Electronic Structure,Magnetic And Spin-Dependent Transport Properties Of Fe₃O₄ Films Near Verwey Transition

Verwey transition in Fe₃O₄ is the first known metal-insulator transition induced by charge ordering.The mechanism of Verwey transition including lattice distortion,change of magnetic and electronic transport properties is one of the hot topics in the fields of condensed matter physics and materials science.In this dissertation,the electronic structure,ferroelectric,magnetic and spin-dependent transport properties of Fe₃O₄ near Verwey transition are investigated by experiments and theoretical calculations.The results provide the basis for understanding the physical mechanism of Verwey transition.The main results of this dissertation are as follows:The distortion of Fe₃O₄ lattice induced by the in-plane biaxial strain tunes the charge ordering and the band gap in the low-temperature phase（LTP）Fe₃O₄ with P2/c and Cc space group.The distribution of trimerons can also be affected by strain.It is found that the spontaneous ferroelectricity of LTP Fe₃O₄ derives from charge ordering.The magnitude of ferroelectric polarization can be modulated by the strain-tuned charge ordering.These results provide the theoretical basis for understanding the strain effects on Verwey transition and the ferroelectricity of LTP Fe₃O₄.The epitaxial Fe₃O₄ films were fabricated on Mg O（001）and Al₂O₃（0001）substrates.The magnetoresistance（MR）and anisotropic MR（AMR）of Fe₃O₄ films were investigated.The MR of epitaxial Fe₃O₄ films increases below Verwey temperature（T_V）because of the antiferromagnetic coupling between the octahedral Fe sites near twin boundaries.As the trimerons are parallel to the magnetic field,the resistivity of Fe₃O₄ films decreases due to the uniaxial magnetic anisotropy of trimerons,which reveals the mechanism of AMR symmetry evolution below T_V.The Raman active modes of LTP Fe₃O₄ appear below T_V,which suggests that Fe₃O₄undergoes the charge ordering and lattice distortion step by step with the decrease of temperature.The polycrystalline Fe₃O₄ films were deposited on Si wafer and quartz by reactive sputtering.The temperature-dependent MR of polycrystalline Fe₃O₄ film is not similar to the epitaxial Fe₃O₄ films,where MR of polycrystalline film is dominated by the spin dependent transport across grain boundaries.Epitaxial Fe₃O₄ films on bicrystal Sr Ti O₃（001）substrates were fabricated by reactive sputtering.The electronic structure of Fe₃O₄ at grain boundaries were investigated by theoretical calculations and experiments.The artificial grain boundary breaks the cubic symmetry of Fe₃O₄,which increases the saturation magnetization and T_V of Fe₃O₄ epitaxial films.The magnetic anisotropy of Fe₃O₄ bicrystal films is altered due to the macroscopic rotation of Fe₃O₄ lattice.Together with the enhanced spin-dependent scattering at the artificial grain boundaries,the magnitude of AMR increases.Polycrystalline Fe₃O₄ films were fabricated on n-Si and p-Si wafers with naturally oxidized Si O₂ layer.The electronic transport properties of the heterostructures were investigated with in-plane and out-of-plane geometry.The in-plane conductive channels switch from Fe₃O₄ to p-Si due to the increased resistivity of Fe₃O₄ film with the decrease of temperature,which leads to the negative differential resistance and the change of spin-dependent transport properties.