Controlled inversion and surface disorder in zinc ferrite nanocrystallites and their effects on magnetic properties

A new strategy to control the inversion of zinc ferrite nanocrystallites is demonstrated, while the correlation of process–structure–property is understood with the help of thorough structural and magnetic characterization. It is found that a very high degree of inversion (>0.5) could be induced by rapid microwave-assisted synthesis carried out below 100 °C. Rietveld refinement of high resolution X-ray diffraction patterns of various annealed samples has been employed to investigate the influence of the nature of post-synthesis annealing and the temperature of annealing on the degree of inversion. It is found that magnetization follows the degree of inversion more closely than it does the size of the nanocrystals. Furthermore, slow and prolonged (2 h) annealing results in very different magnetic characteristic than short pulse (2 min) thermal treatment does. Temperature-dependent magnetization (M–T plots) studies confirmed the superparamagnetic nature of all annealed samples which displayed relatively high blocking temperatures (25 K to 56 K) compared to bulk zinc ferrite (10 K). Coercivity follows the trend of blocking temperature closely. Samples subjected to rapid annealing are found to be associated with a degree of surface disorder that influences the coercivity profoundly. Magnetic measurements suggest that rapid annealing can effectively control the surface disorder in zinc ferrite nanocrystallites, which can screen the interparticle dipolar interaction and thus coercivity. Therefore, a combination of microwave-assisted synthesis to induce a high degree of inversion, followed by different annealing protocols to tune the inversion, can deliver magnetic ferrites of desired characteristics to meet futuristic applications.