Synthesis of highly ordered 30 nm NiFe2O4 particles by the microwave-combustion method

• Highly ordered 30 nm NiFe2O4 was synthesized by a microwave combustion method and annealing.
• The crystallite size (D) increases with increasing the annealing temperature.
• Reduced activation energy was observed for grain growth.
• Hc increases and then decreases with increasing D, whereas Ms has increased.
• Mössbauer spectra elucidate the nature of formed phases and cation distribution.

NiFe2O4 of 30 nm average size was synthesized by microwave combustion and subsequent solid state reaction at 1273 K. The materials were characterized by X-ray diffraction, TEM, vibrating sample magnetometery and Mössbauer spectroscopy. The microwave combustion produced materials were comprised chemically of ferrites and a smaller amount of hematite. The NiFe2O4 particles have the cubic spinel structure with crystallites of sizes less than 10 nm, and were found to have low magnetization, and essentially no hysteresis loop; characteristics of superparamagnetism. Upon annealing at temperatures 973 K and below, crystallite growth was accompanied by increase in both coercive field and magnetization. The coercive field was a maximum for the sample annealed at 973 K. On the other hand, crystallite growth at higher annealing temperatures yielded mainly ferrites and improvement in soft magnetic properties. Mössbauer and magnetization measurements indicate that the fine NiFe2O4 particles produced at the annealing temperature of 1273 K are in good chemical and magnetic order, excluding the spins arrangement at the surface of the particles which show spin glass-like behavior.