Effect of sintering temperature on structural, magnetic, magnetocaloric and critical behaviors of Ni-Cd-Zn ferrites prepared using sol-gel method

We have investigated the effect of sintering temperature on structural, magnetic, magnetocaloric and critical behaviors of Ni_0.4Cd_0.3Zn_0.3Fe₂O₄ ferrites synthesized using sol-gel method at 900 °C and 1100 °C. X-ray diffraction patterns indicated that samples crystallize in the cubic spinel structure (Fd3‾m space group) with an increase of lattice constant and average crystallite size as the sintering temperature increases. Magnetic measurements revealed that that the prepared samples undergo second-order ferromagnetic (FM) to paramagnetic (PM) phase transitions. An increment in magnetization, Curie temperature, magnetic entropy change and relative cooling power has been observed with increasing the sintering temperature. The magnetic entropy change reached maximum values of about 1.11 J.kg⁻¹.K⁻¹ and 1.62 J.kg⁻¹.K⁻¹ for μ₀H = 5 T corresponding to relative cooling power (RCP) of 152.09 J.kg⁻¹ and 253.65 J.kg⁻¹ for samples sintered at 900 °C and 1100 °C, respectively. These values are comparable favorably with those of some others ferrites considered as possible candidates for magnetic refrigeration. The estimated critical exponents obtained for sample sintered at 1100 °C are close to the tricritical mean-field model; whereas for sample sintered at 900 °C, the exponents belong to a different universality class. This change of critical exponents is mainly due to the increase of average crystallite size as the sintering temperature increases.