Magnetotransport in half-metallic ferromagnetic oxides

Magnetotransport properties are described for thin films of two different ferromagnetic oxide material systems that are thought to be half-metallic , with possible applications in spintronics: the doped lanthanum manganites, La_1−xA _xMnO₃ where A = (Ca,Sr), and CrO₂. Magnetotransport properties dominated by extrinsic (microstructure-dependent) effects are described for the manganites. The degree of crystalline texture is correlated to large changes in the magnetotransport, with less textured films showing a magnetoresistance (MR) that is both large and relatively temperature-independent over a wide temperature range, and exhibits a hysteretic peak structure in low fields. Microscopic disorder induced by ion irradiation in a highly-textured Sr-doped film induces a similar hysteretic peak structure in the MR. A mesoscopic model is described in which ion damage creates small regions of reduced magnetic properties, giving an enhanced MR for diffusive transport through those regions.; Intrinsic magnetotransport properties are described for CrO₂. At low temperatures, highly-textured CrO₂ films grown by the high-pressure thermal decomposition of CrO₃, exhibit record spin polarization and good metallic properties. As the temperature is raised through a characteristic value Δ = 80 K the character of transport changes because of the onset of strong magnetic scattering and a T2 law in the resistivity. The Hall effect data at low temperatures exhibit a sign reversal in low fields which is interpreted within a simple two-band model as indicating the presence of highly mobile holes and less mobile but more numerous electrons. The growth of CrO₂ films by chemical vapor deposition (CVD) is described. X-ray diffraction data show that the CVD films—especially a film on (100) TiO₂—are generally more strained than the high-pressure film. Magnetotransport measurements show much the same behavior as for the high-pressure film, except that the Hall effect reverses sign at a much higher field for the CVD film on (100) TiO₂. For planar junctions with superconducting counter electrodes, a polarization of 97% was found in a metallic contact between CrO₂ and Pb, and Zeeman-splitting the superconducting Al density of states in a CrO₂/Al junction showed no evidence of minority spins.