Deviations from Saturation and Phase Changes of the Magnetization in a Uniaxial Ferromagnet

Abstract

A macroscopic Hamiltonian is used to describe and calculate the deviations from saturation and phase changes of the magnetization in a ferromagnet with a uniaxial magnetic anisotropy either parallel or perpendicular to an applied magnetic field. Using Green's-function techniques, we have solved for the component (time, spatially, and thermally averaged) of the magnetization in the field direction, and we have determined the static long-wavelength transverse and longitudinal susceptibilities (called the isolated susceptibilities). The stability limits, or critical fields, determined from the singularities in these susceptibilities occur under the conditions for which the elementary excitation energy vanishes. We found that the singularities in the transverse and longitudinal susceptibilities yielded the same stability limit. Under certain conditions (field applied along an easy axis of magnetization and for temperatures near the ordering temperature) the stability limit is determined by the singularity in the isothermal susceptibility. Results of the calculations for the magnetization as a function of applied field, at various temperatures and with various uniaxial anisotropies, are given.