Optical properties and magnetic properties of antisite-disordered Ni1-xCoxCr2O4 spinels

From the UV-Vis absorption spectra, the FT-IR absorption spectra and the Raman spectra, it is deduced that Co ions primarily occupy the tetrahedral (A) site, with a minor number of them entering into the octahedral (B) site in the Ni1-xCoxCr2O4 compounds. The origin of the position disorder of the Co ions is consistent with the similar ionic radii of the Co ion (0.65 Å) and the Cr ion (0.62 Å) at B site. The FT-IR peak at about 510 cm−1 shifts towards high frequency side with the increasing cobalt content. It is resulted from the reduction of the cation-oxygen distance in the octahedron by the replacement of the Ni2+ with the Co2+ ions. The magnetic measurement shows that Curie temperatures (TC) are 75 and 90 K for the compounds with x=0.2 and 0.8, respectively.