THE INITIAL PERMEABILITY OF POLYCRYSTALLINE MNZN FERRITES - THE INFLUENCE OF DOMAIN AND MICROSTRUCTURE

The validity of nonmagnetic grain-boundary (NMGB) models for the initial permeability of polycrystalline ferrites is examined. The domain size in a series of wet-chemically prepared polycrystalline MnZn ferrites, in the demagnetized state, has been determined by neutron depolarization. A transition in the intragranular domain structure from mono- to two domain is observed at grain size D almost-equal-to 4 mum. An expression for this transition has been derived for a dense magnetic material. The grain size dependence of the initial permeability of the ferrites studied, particularly for monodomain grains, is consistent with the NMGB model. The grain-boundary width and composition have been determined with a nanoprobe (1.5 nm resolution). Composition variations extend almost-equal-to 10 nm into the grain. The findings suggest that, although the NMGB model considers ''nonmagnetic'' grain boundaries, in practice the grain boundary may be hard magnetic.