Crystallization and magnetic behavior of nanosized nickel ferrite prepared by citrate precursor method

NiFe₂O₄ nanoparticles have been synthesized by citrate precursor gel formation with subsequent heat treatment. Differential thermal and thermogravimetric (DTA/TG) analyses show that the metal citrates decomposed around 230 °C followed by crystallization of the ferrite. X-ray diffraction (XRD) patterns reveal the formation of the cubic spinel phase in the samples after sintering the gel at 350 °C, 500 °C and 700 °C. For the samples annealed at 350 °C and 500 °C a small amount of α-Fe₂O₃ was detected whereas single phase was obtained for the sample annealed at 700 °C. The lattice constant a for all the samples is comparable to the value of the bulk material. The mean crystallite size D_XRD of the samples determined from XRD line broadening is 26.2-28.5 nm. Transmission electron microscope (TEM) analysis shows that the single-phase particles form clusters with the particle size in the range of 21-82.5 nm and the most probable value D_TEM of 55.4 nm. Magnetic measurements show that its Curie temperature T_C is close to the bulk value while the spontaneous magnetization M_s at 5 K is lower than that of the bulk. The thermal variation of M_s in the temperature range from 5 to 300 K can be best fitted to a modified Bloch T ^α law with the exponent value α ≈ 2. The magnetization data are explained with reference to the disordered surface spins and the finite size effects. In this investigated temperature range, the coercive force H_c decreases linearly with increasing temperature. The coercivity mechanism in the nanoparticle sample with broad particle size distribution is expected to be complex and different factors which affect the H_c value were proposed.