Optical Properties of Thin-Film High-Temperature Magnetic Ferrites

Strongly-correlated electron materials reveal rich physics and exotic cross-coupled electronic and magnetic properties, with important fields results e.g. superconductivity and multiferroics. 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 iron oxides under external stimuli of magnetic field, electric field, and temperature. The compounds of interest include NiFe₂O₄ [nickel ferrite], CoFe₂O₄ [cobalt ferrite], hLuFeO₃ [hexagonal lutethium ferrite], and LuFe₂O₄ [lutethium ferrite]. These materials are attractive systems in the fields of multiferroics and high-temperature magnets for investigating optical band gap tunability, lattice and charge dynamics, spin-charge coupling, and optically-enhanced magnetoresistive effect. In these works, we have combined optical spectroscopy, magnetic circular dichroism (MCD), and (magneto-)photoconductivity, with high-quality thin-film growth, and first-principles calculations to reveal the nature of the optical excitations within these strongly correlated iron oxides. NiFe₂O₄ we found that optical excitations offer the opportunity for producing spin-polarized current. In CoFe₂O₄ we showed that the band gap is robust with temperatures up to 800 K. We found that the direct-gap excitation of LuFe₂O₄ is highly sensitive the strain induced by epitaxial growth.