Modeling the magnetocaloric effect and critical behavior near the ferromagnetic-paramagnetic transition temperature in Zn0.5-xNixMg0.5Fe2O4 (x=0, 0.125 and 0.250) spinel ferrites

The Landau model allows to estimate the Landau coefficients A(T), B(T) and C(T), which can be determined by fitting the Arrott plots in Zn0.5-xNixMg0.5Fe2O4 (x = 0, 0.125 and 0.250) ferrites. These coefficients are derived to determine $-\Delta S_M\lpar T \rpar$-Delta SM(T) curves. A good correlation between $-\Delta S_M\lpar T \rpar$-Delta SM(T) curves determined using Landau theory and the experimental curves estimated using the Maxwell relation was found. Then, the second-order magnetic phase transition in Zn0.5-xNixMg0.5Fe2O4 compounds was investigated. The critical properties of ferrites prepared by the double ceramic sintering process were investigated. Critical behavior near the FM-PM phase transition was studied from the magnetic data, through various techniques such as the modified Arrott plots (MAP), the Kouvel-Fisher method (KF) and critical isotherm (CI) analysis. The determined critical parameters were found to be close to the value of the mean field theory. On the other hand, the validity of the critical parameters was confirmed by the Widom scaling relation.