Abstract
The paper is aimed at clarification of nature of unusual transport properties of the layered manganites. Magnetic and magnetotransport properties of the Mn-deficient quasi-two-dimensional manganites La1,2Sr1,8Mn2(1−z)O7 are investigated at 5–400K in magnetic fields up to 9Т. The studied single crystals show properties typical to layered manganites. Semiconductor-metal-like transition near the Curie temperature (TC), non-metal type of resistance ρ(T) at low temperatures, enormous magnetoresistance near TC, the negative magnetoresistance in the studied temperature range are observed in these single crystals. The obtained results are discussed in the framework of the magnetic polaron conductivity. It is assumed that in quasi-two-dimensional manganites the polarons are preserved in the magnetically ordered state as well, and that small magnetic polarons become big polaron at low temperatures. The increase of size of the magnetic polaron under magnetic ordering or application of a magnetic field explains the basic transport properties of layered manganites La1.2Sr1.8Mn2O7. The temperature and field dependencies of the polaron sizes, defined from the magnetotransport properties, are reasonably described in the model of phase separation.
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