Study of superparamagnetic nano particles of Mn _x Co_{1− x} Fe₂O₄ ferrite system prepared by co-precipitation technique

The ferrites with magnetic nano-particles of Mn _x Co_{1− x} Fe₂O₄ (with x   =  0.0 to 1.0, step 0.2) system are prepared by the chemical co-precipitation method. Structural and magnetic properties of Mn²⁺ doped cobalt nano ferrites have been studied by x-ray diffraction (XRD), scanning electron microscopy (SEM), saturation magnetization and low field AC susceptibility. The crystallite size is determined from the FWMH of the higher angle Bragg line (4 0 0) using the Scherer's formula. The saturation magnetization at 300 K in the peak field of 5 kOe is measured using B  →  H loop technique. The observed saturation magnetic moment is found very low compared to the Neel's magnetic moment. The lower value of observed magnetic moment is explained on the basis of high magneto-crystalline anisotropy and exchange disorder. The Curie temperatures are determined through thermal variation of low field (0.5 Oe) AC susceptibility. All the plots of thermal variation AC susceptibility exhibit large hump due to single domain to superparamagnetic transition which is the characteristic of high anisotropic ion present in the system. The ZFC-FC curves exhibit thermo-magnetic irreversibility. The magnetic behavior observed through Mossbauer spectra also supports the concept of fine particle superparamagnetism. (paper)