Study on the magnetic properties and critical behavior of CoFe2−x Al x O₄ (x = 1.0 and 1.2) spinel ferrite

We have examined the behavior of CoFe 2− x Al x O 4 ( x = 1.0 and 1.2) ferrite close to the transition from ferromagnetic to paramagnetic phases ( T C ). The findings indicate that at a temperature of T C = 314 K ( x = 1.0) and T C = 224 K ( x = 1.2), there is a second order magnetic phase transition. We used various methods, including modified Arrott plot, Kouvel–Fisher method, and critical isotherm analysis, to determine the critical exponents which were found to be similar to those expected for the Tricritical Mean-field model ( β = 0.288, γ = 1.057, and δ = 4.665) for the x = 1.0 sample. The critical exponents for the x = 1.2 sample ( β = 0.771, γ = 1.081, and δ = 2.403) belonged to a different universality class. These results suggest that the replacement of Fe ions with non-magnetic Al ions decreases the Co–Fe, Fe–Fe, Co–Co interaction sites in the CoFe 2 O 4 spinel ferrite, increasing magnetic disorder.