Defect-induced spin disorder and magnetoresistance in single-crystal and polycrystal rare-earth manganite thin films

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Abstract
Although theoretical understanding of doped mixed–valence manganites that exhibit colossal magnetoresistance (CMR) is still incomplete, the general observation of a systematic correlation at a given temperature between the magnetization and resistance both above and below the Curie temperature Tc can provide a phenomenological basis for describing the magnetotransport response of defective or magnetically inhomogeneous materials. Defect–related changes in the local magnetization correspond to changes in spin disorder that link to the resistivity through one of several possible spin–dependent transport effects. The different sources and types of defect–induced spin disorder are discussed and an assessment made of transport phenomena that contribute to the magnetoresistive response. Recent measurements of the magnetoresistance of artificial grain boundaries in thin–film bicrystals suggest that spin–polarized tunnelling and spin scattering at interfaces play a relatively minor role, the dominant low–field contribution to magnetoresistance coming from the mesoscale response of magnetic inhomogeneity induced by the grain boundary. The consequence of this finding for other sources of heterogeneity is discussed and the implications for CMR devices and applications are assessed.