ac Field ``Freezing'' and ``Melting'' of Magnetization in Fine-Particle Assemblies

Abstract

Alternating‐current‐freezing curves M_f(H, H′) and ac‐melting curves M_m(H, H′) represent a dynamic recording of the process of storing and erasing, respectively, of magnetic signals under the continuously equilibrating action of an ac‐field H′. The properties of such curves are discussed in connection with measurements on assemblies of acicular γ−Fe₂O₃ particles. Indication is made how these curves are calculated from simple particle models. It is especially shown that the extension of the pair model with equal volumes, equal critical fields, and negative interaction field to unequal critical fields accounts surprisingly well for most details found experimentally.