-driven
into an inter-particle ferromagnetic ground state [3].
[1] R. D. Zysler et al., J. Magn. Magn. Mater. 266, 233 (2003).
[2] R. H. Kodama et al., Phys. Rev. Lett. 79, 1393 (1997).
[3] J. P. Bouchaud and P. G. Zérah, Phys. Rev. B 47, 9095 (1993).
MA 13.84 Di 15:00 Bereich A
Magnetic relaxation of interacting ferromagnetic nanoparticle
systems — •Xi Chen 1 , S. Sahoo 1 , S. Bedanta 1 , O. Petracic 1 ,
W. Kleemann 1 , S. Cardoso 2 , and P. P. Freitas 2 — 1 Angewandte
Physik, Universität Duisburg-Essen, D-47048 Duisburg, — 2 INESC, Rua
Alves Redol 9-1, 1000 Lisbon, Portugal
The relaxation of the thermoremanent magnetisation of granular multilayers
[Co80Fe20(tn)/Al2O3(3 nm)]10 is investigated by SQUID magnetometry
at various nominal thicknesses tn of the CoFe layers. In agreement
with Monte Carlo simulations of random dipolarly interacting ferromagnetic
nanoparticle systems [1] we find asymptotic stretched exponential
decay for superspin glasses at low nanoparticle densities, tn = 0.9
- 1.0 nm [2], while power law decay with finite asymptotic remanence is
observed for superferromagnets at higher densities, tn = 1.2 - 1.6 nm [3].
Efforts are put into localizing the marginal density, tn ≈ 1.1 nm, where
the stretched exponential is supposed to convert into a power law without
remanence [1].
[1] M. Ulrich, J. Garcia-Otero, J. Rivas, and A. Bunde, Phys. Rev. B
67, 024416 (2003).
[2] S. Sahoo, O. Petracic, Ch. Binek, W. Kleemann, J. B. Sousa, S.
Cardoso, and P. P. Freitas, J. Phys.: Condens. Matter 14, 6729 (2002).
[3] Xi Chen, W. Kleemann, O. Petracic, O. Sichelschmidt, S. Cardoso,
and P. P. Freitas, Phys. Rev. B 68, 054433 (2003).
MA 13.85 Di 15:00 Bereich A
Resonant Soft X-Ray Scattering from Magnetic Nanoparticles
— •E. Schierle 1 , E. Weschke 1 , Holger Ott 1 , C. Schüßler-
Langeheine 1,2 , G. Kaindl 1 , J.-M. Tonnère 3 , C. Meyer 3 , B.
Stahlmecke 4 , and G. Dumpich 4 — 1 Institut für Experimentalphysik,
Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany —
2 II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77,
D-50937 Köln, Germany — 3 Laboratoire Louis Néel, CNRS, BP 166,
38042 Grenoble cedex 9, France — 4 Experimentalphysik Arbeitsgruppe
Farle, Fakultät für Naturwissenschaften, Institut für Physik, Universität
Duisburg-Essen Campus Duisburg, Lotharstr. 1, 47057 Duisburg
The application of magnetic nanoparticle arrays as data storage media
requires independently addressable non-interacting dots. A characterization
of such arrays in terms of spatial and magnetic correlations
is therefore important, which can be obtained by scattering techniques.
Particularly useful in this respect is resonant scattering in the soft xray
region, i.e., at the L2,3 edges of the 3d transition metals and at the
M4,5 edges of the lanthanide metals. At these photon energies, the wavelengths
nicely match the distances in these systems and the resonances
provide very high sensitivity to the specific nanoparticle. Furthermore,
the magnetic part of the resonant scattering amplitude allows to study
magnetic correlations. First results from monocrystalline Tb nanoparticles
and from well-ordered arrays of CoPt multilayer dots are discussed.
The setup of a new UHV-compatible x-ray diffractometer will be presented,
which also allows in-situ preparation under UHV conditions. This
work was supported by the BMBF, project 05 KS1KEE/8.
MA 13.86 Di 15:00 Bereich A
Coulomb-Blockade and spin-dependent tunneling in cobalt
nanoparticles — •Herbert Graf, Johann Vancea, Christian
Back, and Horst Hoffmann — Institut für Experimentelle und
Angewandte Physik, Universität Regensburg, 93040 Regensburg,
Germany
We report on the observation of the Coulomb blockade at room temperature
and 77 K in cobalt nanoparticles with sizes ranging between 2
and 5 nm. The Co particle-arrays are contacted via in-plane point contacts.
The sharp point contacts have a separation between 10 and 70
nm. Prior to the deposition of the Co particles, we evaporate a thin 1 -
3 nm thick layer of AlOx onto the gap structures. The particles are then
grown onto the AlOx layer at temperatures between 300 K and 500 K.
The electrical measurements are performed inside the same UHV apparatus.
Magnetoresistance measurement loops with an in-situ electro-magnet
show magnetoresistance values of a few percent at 77 K. An enhanced
magnetoresistance above the threshold voltage of the particle array was
observed. We suppose that this enhancement is related to the Coulomb
blockade effect.