Domain formation and long-range spin disorder in Vitroperm

We present a study of the magnetic-field dependence of the small-angle neutron scattering of the nanocrystalline soft magnet Vitroperm (Fe73Si16B7Nb3Cu1), which was treated so as to exhibit a macroscopic magnetic anisotropy. We show how the field dependences of i) the radial average of the scattering cross-section dΣ/dΩ and ii) the variation of dΣ/dΩ as a function of the orientation of the scattering vector can be analysed to supply separately information on the orientation of the net magnetisation of the domains and on the internal spin disorder within the domains. As the applied magnetic field is decreased starting from saturation, we find that two processes occur independently: the formation of a domain structure with the net magnetisation of the domains aligned along the macroscopic easy axis and the increase in magnitude of static, nanometer-scale fluctuations of the spin orientation.