Electronic and magnetic states in the giant magnetoresistive compounds.

The paramagnetic insulator to ferromagnetic metal transition in Lanthanum manganites and the associated magnetoresistive phenomena is treated by considering the localization due to random hopping induced by slowly fluctuating spin configurations and electron-electron interactions. The transition temperature and its variation with composition is derived. The primary effect of the magnetic field on transport is to alter the localization length, an effect which is enhanced as the magnetic susceptibility increases. Expressions for the conductivity, its variation with magnetic field, and its connection with magnetic susceptibility in the paramagnetic phase are given and can be tested with further experiments.