Thermal Evolution of Cation Distribution/Crystallite Size and Their Correlation with the Magnetic State of Yb-Substituted Zinc Ferrite Nanoparticles

Evolution of the structural and magnetic properties of ZnFe1.95Yb0.05O4 nanoparticles, prepared via a high-energy ball milling route and exposed to further thermal annealing/heating, was assessed in detail and correlation of these properties explored. While as-prepared spinel nanoparticles possess a high degree of inversion, heating of the sample to ∼500 °C is found to rapidly alter the cation distribution from mixed to normal, in agreement with the known cation preferences. Under the same conditions the crystallite size only slowly grows. By further thermal treatment at higher temperatures, the crystallite size is changed more appreciably. An interrelationship among the lattice parameter, octahedral site occupancy, and crystallite size has been established. The observations are (a) both the site occupancy of Fe at octahedral 16d spinel sites (N16d(Fe )) and the cubic lattice parameter rapidly increase with an initial increase of the crystallite size, (b) the lattice parameter increases with increasing occupancy, N16d(Fe ), and (c) there appears to be a critical nanoparticle diameter (approximately 15 nm) above which both the site occupancy and lattice parameter values are saturated. The magnetic behavior of the annealed samples appears to be correlated to the evolution of both the cation distribution and crystallite size, as follows. As-prepared samples and those annealed at lower temperatures show superparamagnetic behavior at room temperature, presumably as a consequence of the Fe distribution and strong Fe(8a)−O−Fe(16d) superexchange interactions. Samples with a nanoparticle diameter greater than 12 nm and with almost normal distributions exhibit the paramagnetic state. The coercive field is found to decrease with an increase of the crystallite size. Partial Yb/Fe substitution is found to increase the inversion parameter and saturation magnetization. Detailed knowledge of the thermal evolution of structural/microstructural parameters allows control over the cation distribution and crystallite size and hence the magnetic properties of nanoferrites.