Magnetotransport Properties Of Perovskite Manganese Oxide La_1-xSr_xMnO₃ Thin Films

Perovskite manganese oxide La1-_xSr_xMnO₃(LSMO)films have widely applications in spintronics.The magnetic and electrical transport properties are sensitive to the film quality.In this dissertation,the RF magnetron sputtering technology was used to fabricate the La1-_xSr_xMnO₃(0.43<x<0.52)thin films on the[001]oriented Sr Ti O₃(STO),Mg O,and Mg Al₂O₄(MAO)single crystal substrates at various substrate temperatures(T_S=750?550°C).The effects of substrate temperature and strain on the structure,magnetic,and transport properties of LSMO thin films were systematically investigated.The X-ray diffraction results show that the LSMO/STO and LSMO/Mg O thin films grow with the[00n]orientation under the different T_S values.While the LSMO/MAO thin films have the polycrystalline structure.With the increasing T_S,the non-[00n]orientated X-ray diffraction peaks were more obvious.Transmission electron microscopy characterizations show that the LSMO/STO thin films prepared in the range of T_S=750?650°C are epitaxial;With the decrease of T_S,the crystal quality of the LSMO thin films decreases,and ion vacancies or dislocations appear.X-ray photoelectron spectroscopy analyses prove that the Sr content and oxygen vacancy density in the LSMO thin film vary with T_S,which further modulate the ratio of Mn⁴⁺/Mn³⁺.With the decrease of T_S,the Curie temperature,saturation magnetization and the metal insulator transition temperature(T_MI)decreases.Compared with the LSMO/STO film,the in-plane tensile stress of the LSMO/Mg O film weakens its saturation magnetization and conductivity.The crystalline quality controlled by the stress and substrate temperature induce different magnetoresistance effect(MR)of the LSMO thin films.The MR value of the LSMO/STO(T_S=750°C)thin film near T_MI is-47%at 5 T.with the measured temperature increases or decreases,and the MR value decreases regularly.The LSMO/STO(T_S=650°C)thin films show a MR value of-98%around T_MI,this big MR comes from colossal magnetoresistance effect(CMR)and spin glass like dominated tunneling effect;The main MR mechanism for the polycrystalline LSMO/MAO thin films is the tunneling effect of electrons between grains.Moreover,the LSMO/STO(T_S=700°C)thin films exhibit an anomalous positive low-field MR effect with double peaks(M-shape)at 50?170 K,which is the co-effect of CMR and anisotropic magnetoresistance effect.