Excitations in the Heisenberg antiferromagnet at finite temperature using coherent potential approximation

Abstract

In a previous paper the dynamical properties of the S=2/1 Heisenberg ferromagnet were discussed using an extension of the RPA. This theory is extended to theS=2/1 Heisenberg antiferromagnet. The method consists of representing the effect of temperature by a fixed array of up and down spins. The spin-wave excitations are calculated in this disordered array using the coherent potential approximation for a system of potentials due to the appropriate distribution of bonds. A relation between the number of reversed spins and temperature is derived from self-consistency and the transition temperature TN is found to be the same as the RPA result. The method gives a reasonable approximation overall to the spin-wave response at T<or=TN, although it is not correct in the low-T limit.