Multilevel relaxation model for describing magnetization curves of nanoparticles

A theory has been developed to describe the magnetization curves of an ensemble of single-domain particles. This theory includes their temperature-induced excitations and is in fact a generalization of the two-level Stoner–Wohlfarth model and its relaxation version, as well as a multilevel relaxation model for describing Mössbauer spectra. A basically new aspect of this theory is the solution of a system of differential equations for nonequilibrium populations of stochastic states. The resultant model includes the physical mechanisms for the formation of magnetization curves of nanoparticles in real situations and describes self-consistently the qualitative features of the transformation of these curves under the variation of the temperature and the strength and direction of an external field.