Magneto-Optical Properties of Complex Oxides

Complex oxides give rise to rich physics and exotic cross-coupled electronic and magnetic properties. This is because of the competing interaction between charge, structure, and magnetism in the materials. In this dissertation I present a spectroscopic investigation of several model complex oxides under external stimuli of magnetic field and temperature. The compounds of interest include BiFeO₃ [bismuth ferrite] nanoparticles and tetragonal film, Bi_1-xNd_xFeO₃ [neodymium doped bismuth ferrite], α-Fe₂O₃ [hematite], Ni₃V₂O₈ [nickel vanadate], and RIn_1-xMn_xO₃ [manganese doped rare earth indium oxide]. These materials are attractive systems for investigating optical band gap tunability, lattice and charge dynamics, spin charge coupling, and magnetochromic effect. Systematic intensity and frequency trends in selected vibrational modes show that the paraelectric phase is Pm-3m and the lowest frequency A₁ feature is the soft mode that drives the first order transition. Finite length scale effects are also evident in the electronic structure. It turns out that band gap red shifted ~0.3 eV in the nanoscale particles and blue shifted ~0.4 eV in the tetragonal film. With Nd³⁺ substitution, critical field of cycloid → homogenous (canted antiferromagnet) ordering transition decreases in Bi_1-xNd_xFeO₃. Spiral magnetic order is quenched when x~0.2. Furthermore, we discovered that α-Fe₂O₃ appears more red in applied magnetic field, an effect that is amplified by the presence of the spin flop transition. Analysis of the exciton pattern reveals C2 monoclinic symmetry in the high field phase. Our measurements reveal field-induced blue shifts of the band gap in Ni₃V₂O₈ that are much larger than those driven by temperature, anticipating a more greenish appearance in the fully polarized state. This change is found to emanates from charge density differences around the Ni and O centers in high field. The result on RIn_1-xMn_xO₃ shows a large magnetochromic effect in TbInO₃. The f electron excitations in DyInO₃ change dramatically in the field, a result from the Zeeman effect. When doped with Mn ions, magnetic response of the material is increased, and Mn d-d excitations emerge in the visible range. These findings advance our understanding of spin charge coupling and motivates fundamental research on other functional complex oxides under extreme conditions.