We have developed the theory which describes the magnetization curves of an ensemble of single-domain particles. This theory takes into account their thermal excitations, and it is essentially a generalization of the two-level Stoner–Wohlfarth model and its relaxation variant, as well as a multilevel relaxation model for describing Mössbauer spectra. A fundamentally new feature of our theory is the solution of a system of differential equations for nonequilibrium populations of stochastic states. The resulting model includes the physical mechanisms underlying the formation of nanoparticle magnetization curves in real-world situations and self-consistently describes the qualitative features of the transformation of these curves depending on temperature, direction, and external field strength.
Lobachev et al. (Mon,) studied this question.