Quantum spins in the double exchange model of manganites

Tn the simplest double exchange model of manganites local spins S associate d with Mn ions interact with electrons in the conduction band via strong fe rromagnetic Hund's rule coupling. A many-body coherent potential approximat ion (CPA) is developed which reproduces the exact atomic limit of the model , for all S and arbitrary filling of the band, and which reduces to an earl ier one-electron CPA in the low density limit. The electronic structure and resistivity are calculated in the paramagnetic state and it is found that the resistivity arising from spin scattering is far too small to explain th e large values observed in the neighbourhood of the Curie temperature. Magn etic states are most easily investigated for S = 1/2 and it appears that th e many-body CPA does not give a transition to ferromagnetism in this case. (C) 1999 Elsevier Science B.V. All rights reserved.