Magnetic-Interaction Effects in Fine-Particle Assemblies and in Thin Films

In describing interaction effects, both the mean field and cooperative phenomena are pertinent. It is shown for different groups of materials how the interaction behavior is derived from the characteristic structural features of a material by suitably combining mean field and cooperative effects. With respect to interaction behavior there are two main types of materials: type I where the anhysteretic susceptibility χα is finite, and type II where χα=∞. As for single-domain particles, suspensions are type I when the dynamic mean interaction field satisfies a certain condition; otherwise they are type II; precipitation alloys are either type I or type II depending on a criterion for particle axes alignment with respect to field direction, or on coercivity. Assemblies of multidomain particles are always type I. Uniaxial 19Fe81Ni evaporated films behave analogously as suspensions. The effects of interactions on χα and on remanence curves of these materials are treated in some detail, also with regard to the effects of packing fraction p and field orientation.