Defects and magnetic studies of nanocrystalline cobalt substituted manganese ferrite probed by positron annihilation lifetime spectroscopy and magnetic measurement.

Cobalt substituted manganese ferrite nanoparticles, MnCoFeO (x = 0.0, 0.1, 0.25 and 0.5) were prepared by co-precipitation procedure. The structural and magnetic properties of ferrite nanoparticles were measured by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and vibrating sample magnetometry (VSM). The lattice constant and cation distribution of ferrite samples were extracted from XRD patterns using the software package MAUD. The crystallite size of the samples was determined by the Scherrer equation and indicated that all of the ferrite samples were nanocrystalline. The defects in the samples were studied by employing positron annihilation lifetime spectroscopy (PALS). From the analysis of the positron lifetime spectrum, three components τ, τ, and τ with corresponding intensities I, I, and I were obtained. The mean lifetime of the annihilated positrons is maximum in the case of x = 0.25. This means that the defect concentration for this sample is greater than that for other samples. Magnetic measurements show a significant increase in the saturation magnetization from 20.62 to 36.03 emu/g, as the cobalt content (x) increased. The coercivity (H) of ferrite nanoparticles increased with the increasing cobalt ion substitution up to x = 0.25, and decreased for x = 0.5. This behavior of the H variation in samples is similar to the variation of average concentration of defects, as indicated by the mean positron lifetime τ, Therefore, it is concluded that the variation in the defect concentration affects the coercivity of the samples.