CPA susceptibility of the double exchange model

We use a many-body coherent potential approximation (CPA) to study the para magnetic state of the double exchange model with finite local spins S. Our CPA treats dynamical effects due to the quantum nature of the local spins i n a local approximation, becomes exact in the atomic limit. and reduces to Kubo's one-electron CPA in the empty band limit and to dynamical mean field theory (DMFT) in the classical S --> infinity limit. We solve the CPA equa tions of motion for all S in the strong Hund coupling limit, determining al l quantities self-consistently, We show that the CPA does not give a Ferrom agnetic transition for any S; this is due to the local spin probability dis tribution Function P(S-z) being inaccurately obtained. We bypass this diffi culty by extrapolating P(S-z) From the exact (within DMFT)S = infinity resu lt, and obtain a magnetic transition for all S with the expected Curie laws at band filling n = 0, 1. (C) 2000 Elsevier Science B.V. All rights reserv ed.