BOUND FERROMAGNETIC AND PARAMAGNETIC POLARONS AS AN ORIGIN OF THE RESISTIVITY PEAK IN FERROMAGNETIC SEMICONDUCTORS AND MANGANITES
Nagaev E.L.

Received: June 16, 1999

PACS: 75.50.Pp, 75.70.Pa

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A theory of resistivity is developed for ferromagnetic semiconductors, including possibly manganites. It is based on the interaction of the free and bound charge carriers with the magnetization of the crystal. The temperature dependence of free energy for nonionized donors and free electrons is calculated for the spin-wave and paramagnetic regions. In addition to the trapping by the ferromagnetic fluctuations (the ferromagnetic polarons), the electron trapping by the random magnetization fluctuations as T → ∞ 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 T =0 value lower than its value for T → ∞. For degenerate semiconductors, the theory explains a metal-insulator transition that occurs in samples with relatively low charge carrier density with increasing temperature. If the density is larger, a reentrant metal-insulator transition should take place, so that the crystal is highly conductive as T → ∞.