Structural and magnetic properties of Ni1−xZnxFe2O4 (x=0, 0.5 and 1) nanopowders prepared by sol–gel method

• High specific heating rate was observed over Ni0.5Zn0.5Fe2O4 sintered at 1073 K.
• The critical grain sizes were 41 and 24 nm for NiFe2O4 and Ni0.5Zn0.5Fe2O4 respectively.
• The saturation magnetization decreases with temperature as T1.5.
• The temperature dependence of specific heating rate was evaluated.
• All samples have low coercivity.

A series of nanostructured Ni–Zn ferrites Ni1−xZnxFe2O4 (x=0, 0.5 and 1) with a grain size from 24 to 65 nm have been prepared with a sol–gel method. The effect of composition and sintering temperature on morphology, magnetic properties, Curie temperature, specific heating rate at 295 kHz and hysteresis loss have been studied. The highest coercivity of 50 and 40 Oe, were obtained for NiFe2O4 and Ni0.5Zn0.5Fe2O4 samples with the grain size of 35 and 29 nm, respectively. The coercivity of Ni and Ni–Zn mixed ferrites decreased with temperature. The Bloch exponent was 1.5 for all samples. As the grain size increased, the Curie temperature of NiFe2O4 increased from 849 to 859 K. The highest saturation magnetization of 70 emu/g at 298 K and the highest specific heating rate of 1.6 K/s under radiofrequency heating at 295 kHz were observed over NiFe2O4 calcined at 1073 K. Both the magnitude of the hysteresis loss and the temperature dependence of the loss are influenced by the sintering temperature and composition.