Deformed Crystalline Structures of Vanadium Oxide Films with Modified Metal-Insulator Transition and Asymmetric Magnetoresistance

Family of vanadium oxide compounds is known to include many compositional variations suggesting its suitability for possible ‘defect-engineering’. Wide and complicated variations in the material properties of vanadium oxides might be attributed to the four different possible valence states of vanadium ions, V²⁺, V³⁺, V⁴⁺, and V⁵⁺. Many unexpected electric and magnetic properties of nanostructured vanadium oxide have frequently been reported and considered to be a worthy research topic. The phenomenon of metal-insulator transition (MIT) is shared by all vanadium oxide compounds exhibiting various transition temperatures, and they have been explored by many researchers. Of these compounds, vanadium sesquioxide (V₂O₃) has been touted to be an exemplary material that exhibits simultaneous transitions in magnetic, structural, and electrical properties. Concretely, V₂O₃ is well-known to exhibit a structural transformation from rhombohedral corundum structure at room temperature to monoclinic structure at around 165 K accompanied with up to 6 orders of change in electrical conductivity. It is predicted to be extremely sensitive to small changes in crystallographic symmetry. Therefore, the understanding MIT with respect to the deformations in the crystallographic structure of V₂O₃ is obviously desirable.<br/>Present work reports our investigation on the controlled MIT in V₂O₃ thin films. The defect density in a textured polycrystalline V₂O₃ film was controlled by adjusting the ambient concentration of oxygen during the deposition by reactive sputtering process. It was confirmed by transmission electron microscopy (TEM) that the controllable amount of stacking faults could be inserted within the body of V₂O₃ grains. As the oxygen partial pressure decreased, defect density was increased. Correspondingly, d-spacing between of (110) atomic planes increased compared to bulk V₂O₃ resulting the development of tensile stress in the film. The measurement of electron energy loss spectroscopy (EELS) established that crystallographic defects on the basal planes of V₂O₃ crystal lattice could alter formation of local atomic order similar to that of VO oxide phase. EELS scans were carried out repeatedly at different regions to reveal the distinguishable changes in the V-L edges and O-K edge. The results evidenced the weak hybridization of V-3d and O-2p orbitals due to increased V-O distances at the stacking faults. Therefore, a large distortion is expected at a stacking fault applying a tensile stress on the V₂O₃ crystal lattice. It was found that the strain on the lattice could be correlated with the changes in the electrical properties of V₂O₃ films. MIT temperatures were measured to vary depending on the expansion of <i>a</i>-lattice parameter in correspondence to the defect density.<br/>Upon further deformation of V oxide crystal structure, as mentioned above, noncrystalline V oxide could be fabricated by controlling the oxygen partial pressure during deposition procedure, and V ions were found to show anomalous magnetic order. It was then coupled to the ferromagnetic order of the ultra-thin Co film by depositing Pt (5 nm)/Co (1.4 nm) on VO_x heterostructure to further investigate the magnetic nature of V oxide. We found an unexpected asymmetric magnetoresistance (MR) response in the Pt/Co/VO_x with respect to the externally applied magnetic field. It is postulated that the measured asymmetry is due to the nonconventional magnetic order in the deformed crystalline structures of VO_x layer.<br/>These results demonstrate the strong correlation between the electric and magnetic properties with the atomic lattice deformation and proves the potential role of defect engineering in material science.