Electrical and magnetic properties of manganates with perovskite-related structure

Multi-metal transition metal oxides with perovskite and related structures are known to exhibit a variety of interesting electrical and magnetic properties. In the work presented in this dissertation two systems were chosen to highlight the influence of lattice distortion and structure on those properties.; The first system investigated was a series of charge ordered Ruddlesden-Popper (layered perovskite) manganates with a composition LnSr₂Mn ₂O₇, where Ln is a rare earth. The degree of distortion of the structure was modified by changing the rare earth cation of the perovskite (La, Nd and Gd). Thin films were deposited on single crystal substrates by nebulized spray pyrolysis. It was observed that by reducing the dimensionality of the structure the magnetic exchange becomes antiferromagnetic, replicating the behavior of three-dimensional perovskites with higher distortion. It was further observed that the charge ordered behavior of all films was insensitive to magnetic fields up to 7 T, and also that application of stronger current while measuring transport properties (100 μA) would eliminate the charge ordering signature in the measurement. Both these results establish a clear connection between dimensionality of the structure and the level of distortion of the parent perovskite structure.; The second system chosen was BiMnO₃, one of the few perovskites known to be ferromagnetic (T_c = 105 K) which has recently been indexed in a polar C2 (monoclinic) space group consistent with ferroelectricity. The synthesis of this compound involves high pressures and temperatures. The samples were prepared both by cubic anvil cell and a Paris-Edinburgh setup. Ferroelectric hysteresis loops were measured confirming that BiMnO₃ is the first true multiferroic in the perovskite family, i.e. simultaneously exhibits ferroelectricity and ferromagnetism. Samples of this oxide were also synthesized by nebulized spray pyrolysis and pulsed laser deposition, both conventional ambient pressure techniques.; The magnetic structure of BiMnO₃ was determined through neutron diffraction and it revealed a collinear arrangement of the spins along [010], the unique axis of the monoclinic cell. A careful observation of six Mn-O-Mn pathways in the crystal structure showed that four are ferromagnetic and two weakly antiferromagnetic, supporting the belief that the particular orbital ordering observed in BiMnO₃ is responsible for its ferromagnetism.