Study On Electronic And Magnetic Property Of Half-Doped CMR Manganese (Bulk And Thin Film)

In this dissertation, we investigate the crystal structure, electronic and magnetic phase transitions and colossal magnetic effect (CMR) of the A_0.5-xLa_xSr_0.5MnO₃ (ALSMO, A=Pr, Nd) bulk and thin film.At first, the Pr_0.5-xLa_xSr_0.5MnO₃ (PLSMO) and Nd_0.5-xLa_xSr_0.5MnO₃ (NLSMO) bulk are prepared and characterized. We find that both PLSMO and NLSMO changes first from antiferromagnetic to ferromagnetic at Neel temperature T_N, and then become paramagnetic at Curie temperature T_C, as the temperature increases from 5 K to 300 K. The more important discovery is that the PLSMO bulk shows two kinds of CMR effect at T_N and T_C, respectively, in a lower applied magnetic field, 1T.In addition, we succeeded in the fabrication of the epitaxial (single crystal-like) Pr_0.4La_0.1Sr_0.5MnO₃ (PLSMO) and Nd_0.35La_0.15Sr_0.5MnO₃ (NLSMO) thin films. The electric and magnetic properties of the thin films were examined. We are noticed that both PLSMO and NLSMO have their own optimum deposition temperature (T_o) to grow epitaxial thin films. When the deposition temperature is higher than T_o, a c-axis oriented but polycrystalline thin film grows; when the temperature is lowered than T_o, the thin film tends to be α-axis oriented and also polycrystalline.The third important discovery is that only for the epitaxial PLSMO and NLSMO thin films the electronic phase transitions are closely consistent with the magnetic phase transitions, i.e., an antiferromagnetic phase corresponds to an insulating state, a ferromagnetic phase corresponds to a metallic state and a paramagnetic phase corresponds to a semiconducting state. While for the polycrystalline thin films the electronic phase transitions are always not consistent with the magnetic transitions, due to the grain boundaries existing in the polycrystalline thin films, rather than in the epitaxially grown film.By compares the CMR effect of the PLSMO bulk and that of the PLSMO epitaxial thin film, we can find that the part of the CMR effect of the PLSMO bulk at T_N comes from the antiferromagnet-ferromagnet transition, another part results from the spin disorder which induced by the grain boundaries existing in the polycrystalline bulk. The CMR effect at T_C of both the bulk and epitaxial thin film are all comes from the ferromagnet-paramagnet transition.