| Three half-metallic materials, La0.67Sr0.33MnO 3 (LSMO), Fe3O4 and CrO2 have been studied for the magnetoresistance (MR) effect, both in polycrystalline thin film form and in tunnel junction form, in search for an ideal material for magnetoelectronics device applications. We have put great effort to ensure the samples have high crystalline quality, by using TEM, SEM, RBS ion channeling, AFM, and x-ray theta-2theta scans to study the crystallinity. Especially in the case of CrO2, few literatures have reported the deposition technique and the growth condition of epitaxial thin films are not well known.; Low field magnetoresistance in polycrystalline form of LSMO and CrO 2 are found to correlate to the grain magnetization. Fe3O 4 do not have a similar low field magnetoresistance as expected from its half-metallic behavior of the conduction electrons.; Trilayer tunnel junctions with half-metallic materials LSMO/SrTiO 3/LSMO and Fe3O4/MgO/Fe3O4 have been successfully fabricated and studied. LSMO tunnel junctions show a very large magnetoresistance at low temperature 4.2K, about 250%, consistent with the half-metallic picture with a near 100% spin polarization. Fe3O4 junctions show only 1.5% at 150K. Tunnel junctions with CO/CrO2 as the electrodes and the natural surface of CrO 2 as the barrier layer have been fabricated. A 8% inverted MR has been observed at 4.2K. Bias and temperature dependence of those tunnel junctions have also been studied, the MR drops quickly as bias and temperature increase.; We demonstrated that tunnel junctions made of half-metallic materials could exhibit very large MR. However, the magnetoresistance drop to very small values at room temperatures, much work needs to be done to improve MR behavior at higher temperatures. |
