Magnetic And Electronic Transport Properties Of Reactive Sputtered Epitaxial M_xFe（3-x）O₄Films

Spinel ferrites MFe₂O₄（M=Fe, Co, Ni, Mn, etc.） are the most valuable materialsfor spintronics due to the high Curie temperature and rich variety of electrical andmagnetic properties. Among them half-metallic Fe₃O₄is considered as the promisingcandidate for spin injection and Ni（Co）Fe₂O₄can be used as the spin filters for theproduction of spin polarized currents. In this thesis, non-magnetic elements （Ti and Zn）and magnetic elements （Co and Ni） doped Fe₃O₄polycrystalline and epitaxial films andtheir heterostructures were fabricated by using reactive cosputtering. The structure,magnetic and transport properties were investigated systematically.The non-magnetic element Ti doped Fe₃O₄epitaxial films （TixFe3xO4） exhibitincreasing lattice constant, saturation magnetization and magnetoresistance withincreasing x from0to0.09, indicating that Ti might occupy the A site in spinel.Theoretical calculations based on the density-functional theory reveal that the doped Tiions on the A sites indeed increase the lattice constant and enhance the magneticmoment. Meanwhile, the calculated results also imply that TixFe3xO4has a high spinpolarization of about100%. Epitaxial ZnFe₂O₄films exhibit room-temperatureferrimagnetism and semiconducting behaviors. A large magnetoresistance of21.2%was observed at75K under the magnetic field of50kOe. The room-temperatureferrimagnetism and large magnetoresistance are considered to be caused by the cationdisordering, which further leads to spin canting.The investigations on the magnetic elements Co and Ni doped Fe₃O₄films showthat the observed shrinking characteristics in the M H loops of epitaxial CoFe₂O₄filmsare attributed to the presence of two different magnetic phases. One of the magneticphases represents the surface and interface layers, which has much smaller HCand MSthan those of the bulk CoFe₂O₄films, due to the structure defects such as oxygendeficiency, misorientation or dislocations induced by the vacuum annealing or mismatchstrain. The other magnetic phase comes from the middle part of the CoFe₂O₄layer andhas the magnetic properties similar to those of bulk materials.The angular-dependent anisotropic magnetoresistance （AMR） of the epitaxialCo（Ni）xFe3xO4films has been investigated systematically. Four-fold symmetric AMRwas found in both （100）-and （110）-oriented Co_xFe_3-xO₄films. The six-fold symmetric AMR was observed in the （111）-oriented epitaxial Co_xFe_3-xO₄films while no six-foldterm appeared in epitaxial NixFe3xO4films. The six-fold characteristics observed in theepitaxial Co_xFe_3-xO₄films are considered to be induced by the large magnetocrystallineanisotropy caused by Co addition.Room-temperature resistive switching （RS） characteristics were found in MFe₂O₄（M=Co, Ni） films. We have demonstrated that the oxygen vacancies determine the pathof electrical conduction in the MFe₂O₄films. The drift of the oxygen vacancies fromhigh-density region to low-density one makes the conductive path formed between theelectrodes. The RS effect can be attributed to the formation and rupture of theconductive path under the positive and negative bias. Furthermore, the I V relation ofthe epitaxial NiFe₂O₄/Nb:SrTiO3junctions exhibits RS and rectifying effects at roomtemperature. The rectifying and hysteresis characteristics in the I V curves can beunderstood as the coeffect of Schottky diode and interfacial capacitance. It is expectedthat the RS characteristics observed in this work can make it possible to integrateCoFe₂O₄and NiFe₂O₄films into nonvolatile memory devices.