Hole-doped manganite La0.7Ca0.3MnO3 (LCMO) was extensively studied because of its colossal magnetoresistance (CMR) characteristic in a magnetic field. Recently, a new member of CMR family La0.7Ce0.3MnO3 (LCeMO), an electron-doped manganite, raises a new wave of attention for possible application in p-n junction. In this present study, LCMO and LCeMO single layer and bi-layer were grown on SrTiO3 (100) substrate by a pulse laser ablation technique. Due to the neutralization at the p-n junction a possible insulating layer with the anti-ferromagnetic (AFM) property is expected. There is no systematically study of this matter up to date, thus, it is worth to systematically investigate the physical properties of this junction. The result indicates the possible neutralization layer exhibits huge resistance comparison with two lateral layers, the bias current is constrained on the limited thickness of the top layer, which implies the neutralization layer forms a depletion layer that block the current to flow through to the bottom layer. Its electric and magnetic properties may similar to the parent compound LaMnO3 with insulating and anti-ferromagnetic characteristics. Separated by this possible layer, the magnetic coupling between lateral layers is weak. However, the possible AFM layer does pin the magnetic moment of the top layer along the direction perpendicular to the substrate that make a distinct magnetoresistance at low magnetic field.

This dissertation reports on the synthesis, structural and thermal characterization and electrical and thermal transport properties of a variety of strongly correlated spinels. General structure property relationships for electrical and thermal transport are discussed. However, the relationship between thermopower and features of the crystal structure such as spin, crystal field, anti-site disorder, and structural distortions are explored in depth. The experimental findings are reported in the context of improving existing oxide thermoelectric materials, screening for new materials or using thermopower as a unique characterization tool to determine the cation distribution in spinels. The need for improved n-type oxide thermoelectric materials has led researchers to consider mixed valence \((+3/+4)\) manganese oxides. Contrary to previous findings we report herein that the \(LiMn_2O_4\) compound reaches the relatively large n-type thermopower of \(-73 \mu V/K\) which is three times larger than the value observed in other manganese oxides, \(-25 \mu V/K\). The cause of this increase in thermopower is shown to be the absence of a Jahn-Teller distortion on the \(Mn^{3+}\) ions in \(LiMn_2O_4\). By avoiding this structural distortion the orbital degeneracy is doubled and the Koshibae et al.’s modified Heikes formula predicts a thermopower of \(-79 \mu V/K\) in good agreement with the experiment. Altering the \(Mn^{3+/4+}\) ratio via aliovalent doping did not affect the thermopower and is a second evidence of universal charge transport first reported by Kobayashi et al. The role of anti-site disorder was further examined in \(Fe_xMn_{1-x}NiCrO_4\) x=0, ½, ¾, 1 spinels but the effect on thermopower was inconclusive due to the presence of impurity phases. Next, the thermopower as a function of temperature in \(Co_3O_4\) was investigated as a means whereby the Wu and Mason’s 30 year old model for using thermopower to calculate cation distribution in spinels could be revisited. We report evidence that Wu and Mason’s original model using the standard Heikes formula and considering octahedral sites alone leads to a stoichiometrically inconsistent result at high temperatures. Alternate models are evaluated considering Koshibae et al.’s modified Heikes formula and accounting for tetrahedral site contributions. Furthermore, the effect of a possible spin state transition is considered. / Engineering and Applied Sciences