Study Of B-site Doped Lanthanum Calcium Manganite

Perovskite manganites have drawn more and more attention due to their unusual electric and magnetic properties. A lot of theoretical and experimental work has been done during the last several decades, yet we still known little about them, especially about their transport properties of spin polarization. Transition metal ions have magnetic moment because of their incomplete d-shells, there is no direct exchange between these magnetic moments in these metal oxides, but there is supper exchange effect via exited oxygen ions, and magnetic ordering structures are formed. Ferromagnetic and metallic phases often coexist in mix-valence perovskite oxides. In this thesis we investigated the structure, electric and magnetic resistance properties of B-site doped lanthanum calcium manganite.Our Cu- and Ti-doped species were prepared by solid state reaction, we confirmed that Cu and Ti atoms occupied B site successfully by XRD and IR measurements. And the system is single-phased. We also measured the conductivity and magnetic resistance at low temperatures.We studied La_0.7Ca_0.3Mn_1-xCu_xO₃ system and find that Cu doping strongly affected the transport property of the system. The system turns from metal to insulator at lower temperatures with Cu concentration increasing. For low doping levels, Cu substituted Mn with the valence state of Cu³⁺, with the increase of doping level, Cu²⁺and Cu³⁺ coexisted. For high doping levels, Cu existed in the state of Cu²⁺. The length and angle of M-O-M bonds, the same with the valence of Cu, varied when doping level increases, which eventually lead to the change of cell parameters. The radius of Cu³⁺ is a little smaller than Mn³⁺,so if Mn³⁺ is substituted by Cu³⁺,the cell parameter should be smaller, but in our species we found that the cell parameter increased with doping. So we think that in our species Cu existed in mix-valence states of Cu²⁺and Cu³⁺. We also discussed the transport mechanism of manganite. Our experiment results agreed well with the Nearest-neighbour hopping modelFor La_0.7Ca_0.3Mn_1-xTi_xO₃ system, electrical resistance increases with doping. When the concentration is less then 0.05, there is a maximum of resistance, and it shifts to low temperature regime with doping increasing. When magnetic field increases, resistance decreases. The magnetic resistance of La_0.7Ca_0.3Mn_1-xTi_xO₃ has a maximum at Tc, and Tc decreases with the increasing of magnetic field. Magnetic resistance increases with the applying field, further more, with the doping increasing, the maximum of magnetic resistance shifts to lower temperature regime. Ti doping isolates the ferromagnetic domains, weakens the ferromagnetic phase coupling, and eventually leads to the transition from ferromagnetic metal phase to paramagnetic insulator phase.