Study On The Magnetism Of Phase Separation And Magnetocaloric Effect Of Rare-earth Manganites

The perovskite-like manganites have attracted a great interest from researchers in recent years not only due to their rich physics but also the application potential, such as magnetic storage, magnetic sensor, magnetic refrigeration and so on. The spin, orbit, charge, and lattice of the manganite are strongly coupled, which leads to very complex electronic and magnetic phase diagram and a lot of interesting phenomena, such as charge ordering and phase separation, etc.. The phase separation is observed by various direct or indirect manners, but a perfect explain about the origin is absent. Different magnetic phases may be coupled, which leads to an exchange bias effect. The manganites have high saturation magnetization and small coercive force, and the Curie temperature of the system can be tuned in a large temperature range by different doping level, which makes it valuable in the magnetic refrigeration. The thesis mainly focuses on the exchange bias effect based on the phase separation and its influence on the transport properties; meanwhile the magnetocaloric effect is investigated in different direction of the La_2/3Sr_1/3MnO₃ single crystalline. Furthermore, the magnetic transport properties and electron paramagnetic resonance (EPR) behaviors were studied in La_2/3Ca_1/3Mn_1-xFe_xO₃(x = 0, 0.1, 0.2). The main contents in the disstation are summarized as follows:In chapter 1, the research progress of the manganite, including the crystal structure, the main magnetic and electronic transport physical mechanism, the electronic and magnetic phase diagrams of La_1-xCa_xMnO₃ and other systems, the exchange bias effect based on phase separation, the research progress of the magnetocaloric effect as well as the result of the electron paramagnetic resonance were briefly introduced.In chapter 2, the structure and magnetism of La_1-xCa_xMnO₃ (0.80≤x≤0.95) were discussed in detail. The analysis of the structure show that all samples are of orthorhombic phase at room temperature. With the temperature decreasing, part of orthorhombic phase transforms to monoclinic phase in compounds for 0.85≤x < 0.95. The exchange bias effect is observed in the samples, which shows a peak around for x = 0.90 and vanishes for only pure spin canted G type anti-ferromagnetism (AFM) existing in system (x = 0.95). We came to a conclusion that the exchange bias effect in the C-AFM due to the electron pahse separation and the pinning of C-AFM to spin canted G-AFM both contribute to the exchange bias effect observed in the system. The magnetic transport result indicated that the interfacial spins play an important role in the system. In chapter 3, the crystal growth by optical floating-zone was studied. The annealing effect on the structure and the magnetocaloric effect were investigated along different directions in the single crystal. There exists some missarrangment of the a and c-axes in the La_2/3Sr_1/3MnO₃ single crystal without annealing. After annealing, the missarrangment disappeared in the ac-plane while the crystal dissociated into sheet crystal along the b-direction. A relatively large magnetic entropy change in low magnetic field was found, which show a small anisotropy. The result shows that La_2/3Sr_1/3MnO₃ single crystal is potential valuble in the magnetic refrigeration.In chapter 4, the magnetic transport properties and the electron paramagnetic resonance spectrum of La_2/3Ca_1/3Mn_1-xFe_xO₃ (x = 0, 0.1, 0.2) polycrystals were investigated. The doping of Fe destroyed the double exchange and induced colossal magnetoresistance. The paramagnetic resonance result shows inhomogeneous magnetism in the system, which results from the anti-ferromagnetic coupling of Fe with the surrounding Mn ions.