Time-resolved Magneto-optical Kerr Measurement System Establishment And The Study Of Perovskite Manganite Oxides

Magnetic materials,as the most important information storage media,are demanded to manifest better performance by the rapid development of modern information science.It has always been a key issue in the research field of spintronics to investigate the behaviors of magnetic moment in magnetic materials on ultrafast time scale,in persuit of faster manipulation of the magnetization order in these materials.Femtosecond pulse laser can be used to excite magnetic materials to highly non-equilibrium state,followed by ultrafast processes like demagnetization and precession.However,systematic and deep-going understanding of these novel non-equilibrium behaviors are still in lack.Hence,observing and investigating the ultrafast laser-induced magnetization dynamics in magnetic materials is of great significance at both fundermental and application levels.On the other hand,perovskite transition metal oxides exhibit intriguing phenommena such as the colossal magnetoresistance,which has attracted extensive attention in the field of basic research.Study on the physical properties involving optical,magnetic,and oxygen vacancy effects on such materials will help better understanding the intrinsics of these typical correlated electron systems and improving the potential application value.The main results of this dessertation are listed as follows:1.A time-resolved magneto-optical Kerr effect（TR-MOKE）measurement system is established based on the pump-probe spectropy technique.We acquired the hundred femtosecond time resolution and mrad angle resolution of magneto-optical Kerr rotation by employing the femtosecond pulse laser,belanced optical detection,optical modulation and lock-in amplification.Efforts are paid to implement the weak signal detection,signal width extension,and eliminating the pump interference.We also measured the ultrafast optical spin relaxation in the GaAs semiconductor.Comparing the results with previous reports,we tested and confirmed the reliability and stability of our system.2.A series of Sr doped LaMnO₃ thin films and BiFeO₃/La_0.67Sr_0.33MnO₃ heterostructures were fabricated on SrTiO₃（001）substrates under different growth conditions using the laser molecular beam epitaxy.The characterasation of X-ray diffraction indicate the thin films were heteroepetaxial growth at single crystalline phase with high epitaxial quanlity.3.With the TR-MOKE measurement system established,we measured the ultrafast laser induced magnetization dynamics of La0.7Sr0.3MnO₃ thin films.We observed the ultrafast demagnetization and magnetization precession,and measured the precessional behavior influenced by external magnetic field,temperature,laser power and polarization.Theoretical expression of the precession frequency was derived when considerting the in-plane anisotropy field in LSMO films.We have also investigated the the ultrafast laser-excited magnetization dynamics of 10-nm-thick La_0.67Sr_0.33MnO₃（LSMO）thin films with BiFeO₃（BFO）coating layers.Uniform magnetization precessions are observed in the films under an applied external magnetic field by measuring the TR-MOKE signal.The magnetization precession frequencies of the LSMO thin films with the BFO coating layers are lower than those of uncoated LSMO films,which is attributed to the suppression of the anisotropy field induced by the exchange interaction at the interface between the antiferromagnetic order of BFO and the ferromagnetic order of LSMO.4.We have also investigated the effects of oxygen vacancies on the structural,magnetic,and transport properties of La_1-xSrxMnO₃ grown around acritical point（without/with oxygen vacancies）under low oxygen pressure and high oxygen pressure.We found that all films exhibit ferromagnetic behavior below the magnetic critical temperature,and that the films grown under low oxygen pressures have degraded magnetic properties with lower Curie temperatures and smaller magnetic moments.These results are attributed to the supressed double exchange effect in the films by introducing oxygen vacancies,indicating that in epitaxial La_1-xSr_xMnO₃ thin films,the magnetic and transport properties are very sensitive to oxygen vacancies.