| Half-metallic ferromagnetic materials, which have unique band structure and high spin polarization, have attracted widespread attention in the field of spintronics. As novel spintronic materials, they have promising applications. As early as in 1975, Julliere found TMR effect in a sandwich structure FM(Co)/I(Ge)/FM(Fe). The MR in this structure increases with increasing spin polarization of the ferromagnetic electrodes. Then in 1988, Fert’s group discovered the giant magnetoresistance (GMR) effect in Fe/Cr multilayers. These effects are closely linked with the magnetism and spin polarization of the ferromagnetic materials. Since 1990s, the half-metallic ferromagnetic materials with high spin polarization have attracted more attention for application in spintronic devices. Among many kinds of half-metallic ferromagnets, CrO2 has 100% spin polarization, high Curie temperature and high electric conductivity, which makes it one of the best candidates for spintronic application. However, CrO2 is metastable at normal temperature and pressure, and its preparation is facing great difficulties. By now CrO2 powder compacts, films and its nanostructures have been prepared by the atmospheric pressure chemical vapor deposition (CVD) and other methods in some research work. But except for the substrates of Al2O3 (0001) and TiO2 (100), little research has been done to study the growth of CrO2 films on other substrates. In this thesis, we first studied the structure, morphology, magnetic and transport properties of CrO2 epitaxial thin films grown on (110) and (001)-oriented TiO2/MgF2 substrates. Secondly we prepared nanoscale CrO2 shell on the TiO2 nanorods/FTO substrate and studied its structure and morphology characteristics. All work and results are summarized as follows.(1) CrO2 epitaxial thin films.Structure and morphology:The (110) and (001)-oriented CrO2 films were deposited on (110) and (001)-oriented TiO2 films respectively. Because of the different lattice mismatch between the film and the substrate, the (110)-oriented CrO2 film has no diffraction peak shift in XRD, while (001)-oriented CrO2 film has diffraction peak shift compared with the bulk materials. According to the AFM results, the (110) and (001)-oriented CrO2 films were layer-by-layer and island growth mode respectively. The surface roughness of (110)-oriented CrO2 film is less than that of (001)-oriented CrO2 films, and there are much more grain boundaries in (001)-oriented CrO2 films.Magnetic and transport properties:In (110)-oriented CrO2 thin films, strong magnetic anisotropy was observed. The saturation magnetization of (110)-oriented CrO2 thin film (growth time of 40 mins) is 267emu/cm3. In the low temperature region, the MR is mainly affected by the Lorentz force (high field) and grain boundaries tunneling effect of spin-dependent scattering (low field), which is related to the grain boundary density and film quality. So the MR of (110)-oriented CrO2 thin film is smaller than (001)-oriented CrO2 thin film at low temperature. The MR of the film is linear with magnetic field at room temperature, which can be explained by the presence of double-exchange mechanism in CrO2. At room temperature, the normal Hall Effect dominates, while at low temperature anomalous Hall Effect plays a dominant role. In addition, metallic like conductivity of the film is observed by the increase of the resistance with increasing temperature.(2) TiO2/CrO2 core/shell structures.Structure and morphology:the CrO2 shells successful grown on the TiO2 nanorods/FTO substrate are polycrystalline and rutile phase structure and there is no preferred orientation. CrO2 shells are grown on top of TiO2 nanorods, it is epitaxially grown on the TiO2 nanorod.Magnetic properties of CrO2 nanorods:compared with CrO2 thin film, the coercivity and saturation field of CrO2 nanorods are significantly larger. The saturation magnetization of both is the same. |
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