Magnetic localization in mixed-valence manganites

The metal-insulator transition is mixed-valence manganites of the (La0.7Ca0.3)MnO3 type is ascribed to a modification of the spin-dependent potential J(H)S.S associated with the onset of magnetic order at T-C. Here J(H) is the on-site Hund's-rule exchange coupling of an e(g) electron with s=1/2 to the t(2g) ion core with S =3/2. Above T-C, the e(g) electrons are localized by the random spin-dependent potential and conduction is by variable-range hopping. Over the whole temperature range, the resistivity varies as ln(rho/rho(infinity)) = [T-0{1-(M/M-S)(2)}/T](1/4), where M/M-S is the reduced magnetization. The temperature and field dependence of the resistivity deduced from the molecular-field theory of the magnetization reproduces the experimental data over a wide range of temperature and held.