Effects of lattice distortion and Jahn - Teller coupling on the magnetoresistance of and epitaxial films

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Abstract
Studies of epitaxial films on substrates with a range of lattice constants reveal two dominant contributions to the occurrence of colossal negative magnetoresistance (CMR) in these manganites: at high temperatures (, being the Curie temperature), the magnetotransport properties are predominantly determined by the conduction of lattice polarons, while at low temperatures , the residual negative magnetoresistance is correlated with the substrate-induced lattice distortion which incurs excess magnetic domain wall scattering. The importance of lattice polaron conduction associated with the presence of Jahn - Teller coupling in the manganites is further verified by comparing the manganites with epitaxial films of another ferromagnetic perovskite, . Regardless of the differences in the substrate-induced lattice distortion, the cobaltite films exhibit much smaller negative magnetoresistance, which may be attributed to the absence of Jahn - Teller coupling and the high electron mobility that prevents the formation of lattice polarons. We therefore suggest that lattice polaron conduction associated with the Jahn - Teller coupling is essential for the occurrence of CMR, and that lattice distortion further enhances the CMR effects in the manganites.