This study presents results on growth and characterization of spinel Fe$_3$O$_4$ and ZnFe2O4 thin films on SrTiO3, MgAl2O4 and GaN substrates with single-layer-, or bi-layer structures. The spinel films were grown in argon or oxygen background pressure at different substrate temperature by using pulsed laser deposition (PLD) technique, and metal electrodes were deposited by sputtering technique. Structural, microstructural, electrical and magnetic properties of the samples were investigated by various characterization methods, such as atomic force microscopy (AFM), X-ray diffraction (XRD), magneto-optic Kerr effect (MOKE), magnetoresistance (MR), temperature dependence resistance R(T), and transmission electron microscopy (TEM).
Epitaxial Fe3O4 films on (110)-oriented SrTiO3 and MgAl2O4 were prepared to investigate the in-plane-direction dependence of the interfacial lattice disorder and its effect on the magnetic property. TEM images of (110) Fe3O4/SrTiO3 showed that lattice spots at the interface along the in-plane [-110] direction were quite regular, but those along the in-plane [001] direction exhibited a distinctive lattice disorder. In contrast, the (110) Fe3O4/MgAl2O4 exhibited quite regular and smooth interfaces along both in-plane directions. The (110) Fe3O4 /SrTiO3, which keeps structural symmetry across the interface along the in-plane [-110] direction but not across the interface along the in-plane [001] direction, exhibited improved properties: The in-plane uniaxial magnetic anisotropy was about five times larger than those in the previous reports, and the maximum magnetoresistance was about twice that of the Fe3O4/MgAl2O4. This suggests that the microscopically anisotropic lattice disorder at the interface can be used to control the uniaxial magnetic anisotropy and the magnetoresistance of a magnetic film for spintronic applications.
25-nm-thick Fe3O4 films without and with 1.5-nm-thick ZnFe2O4 capping layer was deposited on (110) SrTiO3 substrate, and their structural and magnetic properties were studied. XRD data showed that the position of the (110) peak shifted marginally to a higher angle after deposition of the capping layer in oxygen environment. TEM images showed that the Fe3O4 film on the (110) SrTiO3 substrate appeared epitaxial, and the capping layer on the Fe3O4 film appeared polycrystalline. Reciprocal space mapping and pole-figure measurements revealed that a structural distortion of the Fe3O4 layer from cubic to tetragonal-like was presumably induced by a slight change in oxygen content during the deposition of the capping layer in oxygen environment. Compared with the cubic Fe3O4 film, magnetic hysteresis loops of the tetragonal-like Fe3O4 showed increased coercivity and enhanced uniaxial magnetic anisotropy, which were presumably attributed to an increase of crystalline anisotropy level in the tetragonal-like structure.
The structural and electrical properties of the half-metallic oxide Fe3O4 film grown on an n-type semiconducting gallium nitride GaN were studied. X-ray diffraction and transmission electron microscopy analyses confirmed the heteroepitaxial growth of the (111) Fe3O4 film on (0001) GaN with a clear interface. The current-voltage measurement for the Fe3O4/GaN heterojunction indicated a strong temperature dependence of the Schottky diode behavior. At room temperature, the Fe3O4/GaN heterojunction exhibited a weak diode-like behavior for the bias- voltage sweep. However, as the temperature decreased from room temperature to 30 K, the current in negative-bias voltages decreased significantly, but its change in the positive bias regions was small. Thus, the rectification ratio increased by 104 times, while the barrier height decreased and the ideality factor increased. The barrier height inhomogeneities at the interface could be an origin for such distinctive temperature dependences of barrier height and ideality factor.
Au/Fe3O4/ZnFe2O4/GaN and Au/ZnFe2O4/GaN metal-insulator-semiconductor (MIS) structures were fabricated to investigate the I-V characteristics and photovoltaic effect. The rectification ratio and the barrier of Au/ZnFe2O4/GaN junction was larger than the Au/Fe3O4/ZnFe2O4/GaN. Both structures show the photovoltaic effect and their I-V curve were shifted remarkably after illuminating the UV light. The diode properties and photovoltaic effect in Au/ZnFe2O4/GaN were much better than the Au/Fe3O4/ZnFe2O4/GaN structure.
Thesis Advisor: Prof. Joonghoe Dho