Spin waves in a Heisenberg antiferromagnet at finite temperature using the coherent potential approximation

Abstract

A method developed originally for the insulating Heisenberg ferromagnet with S=1/2 [1] has been extended to the antiferromagnetic case. This method provides the response function at finite T using an ’’alloy analogy’’ in which the system is regarded as an array of spins with Sz=±1/2. The Green’s function have been determined using the coherent potential approximation following the lines for treating dilute systems at T=0 [2] and considering the crystal as an array of uncorrelated bonds [3]. The relation between T and the concentration of ’’wrong spins’’ at T=0 is found from a sum rule for the self-correlation functions. It turns out that the critical temperture TN is the same as the RPA result. The description is reasonable for the spin wave response at T?TN, although it is not correct in the low T limit.