Cationic site substitution effect on magnetic properties of NiCr₂O₄ nanoparticles

Spinel ferrimagnetic compounds of the form MCr₂O₄, where M is a divalent metal cation are of great interest due to the structural and magnetic phase transitions. In this class of materials, NiCr₂O₄ and CoCr₂O₄ are well studied in bulk or single crystals. NiCr₂O₄ demonstrates low temperature magnetic phase transitions together with structural phase transitions. However, CoCr₂O₄ displays only magnetic phase transitions at low temperature retaining stable cubic phase across the magnetic phase transition temperatures. Exchange interaction between the magnetic cations situated at the tetrahedral or octahedral site determines the net magnetic properties of the material. By site mixing with different cations, drastic changes in the magnetic exchange interaction leading to change in magnetic properties are observed. The present work focuses on the structural and magnetic properties of (Ni_0.5Co_0.5)(Cr_0.75Fe_0.25)₂O₄ nanoparticles. Co²⁺ and Fe³⁺ have been substituted at Ni and Cr sites, respectively, to modify the magnetic interactions. Rietveld refinement of the x-ray diffraction (XRD) data confirmed the cubic Fd3-m space group of the samples. In addition to the site substitution the size of the particles was exclusively in the nano range. Transmission electron microscopy (TEM) analyses showed the average particle size DTEM = 82 ± 2 nm. MZFC and MFC demonstrate irreversibility at low temperature. Energy dispersive X-ray spectroscopy (EDS) confirmed the presence of Co, Ni, Cr, Fe and O without trace of any impurities in the samples. TC of the sample was found to be 138 ± 2 K by differentiating the MZFC data with respect to temperature measured with a probing magnetic field of 0.1 T. Hysteresis loops measured at 3 K demonstrated kinks around zero field range.