Bound ferromagnetic and paramagnetic polarons as an origin of the resistivity peak in ferromagnetic semiconductors and manganites

A theory of resistivity is developed for ferromagnetic semiconductors, poss ibly, including manganites. The theory is based on analysis of the interact ion of the free and bound charge carriers with the magnetization of the cry stal. The temperature dependence of free energy for nonionized donors and f ree electrons is calculated for the spin-wave and paramagnetic regions. In addition to the trapping by the ferromagnetic fluctuations (the ferromagnet ic polarons), the electron trapping by the random magnetization fluctuation s as T --> infinity is taken into account (the paramagnetic polarons). For the nondegenerate semiconductors, the theory makes it possible to explain a nonmonotonic temperature dependence of the activation energy, with the val ue for <T = 0 being lower than that for T --> infinity. For degenerate semi conductors, the theory explains a metal-insulator transition that occurs wi th increasing temperature in samples with relatively low charge carrier den sity. If the density is larger, a reentrant metal-insulator transition shou ld take place, so that the crystal is highly conductive as T --> infinity. (C) 2000 MAIK "Nauka/Interperiodica".