Magnetic Properties in the Ordered Phase

Abstract

In our theoretical framework, the magnetic isotherm is determined by solving a differential equation of the inverse of magnetic susceptibility $y(\sigma ,t)$ as a function of $\sigma$. Its initial condition is equivalent to determine the temperature dependence of the magnetic susceptibility in the absence of the external magnetic field. In the ordered phase, the same condition corresponds to finding the temperature dependence of spontaneous magnetization $\sigma _0(t)$. A subtle difficulty is, however, present in this case, because there exist two unknown parameters to be determined. In this chapter, we show it is related with the analyticity of the transverse thermal spin fluctuation amplitude. It is also shown how to get rid of the difficulty and therefore to evaluate various magnetic properties as functions of temperature and external magnetic field in the ordered phase.