Magnetic and electrical properties of the 'colossal magnetoresistance' perovskite manganites

Interest in the doped perovskite manganite materials (La{dollar}sb{lcub}rm 1-x{rcub}{dollar}Ca{dollar}sb{lcub}rm x{rcub}{dollar}MnO{dollar}sb3){dollar} renewed recently when a "colossal" negative magnetoresistance, more than a million-fold decrease, was discovered at x = 0.3 doping. The physics behind this phenomenon is not yet understood. This dissertation presents the results of an extensive series of magnetism and magnetoresistance measurements on La{dollar}sb{lcub}0.67{rcub}{dollar}Ca{dollar}sb{lcub}0.33{rcub}{dollar}MnO{dollar}sb3{dollar} single crystal thin films grown by molecular beam epitaxy. The major undertaking was to correlate the magnetization, resistivity, and temperature over the temperature range 5 - 400 K, and in applied fields up to 14 T. The results indicate that the transition in the transport properties from insulating to metallic occurs at a critical temperature independent of the magnetization. Anisotropic effects were also investigated. Bi-axial in-plane magnetic anisotropy with {dollar}langle{dollar}100{dollar}rangle{dollar} easy axes, and a uniaxial c-axis "easy plane" anisotropy was discovered. Anisotropy constants were estimated by fitting to experimental magnetization curves. The resistivity was also found to be anisotropic, depending on the direction of the magnetization relative to (1) the current and (2) the crystal axes. This anisotropic magnetoresistance was characterized over its entire temperature range, and in applied fields up to 1 Tesla. Finally, the combined effects of magnetic anisotropy, anisotropic magnetoresistance and colossal magnetoresistance are shown to explain both the observed positive magnetoresistance in applied field perpendicular to the film geometry, and the hysteretic magnetoresistance observed at small applied fields.