Co(II) Optical Absorption in Spinels: Infrared and Ligand-Field Spectroscopic Study of the Ionicity of the bond. Magnetic Structure and Co²⁺→Fe³⁺MMCT in Ferrites. Correlation with the Magneto-Optical Properties

Abstract

The analysis of the infrared and ligand field spectra of COM₂O₄ spinels reveals that the ionicity of these compounds varies in the following order aluminate > gallate > ferrite and chromite > rhodite and cobaltite. A linear relation has been established between the Δ(LO-TO)₁ splitting, Racah parameter and the ionic-covalent parameter SSp=ΣICP+tetra∑ICPocta. The influence of strong superexchange interactions on the optical spectrum of cobalt ferrites has been studied. The cation distribution has been established by EXAFS and XANES measurements. The cluster (CoFeO₁₀)^15– is characterized by a large MMCT transition Co²⁺→Fe³⁺ at 1.65–1.7 eV (FWMH: 1.35–1.95 eV). The 4A2→4T1 (P) tetrahedral cobalt(II) in ferrimagnetic compounds is overlapped by the MMCT band. This study and the reinvestigation of the iron(III) electronic spectrum is ferrites may explain the magneto-optical properties of mixed cobalt-ferrites.