Structural and Mossbauer studies of nanocrystalline Mn4+-doped Li0.5Fe2.5O4 particles prepared by mechanical milling

The structure and magnetic properties of spinel-related Mn4+-doped Li0.5Fe2.5O4 nanocrystalline particles of the composition Li0.5Fe2.25Mn0.1875O4, prepared by milling a pristine sample for different times, were investigated. The average crystallite and particle size, respectively, decreased form similar to 40 nm to similar to 10 nm and similar to 2.5 mu m to similar to 10 nm with increasing milling time from 0 h to 70 h. Rietveld refinement of the XRD data of the non-milled sample show the Mn4+ dopant ions to substitute for Fe3+ at the octahedral B-sites of the spinel-related structure. The Mossbauer spectra of the milled ferrites indicate that more particles turn superparamagnetic with increasing milling time. The M " ossbauer data collected at 78 K suggest that while in the non-milled sample the Mn4+ ions substitute for Fe3+ at the octahedral B-sites, this is reversed as milling proceeds with doped Mn4+ ions, balancing Fe3+ vacancies and possibly Li+ ions progressively migrate to the tetrahedral A-sites. This is supported by the slight increase observed in the magnetization of the milled samples relative to that of the non-milled one. The magnetic data suggest that in addition to the increasing superparamagentic component of the milled particles, thermal spin reversal and/ or spin canting effects are possible at the surface layers of the nanoparticles.