Abstract
The dynamical properties of the S=1/2 Heisenberg ferromagnet are studied using an extension of the RPA method. The effect of temperature is represented by a fixed array of up and down spins, including short-range-order effects. The spin wave excitations are calculated in this disordered array using the coherent-potential approximation (CPA) 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. The method gives a satisfactory approximation overall to the spin wave response at T<or=Tc, although it is incorrect in the low-T limit. The transition temperature in the model lies between the molecular-field and RPA results and is reduced by the introduction of a self-consistent value of the short-range order.
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