Correlation between structure and intervalence charge-transfer transitions in nanocrystallineCoFe2−xMxO4(M=Mn,Al,Sc)thin films

Abstract

The structures, magnetisms, and magneto-optical (MO) Kerr spectra of nanocrystalline ${\mathrm{CoFe}}_{2\ensuremath{-}x}{M}_{x}{\mathrm{O}}_{4}$ $(M=\mathrm{M}\mathrm{n},\mathrm{A}\mathrm{l},\mathrm{S}\mathrm{c})$ thin films have been systematically investigated by means of x-ray diffraction, atomic force microscopy, alternating gradient magnetometer, and MO Kerr spectrometer. The results revealed that the intervalence charge transfer (IVCT) transition of $[{\mathrm{Co}}^{2+}]{t}_{2g}\ensuremath{\rightarrow}{\mathrm{O}}^{2\ensuremath{-}}\ensuremath{\rightarrow}[{\mathrm{Fe}}^{3+}]{t}_{2g}$ in those nanocrystalline films had a strong correlation to the structures created by doping with those trivalent metal ions, within the miscibility range of spinel structure. Doping with ${\mathrm{Al}}^{3+}$ and ${\mathrm{Sc}}^{3+}$ ions led to the increase and decrease of the energy of the IVCT transition, respectively. On the other hand, doping with ${\mathrm{Mn}}^{3+}$ has weak effect on the energy. The molecular orbital theory of IVCT transitions was employed to account for the dependence of the transition energy on the structures. It is concluded that the position of the Kerr rotation peak of the IVCT transition can be adjusted by doping with different trivalent cations and controlling the doping content due to doping induced structural changes.