Superconductivity in spinel oxide LiTi2O4 epitaxial thin films

LiTi2O4 is a unique material in that it is the only known oxide spinel superconductor. Although bulk studies have demonstrated that superconductivity can be generally described by the Bardeen-Cooper-Schreiffer theory, the microscopic mechanisms of superconductivity are not yet resolved fully. The sensitivity of the superconducting properties to various defects of the spinel crystal structure provides insight into such mechanisms. Epitaxial films of LiTi2O4 on single crystalline substrates of MgAl2O4, MgO, and SrTiO3 provide model systems to systematically explore the effects of lattice strain and microstructural disorder. Lattice strain that affects bandwidth gives rise to limited variations in the superconducting and normal state properties. Microstructural disorder such as antiphase boundaries that give rise to Ti network disorder can reduce the critical temperature, but Ti network disorder combined with Mg interdiffusion can affect the superconducting state much more dramatically. Thickness dependent transport studies indicate a superconductor-insulator transition as a function of film thickness regardless of lattice strain and microstructure. In addition, surface sensitive X-ray absorption spectroscopy has identified Ti to retain site symmetry and average valence of the bulk material regardless of film thickness.Comment: 25 pages, 7 figures, v2 - expanded Fig 1,2,7 with added discussion

The role of magnetic anisotropy in spin filter junctions

We have fabricated oxide based spin filter junctions in which we demonstrate that magnetic anisotropy can be used to tune the transport behavior of spin filter junctions. Until recently, spin filters have been largely comprised of polycrystalline materials where the spin filter barrier layer and one of the electrodes are ferromagnetic. These spin filter junctions have relied on the weak magnetic coupling between one ferromagnetic electrode and a barrier layer or the insertion of a nonmagnetic insulating layer in between the spin filter barrier and electrode. We have demonstrated spin filtering behavior in La0.7Sr0.3MnO3/chromite/Fe3O4 junctions without nonmagnetic spacer layers where the interface anisotropy plays a significant role in determining transport behavior. Detailed studies of chemical and magnetic structure at the interfaces indicate that abrupt changes in magnetic anisotropy across the non-isostructural interface is the cause of the significant suppression of junction magnetoresistance in junctions with MnCr2O4 barrier layers.Comment: 7 pages, 7 figure

Epitaxial growth and characterization of CaVO3 thin films

Epitaxial thin films of CaVO{sub 3} were synthesized on SrTiO{sub 3}, LaAlO{sub 3} and (La{sub 0.27}Sr{sub 0.73})(Al{sub 0.65}Ta{sub 0.35})O{sub 3} substrates by pulsed laser deposition. All CaVO{sub 3} films, independent of epitaxial strain, exhibit metallic and Pauli paramagnetic behavior as CaVO{sub 3} single crystals. X-ray absorption measurements confirmed the 4+ valence state for Vanadium ions. With prolonged air exposure, an increasing amount of V{sup 3+} is detected and is attributed to oxygen loss in the near surface region of the films

Control of the Magnetic and Magnetotransport Properties of La0.67Sr0.33MnO3 Thin Films Through Epitaxial Strain

The influence of epitaxial strain, in the form of tetragonal distortions, on the magnetic and magnetotransport properties of La{sub 0.67}Sr{sub 0.33}MnO{sub 3} thin films was studied. The tetragonal distortion (c/a ratio) was modulated through the choice of the substrate, ranging from c/a=1.007 on (001)-oriented (LaAlO{sub 3}){sub 0.3}(Sr{sub 2}AlTaO{sub 6}){sub 0.7} substrates to 0.952 on (110)-oriented GdScO{sub 3} substrates. In agreement with previous theoretical predictions, these large values of tensile strain cause the Curie temperature and the saturation magnetization to decrease, alter the temperature dependence of the resistivity and magnetoresistance, and increase the resistivity several orders of magnitude

Hydrogen patterning of Ga1-xMnxAs for planar spintronics

We demonstrate two patterning techniques based on hydrogen passivation of Ga1-xMnxAs to produce isolated ferromagnetically active regions embedded uniformly in a paramagnetic, insulating host. The first method consists of selective hydrogenation of Ga1-xMnxAs by lithographic masking. Magnetotransport measurements of Hall-bars made in this manner display the characteristic properties of the hole-mediated ferromagnetic phase, which result from good pattern isolation. Arrays of Ga1-xMnxAs dots as small as 250 nm across have been realized by this process. The second process consists of blanket hydrogenation of Ga1-xMnxAs followed by local reactivation using confined low-power pulsed-laser annealing. Conductance imaging reveals local electrical reactivation of micrometer-sized regions that accompanies the restoration of ferromagnetism. The spatial resolution achievable with this method can potentially reach <100 nm by employing near-field laser processing. The high spatial resolution attainable by hydrogenation patterning enables the development of systems with novel functionalities such as lateral spin-injection as well as the exploration of magnetization dynamics in individual and coupled structures made from this novel class of semiconductors.Comment: ICDS-24, July 2007. 8 pages with 4 figure