Particle size effect on structural and magnetic properties of Co_0.75Ni_0.25Cr₂O₄ composite nanoparticles

This study explores the influence of particle size on the structural and magnetic characteristics of Co_0.75Ni_0.25Cr₂O₄ composite nanoparticles prepared using the sol-gel method, with calcination temperatures varying between 500 °C and 900 °C at the intervals of 100 °C. The XRD results confirm that Co_0.75Ni_0.25Cr₂O₄ has a cubic crystal structure with the space group Fd-3m . A secondary Cr₂O₃ phase was detected in all samples that exhibits a rhombohedral structure with the space group R-3c . The particle sizes ranged from 7 ± 2 nm to 114 ± 7 nm, for 500 °C to 900 °C. Magnetization studies reveal size dependent magnetic transitions, with the Curie temperature, T _C, ranging from 79 K to 89 K. Notably, all samples exhibited a spin-spiral transition temperature, T _S, indicative of an incommensurate magnetic structure in nanocrystalline particles an unexpected result. Interestingly, T _S increased varying between varying between 25 ± 1 and 48 ± 3 K, as particle size decreased. Furthermore, the experimentally determined effective magnetic moment for samples exceeded the theoretical estimate for Co_0.75Ni_0.25Cr₂O₄, highlighting the significant contribution of the orbital moments of Co²⁺ and Ni²⁺ ions to the overall magnetization. Field-dependent magnetization studies, M→(μ₀H→), confirmed ferrimagnetic behaviour in the samples, as evidenced by the presence of hysteresis below T _C. The magnetization curve simulations effectively distinguished the ferrimagnetic, superparamagnetic, and paramagnetic components, revealing how the magnetic phase composition of Co_0.75Ni_0.25Cr₂O₄ evolves with calcination temperature at 80 K. These findings demonstrate that nanoscale size reduction and Ni substitution significantly modify exchange interactions and spin ordering in CoCr₂O₄ based spinel chromites.