Comparative magneto-optical investigation of d–d charge–transfer transitions in Fe3O4, CoFe2O4, and NiFe2O4

Abstract

Magneto-optical transitions in CoFe2O4 and NiFe2O4 spinel ferrites are investigated in comparison with those in isostructural Fe3O4. The Kerr angle and ellipticity of the ferrites have been measured by magneto-optic Kerr effect (MOKE) spectroscopy in the 1.5–5 eV region. The results indicate that the d–d intervalence charge–transfer (IVCT) transitions, Co2+(t2g)→Fe3+(eg), and Ni2+(t2g)→Fe3+(eg), in the octahedral sites in CoFe2O4 and NiFe2O4 are located at about 3.6 and 4.5 eV, respectively, shifted to higher energies by about 1.5 and 2.4 eV, respectively, from the corresponding IVCT transition in Fe3O4, Fe2+(t2g)→Fe3+(eg). The MOKE spectra of CoFe2O4 and NiFe2O4 also indicate that the IVCT transitions, Co2+(t2g)→Fe3+(t2g) and Ni2+(t2g)→Fe3+(t2g), exist at about 2.2 and 3.1 eV, respectively. On the other hand, the intersublattice charge-transfer (ISCT) transitions between the tetrahedral and the octahedral Fe3+ sites in CoFe2O4 and NiFe2O4 are observed at about 2.6 and 4.0 eV, same as the corresponding ISCT transitions in Fe3O4. The observed IVCT and ISCT transitions involving octahedral Fe3+ ion consistently give the crystal-field energy splitting of about 1.4 eV between its t2g and eg orbitals for all the compounds.