Appearance of superparamagnetism on heating nanosize $Mn0.65Zn0.35Fe2O4$

Nanosize particles (average size ∼12 nm) of mixed ferrite $Mn0.65Zn0.35Fe2O4$ were prepared by the hydrothermal precipitation route and studied using x-ray diffraction, transmission electron microscopy, differential scanning calorimetry, magnetization measurements, and Mössbauer spectroscopy. The as-prepared sample was largely ferrimagnetic and, as the sample was annealed at temperatures above 250 °C, it gradually became superparamagnetic. This unexpected behavior is explained by assuming that the cation distribution in the nanosize as-prepared sample is in a metastable state and, as the sample is heated, this distribution changes to a more stable state while the grain size remains nearly the same.