Size dependent structural and magnetic properties of Co_(1−x) Ni_x Cr₂O₄ nanoparticles (x = 0.15, 0.20)

This study examines the structural and magnetic characteristics of Co_(1−x)Ni_xCr₂O₄ (x = 0.15, 0.20) compounds synthesised using the co-precipitation technique and calcined at 500 °C and 900 °C respectively. X-ray diffraction studies reveal the structure of the synthesized samples is face-centered cubic with space group Fd-3m. The particle sizes and morphology were examined by using transmission electron microscopy, and the size ranges from 64 ± 10 nm to 105 ± 15 nm for the 900 °C and 7 ± 2 and 8 ± 2 nm for the 500 °C calcined samples. Field-cooling and zero-field-cooling magnetization measurement protocols were used to investigate the materials' magnetic characteristics. The Curie temperature (T_C) decreases from 91 ± 0.5 K to 85 ± 0.3 K as the Ni concentration increases from x = 0.15–0.20. Additionally, an increase in the T_C value is seen for both compositions when the particle size is decreased. Spiral ordering is observed at a transition temperature T_S for the samples calcined at 900 °C, while no evidence for T_S was seen for samples calcined at 500 °C. Magnetization measurements as a function of field, Mμ₀H, at several constant temperatures, show that the coercivity reduces with an increase in temperature in all samples. The Co_(1−x)Ni_xCr₂O₄ (x = 0.15, 0.20) samples calcined at 900 °C exhibit superparamagnetic behaviour with a small ferrimagnetic hysteresis loop. Furthermore, this paper evaluates the contribution of ferrimagnetic, superparamagnetic, and paramagnetic phases to the total Mμ₀H curve for Co_(1−x)Ni_xCr₂O₄ (x = 0.15, 0.20) calcined at 900 °C through simulation. In addition, a modified Langevin simulation, incorporating a particle size distribution term, is performed for the Co_(1−x)Ni_xCr₂O₄ (x = 0.15, 0.20) samples calcined at 500 °C. From the simulations, it is clear that all samples show evidence of significant superparamagnetic contributions to the total saturation magnetization because of the small particles present.