Magneto-Optical Spectroscopy of Epitaxial CoxFe3-xO4 (001) Thin Films

The magneto-optical (MO) spectra of spinel cobalt ferrite thin films with different distortions were measured in a broad range of 0.59 – 3.54 eV. Instead of using the cation substitution method, we prepared cobalt ferrite thin films with local distortion by using a RF magnetron sputtering method on an Mg₂SnO₄ buffer layer to induce epitaxial strain. We derived the diagonal and off-diagonal elements of the dielectric tensor to investigate the influence of distortion on the MO effect in the films. In the spectra of the off-diagonal dielectric elements, the transitions at 0.8, 1.8, and 2.0 eV, which correspond to the optical transitions of tetrahedral (Co²⁺) and octahedral [Co²⁺], [Fe³⁺], were assigned to crystal field (CF) transitions: (Co²⁺): ⁴A₂ → ⁴T₁(F), ⁴A₂ → ⁴T₁(P) and intervalence charge transfer (IVCT) transition: [Co²⁺]t_2g → [Fe³⁺]t_2g, respectively. This result revealed that the Co ions were present in both the tetrahedral and octahedral sites in the epitaxial CFO thin films. By comparing the relative amplitude of each MO transition of cobalt ferrite thin films with different distortions, we found that MO spectroscopy is a highly promising tool for evaluating local distortions.