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Magnetismus Poster: Do., 13:00–15:30 D-P263
Field-Dependent Magnetic Small-Angle Neutron Scattering in Nanomaterials
Jörg Weissmüller 1,2 , Andreas Michels 2
1 Institut für Nanotechnology, Forschungszentrum Karlsruhe, Karlsruhe – 2 Technische
Physik, Universität des Saarlandes, Saarbrücken
When all spins in a ferromagnetic material are aligned in parallel by a sufficiently
large applied magnetic field H, then magnetic small-angle neutron scattering (SANS)
arises exclusively from the nonuniformity of the magnitude of the magnetization, due
to inhomogeneous density or composition. Often, there is a significant, extra magnetic
scattering when the field is insufficient to saturate the sample. In bulk samples with a
nonuniform magnetic anisotropy, such as nanocrystalline materials, the extra scattering
is large even when the sample is a single (macroscopic) magnetic domain, as exemplified
by the figures below [1,2]. Here, the scattering originates not from the domain
structure, but from the failure of the spins to align perfectly with the direction of the
net magnetization. We shall discuss recent progress in understanding magnetic SANS
from this spin misalignment, and in characterizing the underlying magnetic interaction
terms by analysis of SANS data. The entire H- and q-dependence of d Sigma / d
Omega can be understood quite generally in terms of materials parameters, saturation
magnetization and exchange stiffness constant, and of the Fourier coefficients of the
magnetic anisotropy field, which describe the spatial variation of the magnitude and
direction of the anisotropy. By analysis of field-dependent SANS data in terms of this
approach one can measure these quantities, which is more difficult or even impossible
by other techniques.
[1] J. Weissmüller, A. Michels, J.G. Barker, A. Wiedenmann, U. Erb, and R.D. Shull,
Phys. Rev. B 63 (2001), 214414/1. [2] J. Weissmüller, A. Michels, D. Michels, A.
Wiedenmann, C.E. Krill, H.M. Sauer, R. Birringer; Phys. Rev. B 69 (2004), 054402/1.
Fig. 1: Field-dependent
SANS cross-sections of
nanocrystalline Co (left; [1])
and Tb (right; [2]).