•Andreas Michels^1,2, Jörg Weissmüller^1,2 und Rainer Birringer^2
1Institut für Nanotechnologie, Forschungszentrum Karlsruhe, Karlsruhe, Germany
²Technische Physik, Universität des Saarlandes, Saarbrücken, Germany

Based on a micromagnetics model, we develop a method through which quantitative information on the volume-averaged mean-square magnetostatic stray field á|H^b_d|² ñ_v due to non-zero divergences of the magnetization M within the bulk of a ferromagnetic body can be obtained by analysis of magnetic-field-dependent small-angle neutron scattering data. In the limit of high applied magnetic field H_a, when the direction of M deviates only sligthly from H_a, we have estimated a lower bound for á|H^b_d|² ñ_v as a function of the external field, and we have applied the method to bulk samples of nanocrystalline (nc) electrodeposited Ni and Co and coarse-grained polycrystalline cold-worked Ni. The root-mean-square magnetostatic stray field, which is inherent to a particular magnetic microstructure, shows a pronounced field dependence, with values ranging from about 5 to 50  mT. Even at applied fields as large as 1.7  T, the quantity m₀ á|H^b_d|² ñ_v^1/2 of nc Co is still 24  mT, which suggests that contributions to the total magnetostatic field originating from the bulk are significant in nc ferromagnets; therefore, á|H^b_d|² ñ_v cannot be ignored in the interpretation of e.g. measurements of magnetization or spin-wave resonance. A comparison of á|H^b_d|² ñ_v with the volume-averaged mean-square anisotropy field reveals that both quantities are of comparable magnitude. [Ref.: Eur. Phys. J. B 29, 533-540 (2002).]