EFFECTS OF LATTICE DISTORTION AND JAHN-TELLER COUPLING ON THE MAGNETORESISTANCE OF LA0.7CA0.3MNO3 AND LA0.5CA0.5COO3 EPITAXIAL-FILMS

Studies of La0.7Ca0.3MnO3 epitaxial films on substrates with a range o f lattice constants reveal two dominant contributions to the occurrenc e of colossal negative magnetoresistance (CMR) in these manganites: at high temperatures (T --> T-C, T-C being the Curie temperature), the m agnetotransport properties are predominantly determined by the conduct ion of lattice polarons, while at low temperatures (T much less than T -C), the residual negative magnetoresistance is correlated with the su bstrate-induced lattice distortion which incurs excess magnetic domain wall scattering. The importance of lattice polaron conduction associa ted with the presence of Jahn-Teller coupling in the manganites is fur ther verified by comparing the manganites with epitaxial films of anot her ferromagnetic perovskite, La0.5Ca0.5CoO3. Regardless of the differ ences in the substrate-induced lattice distortion, the cobaltite films exhibit much smaller negative magnetoresistance, which may be attribu ted to the absence of Jahn-Teller coupling and the high electron mobil ity that prevents the formation of lattice polarons. We therefore sugg est that lattice polaron conduction associated with the Jahn-Teller co upling is essential for the occurrence of CMR, and that lattice distor tion further enhances the CMR effects in the manganites.