Structural and magnetic studies of nanocrystalline Mg-doped Li _0.5Fe_2.5O₄ particles prepared by mechanical milling

The structure and magnetic properties of spinel-related Mg-doped Li _0.5Fe_2.5O₄ nanocrystalline particles prepared by milling a pristine sample for different times were investigated. The lattice constant increased initially and later decreased whereas the infrared spectral bands of the material were found to shift with increasing milling time. These results imply strains, bond deformation and cationic migration at the nanoparticles' surface layers. The surface to core volume ratio of the nanoparticles obtained from the Rietveld refinement of their x-ray diffraction data was used in a novel way to fit their 78 K zero-field Mössbauer spectra. This offered a simple tool to estimate the Fe³⁺ ionic distribution at the cores and surfaces of the nanoparticles without the need to use low-temperature in-field Mössbauer spectroscopy. The saturation magnetization and Curie temperature were found to decrease and the material increasingly turned superparamagnetic as milling proceeded. The coercivity and the magnetization increased initially and later decreased at higher milling times. The usefulness of the Mössbauer fitting approach adopted was demonstrated by using the Fe³⁺ ionic distribution obtained for the 50 h milled sample to calculate its magnetization in the framework of Néel's collinear model. The calculated value was found to be consistent with experiment if slight thermal spin reversal and/or canting effects are assumed.