SPIN POLARONS IN THE DOUBLE-EXCHANGE MODEL

We consider the possible formation of spin polarons in the double exch ange model at finite temperatures. We show that the average size of a spin polaron in this model increases as the temperature decreases. Bas ed on this, we argue that the transition to the ferromagnetic phase in the double-exchange model happens when the spin polarons start overla pping. The effect of the external field is shown to reduce the effecti ve temperature and thus to increase the size of the polarons. The theo ry predicts a linear increase of T-c versus the applied external magne tic field, a temperature dependence of the magnetic susceptibility abo ve T-c that deviates from Curie-Weiss behavior, and an increase of T-c as the electron/hole density increases.