| Since the discovery of colossal magnetoresistance effect (CMR) in perovskite oxide, it has sparked considerable renewed interests in these long-known materials with an eye towards both on understanding of the CMR and related properties and potential applications in magnetic information store and low-field magnetic sensors. These materials also exhibit intriguing physical properties such as metal-insulator transition induced by applied magnetic field or photo radiation, spin glass, charge ordering, orbital ordering, phase separation, and spin state transition etc. The full understanding of these properties will definitely stimulated the progress of condensed matter physics.In this work, three new kinds of Ga doping perovskite cobaltites: LaCo1-yGayO3 (y=0, 0.1, 0.2, 0.3, 0.4, and 0.5), La2/3Sr1/3Co1-yGayO3 (y=0, 0.1, 0.2, 0.25, 0.3, 0.35 and 0.4), and La0.7Ca0.3Co1-yGayO3 (y=0, 0.1, 0.2, 0.3, and 0.4), were successfully prepared by standard solid state reaction. Their structure, magnetism, spin glass, Co ion spin state, and related properties have been studied. The main results are as following:The structure and magnetism of Ga doping LaCo1-yGayO3 (y=0, 0.1, 0.2, 0.3, 0.4, and 0.5) were studied. Co ions of all samples have spin state transition around T~100 K. Ga doping mainly reduced the contents of magnetic Co ions. There are a few of Co4+ magnetic ions induced by intrinsic lattice defects, so ferromagnetism and anti-ferromagnetism inset paramagnetic matrix, the samples have complex magnetic structure at low temperature and may have large CMR effect.Poly-crystals of La2/3Sr1/3Co1-yGayO3 (y=0, 0.1, 0.2, 0.3, and 0.4) were prepared by solid state reaction. The magnetization vs temperature and magnetic field were studied carefully. It indicates that Ga doping restrained ferromagnetism in the samples. The ferromagnetic Curie temperature decreases as Ga contents increasing, and the anti-ferromagnetic interaction appears in the high doped samples. Large CMR may be obtained by the competiton of ferromagnetic and anti-ferromagnetic and can be widely used. The paramagnetic Curie temperature and effective magnetic moments of Co ions were gained through Curie-Weiss law fitted with high-temperature susceptibility. The saturated magnetic moments at T~5 K were also obtained through magnetization curves. Their values were compared with the theory's, and the influence of Ga doping on spin states of Co3+ and Co4+ were discussed. It indicates that most of Co ions have mixed spin states. ZFC and FC magnetization curves and ac susceptibility at different frequency of poly-crystals f La2/3Sr1/3Co1-yGayO3 (y=0.25,0.3,0.35,0.4) were studied. The ZFC and FC magnetization have large un-reversible, indicating typical spin glass state and can be explained by double potential well modle. The in-plane and out-of-plant ac susceptibility curves have obvious peaks around freezing temperature (Tf). The in-plane peaks become lower with frequency and the corresponding Tf increases. But the out of plane peaks become higher with frequency and the corresponding Tf also increases. The in-plane ac susceptibility changes with frequency below Tf, but is independence of frequency at high temperature, and vice visa for out of plane ac susceptibility. It is remarkable that the out-of-plane ac susceptibility of y=0.25, and 0.3 samples has two obvious peaks, correspondingly, the in-plane ac susceptibility has a sharp peak at high temperature and a shoulder at low temperature. It indicates that the two samples have two spin glass states. The origin of glass states was dicussed. At last, the Tf was fitted with a critical"slowing down"formulation, and some characteristic factors were obtained.ZFC and FC magnetization curves of poly-crystals La0.7Ca0.3Co1-yGayO3(y=0,0.1,0.2,0.3,0.4) were studied carefully. It indicates that the magnetic transition temperature and magnetization decrease with Ga contents obviously. The Ga doping restrained the ferromagnetic interaction in the samples. It is noticeable that the ZFC magnetization curves of high doped samples have double peaks at low temperature, indicating that the samples change from ferromagnetism to anti-ferromagnetism, afterward to ferromagnetism, and at last to paramagnetism, and the reasons were discussed. The paramagnetic Curie temperature and effective magnetic moments were gained by Curie-Weiss law fitted with high temperature magnetization and compared with theoretical values. The effects of Ga doping on spin states of Co ions are studied. It indicated that most (even all) Co ions of the high doped samples have high spin states.
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