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

Nanosize particles (average size similar to 12 nm) of mixed ferrite Mn0.65Z n0.35Fe2O4 were prepared by the hydrothermal precipitation route and studie d using x-ray diffraction, transmission electron microscopy, differential s canning calorimetry, magnetization measurements, and Mossbauer spectroscopy . The as-prepared sample was largely ferrimagnetic and, as the sample was a nnealed at temperatures above 250 degrees C, it gradually became superparam agnetic. This unexpected behavior is explained by assuming that the cation distribution in the nanosize as-prepared sample is in a metastable state an d, as the sample is heated, this distribution changes to a more stable stat e while the grain size remains nearly the same. (C) 2000 American Institute of Physics. [S0003-6951(00)00204-7].