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Magnetismus Vortrag: Mi., 18:40–19:00 M-V16
Dipolar correlations in nanocomposites
A. Michels 1 , C. Vecchini 2 , O. Moze 2 , K. Suzuki 3 , P. K. Pranzas 4 , J.
Kohlbrecher 5 , J. Weissmüller 1,6
1 Technische Physik, Universität des Saarlandes, Saarbrücken, Germany – 2 CNR-INFM
S3 National Research Center, Physics Department, University of Modena and Reggio
Emilia, Italy – 3 Department of Materials Engineering, Monash University, Melbourne,
Australia – 4 GKSS Research Center, Geesthacht, Germany – 5 Paul Scherrer Institute,
CH-5232 Villigen PSI, Switzerland – 6 Institut für Nanotechnologie, Forschungszentrum
Karlsruhe, Karlsruhe, Germany
We present results for the magnetic-field, temperature,
and neutron-polarization dependence of the recently observed
dipole-field-induced spin disorder in the soft magnetic
Fe-based nanocomposite Nanoperm (Fe89Zr7B3Cu1).
The mismatch of the saturation-magnetization values
between the nanosized Fe particles (particle size: 12 nm)
and the amorphous magnetic matrix gives rise to a dipolar
stray field around each crystallite which induces spin
disorder at the nanoscale and manifests itself as a pronounced
clover-leaf-shaped angular anisotropy in the
magnetic small-angle neutron scattering cross section
(see figure below). We provide an analysis of the spindependent
magnetic neutron scattering cross section of
a multicomponent and multiphase nanocrystalline fer-
Fig. 1: (a) Field dependence
of difference-intensity data of
Nanoperm at T = 298 K.
romagnet which entails anisotropic contributions that
give rise to enhanced magnetic scattering at angles θ =
The total SANS cross section
dΣ/dΩ at µ0H = 1.5 T has
been subtracted from the respective
dΣ/dΩ at the lower field.
45
(b) Temperature dependence of
difference-intensity data.
◦ relative to the direction of an applied magnetic
field. The clover-leaf-shaped anisotropy is observed in
the scattering patterns over a wide range of applied
magnetic fields (∼ 30 − 290 mT) and momentum transfers
(0.08 nm −1 ≤ q ≤ 0.6 nm −1 ) and persists up to
T = 693 K, i.e., several hundred degrees Kelvin higher than the Curie transition temperature
of the amorphous matrix phase (T am
C ∼ = 345 K). Analysis of the radiallyaveraged
spin-misalignment scattering cross section suggests that the characteristic
wavelength of the dipole-field-induced spin disorder is temperature independent and of
the order of at least 80 nm, in other words, dipolar correlations evolve on a length scale
larger than the mean particle size. Measurements with a polarized incident neutron
beam indicate that the dipole-field-associated spin-misalignment scattering contributes
only very little to the polarization dependence.