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

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 T C 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.

Magnetic, transport and microstructural properties of polycrystalline samples with nominal composition of La0.7Ca0.3Mn1−xVxO3 (0⩽x⩽0.2)

“…temperatures around T C . The MR peak near T C arises from the CMR component inside the grains (intrinsic CMR effect), whereas the low-field MR at low temperatures is due to the grain boundaries (extrinsic MR effect) [47][48][49][50][51][52][53][54]. The extrinsic MR effect has been explained in terms of spin-polarized tunneling between grains [47,53] or spin-dependent scattering of polarized electrons at the grain boundaries [48,49].…”

“…6 shows the temperature dependence of the resistance at zero magnetic field for all samples. The resistance of the V-free polycrystalline sample is dominated by the grain boundaries [47][48][49][50][51][52][53][54]. The resistance of the composites is mainly affected by both of the grain size and the resistance of the FM phase, and both of the amount and the distribution of the secondary phases.…”

Magnetic and magnetotransport properties of magnetite films with step edges

“…At low temperatures a large extrinsic magnetoresistance effect was observed which decays much stronger with temperature as is anticipated from the temperature dependence of the saturation magnetization. This was attributed to a reduced interfacial magnetization [7][8][9][10] and up to date no remedy to solve this problem of manganite compounds has been found. Comparative studies of the temperature dependence of the extrinsic magnetoresistance in various half-metallic oxides showed that the decay of the low-field magnetoresistance is much weaker in double perovskites of the type Sr 2 FeMoO 6 than in the manganites and CrO 2 [11].…”

Magnetic field dependence of conductance and conduction noise of an artificial grain boundary junction in a La0.67Ba0.33MnO3 epitaxial thin film

“…In the polycrystalline films and bulk samples electrical transport across the grain boundaries is proposed due to spinpolarized tunneling [5] or due to spin-dependent scattering of polarized electron at the grain boundaries [4,12]. A mesoscopic magnetoresistance model based on a grain boundary region with strongly suppressed T c has also been proposed [13]. The nature of surfaces and interfaces is a central issue for the proper understanding of the grain boundary transport and magnetoresistance effect in these polycrystalline samples.…”