Structural and magnetic properties of (Co_1-xNi_x)Cr₂O₄ (x = 0.5, 0.25) nanoparticles

Nanoparticles of (Co_1-xNi_x)Cr₂O₄, with x = 0.5 and 0.25, were prepared utilizing the sol-gel technique, in order to investigate the effect of Ni substitution at the Co site. The crystal structure of the prepared samples was identified using X-ray diffraction. Transmission electron microscopy images indicate a non-uniform distribution in particle sizes. Temperature dependent magnetization measurements as a function of probing field demonstrate different magnetic transition temperatures to that of both the parent compounds. The magnetization as a function of applied magnetic field shows a wasp-waist like feature for (Co_0.5Ni_0.5)Cr₂O₄ nanoparticles measured at 10 K, which is absent in both NiCr₂O₄ and CoCr₂O₄. This feature diminished for other measurement temperatures below the Curie temperature and was also absent at all temperatures for the (Co_0.75Ni_0.25)Cr₂O₄ nanoparticles. X-ray photoemission spectroscopy results show that the Ni cations prefers the 3+ and Co the 2+ oxidation states, while that of Cr was found to be 3+. However, mixed oxidation states were observed for Ni and Co in both samples, which can influence the magnetic properties.