Oxides of manganese have received considerable attention lately, mainly because of the colossal magnetoresistance they exhibit. After a careful interpretation of the large body of available experimental results, the paramount importance of intrinsic inhomogeneities to the understanding of these materials, can no longer be ignored. A scenario, based on the competition between different ordered phases which are mixed by the intrinsic disorder, is proposed. Several quantities that follow from this scenario can be evaluated and are found to correspond to experiments. / Dissertation / PhD

With growing interest in devices that utilize the spin degree of freedom of the charge carriers, there is an extensive research effort into materials with high spin polarization. Two types of materials that have attracted particular attention are the half metallic (HM) ferromagnets and dilute magnetic semiconductors (DMS). I report on a series of experiments which probe the level spin polarization in HM CrO2 and the DMS Ga1−xMnxAs. In order to accurately determine the spin polarization, P, of CrO2 in a realistic device structure I have developed a method to chemically modify the surface of CrO2 to obtain a consistent and reproducible barrier, which preserves the bulk spin polarization. Using this method I have been able to produce high quality CrO2 based planar junctions with either superconducting (SC) or ferromagnetic (FM) counter electrodes. Analysis of both zero field and Zeeman split conductance data from CrO2-SC junctions consistently yield P values close to 100%, providing unambiguous evidence that the high P of CrO2 is maintained at and across an artificial barrier in a realistic device structure. Magnetic tunnel junctions (MTJ) fabricated with CrO2 and Co electrodes display a low field inverse magneto-resistance with a maximum magneto-resistance (MR) of -24% occurring at 5K. The origin of this inverse sign is discussed in terms of selective spin transport due to variations in the type of interfacial bonding between the electrodes and the barrier. A strong linear bias dependence of the MR, similar to what is seen in the CrO2-SC junctions, is observed. This linear background is attributed to a continuum of inelastic states in the barrier region. Measurements of the MR as a function of temperature display a rapid decrease in MR as temperature increases. Additionally we have carried out the first direct measurement of the degree of spin polarization of the magnetic semiconductor Ga1−xMnxAs using Andreev reflection spectroscopy. Analysis of the conductance spectra of high transparency Ga0.95Mn0.05As/Ga junctions consistently yields an intrinsic value for P greater than 85%. Our experiments also revealed an extreme sensitivity of the measured spin polarization to the nature and quality of the interface for this material. / Dissertation / PhD