Study On The Growth And Physical Properties Of 3d Transition-metal Oxide Single Crystals

Recently, the study on the transition-metal oxides has been one of the hot objects in the condensed matter physics and material sciences due to the discovery of high-TC superconductivity in cuprates and colossal magnetoresistance in manganites. Researchers try to study the structural, magnetic and transport properties changes by changing physical parameters such as chemical substitution by different elements, applying extrinsic pressure, magnetic field, electric field or photo irradiation to investigate the intrinsic physical mechanism. In order to comprehensively understand the properties of the ground state in these materials, this thesis was focused on the element substituting effect and photomagnetic effect in some 3d transition-metal oxide single crystals.A phenomenon of re-entrant metal-insulator transition (MIT) was observed in the ferromagnetic state of the Cu-doped perovskite manganites La1-xPbxMnO3 single crystals. The re-entrant MIT temperature T* increases with increasing Cu doping level and does not be affected by applied magnetic fields. Accompanying the appearance of T*, there exists a large almost constant magnetoresistance (MR) below T*. The observed phenomenon is ascribed to charge carrier localization due to lattice distortion caused by the Cu-doping at Mn sites. There exist three magnetic transitions in the temperature dependence of magnetization M(T) curve of a Co-doped La1-xPbxMnO3 single crystal, which can be described as short-range ferromagnetic correlation in the paramagnetic background, long-rang ferromagnetic ordering in the critical region and the appearance of cluster-glass state in the low-temperature range. The change in the similar temperature range is also observed in the resistive and MR(T) curves. It is shown that there exists intrinsic phase separation in the single crystal. The effect of Ti-doping on the physical properties of Ca3Co4O9 single crystal has obvious anisotropic character. The in-plane magnetic and transport properties change slightly with increasing Ti-doping level x as x≤0.6, and they change sharply as x reaches 0.8. However, the out-of-plane properties vary considerably in the whole Ti-doping range. The observed phenomena can be ascribed to the different Ti occupied sites as Ti concentration varies. In addition, for all Ti doped samples, the out-of-plane thermopower exhibits a steeper upturn at low temperatures and is obviously suppressed by an applied magnetic field. The low-temperature upturn of out-of-plane thermopower is suggested to originate from the enhanced spin fluctuation induced by Ti-doping. The result of the study on the photomagnetic effect in a single crystal of the bilayer perovskite manganite La1.2Sr1.8Mn2O7 (LSMO-327) shows that the three-dimensional (3D) ferromagnetic transition temperature TC 3 D is remarkably reduced under the light (λ=532nm) irradiation and the two-dimensional (2D) FM correlation temperature TC 2 D within the Mn-O bilayer is almost not affected by the light irradiation. The observed photomagnetic effect can be explained based on the exchange anisotropic properties and photoinduced charge-transfer from O-2p (pz) to Mn4+-3d eg r2orbital state in the LSMO-327 single crystal.