Mössbauer studies of multiferroic spinel CoCr1.98Fe0.0257O4

Abstract

In order to elucidate the role of Cr ions in CoCr2O4 exhibiting multiferroic property, we have substituted a small amount of Fe ions for Cr sites and investigated the magnetic behavior of Fe ions on atomic scale, using Mössbauer measurement. Polycrystalline CoCr1.98Fe0.0257O4 compound was prepared by wet-chemical process. The crystal structure was found to be a single-phase cubic spinel with space group of Fd-3m. The lattice constant (a0) and the internal structural parameter (x) of the oxygen were determined to be 8.340 and 0.264Å, respectively. Mössbauer absorption spectra at 4.2K show that the well developed two sextets are superposed with small difference in hyperfine field. Isomer shift values (δ) of the two sextets are found to be 0.34 and 0.35mm∕s relative to the Fe metal, which are consistent with the high spin Fe3+ charge state. With increasing temperature, the sextets gradually split into two subspectra, and then around 28K the absorption line broadening of outer sextet appears rapidly. Above the Néel temperature (TN=97K) the paramagnetic doublets are observed. The sudden change of outer sextet is observed above 28K, which corresponds to the spin transition temperature. Mössbauer measurement results suggest that Cr3+ ions have two different magnetic sites, and the temperature dependent magnetic property is attributable to the different behaviors of magnetic ions in the two sites.