Magnetic and hyperfine properties of nanocrystalline Ni0.2Zn0.6Cu0.2Fe2O4 prepared by a chemical route

Nanoparticles of Ni0.2Zn0.6Cu0.2Fe2O4 were prepared by the standard co-precipitation method. The formation of nanocrystalline mixed spinel phase has been confirmed by x-ray diffractograms. The sizes of the nanoparticles were estimated in the range 7-30 nm, which was confirmed by transmission electron microscopy. Thermal variations of the real part of AC magnetic susceptibilities measured from 450 K down to 80 K and Mossbauer effect measurements at room temperature and down to 20 K clearly indicate the presence of superparamagnetic particles in all the samples. Specific saturation magnetizations measured by VSM are found to increase steadily with the increase of average particle size. The coercive field obtained from low frequency measurements shows that in all the samples a small fraction of particles is not relaxed within the measuring time. For samples showing a less dominating superparamagnetic behaviour, AC magnetic susceptibility data showed the expected increase of blocking temperature with increase in particle size. Magnetic anisotropy energy constants of the nanoparticles were estimated from the blocking temperature and the values cannot be directly correlated with their particle sizes.