The structure and magnetic properties of spinel-related Mn
4+-doped Li
0.5Fe
2.5O
4 nanocrystalline particles of the composition Li
0.5Fe
2.25Mn
0.1875O
4, prepared by milling a pristine sample for different times, were investigated. The average crystallite and particle size, respectively, decreased form ~40 nm to ~10 nm and ~2.5
μm 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 Mn
4+ dopant ions to substitute for Fe
3+ at the octahedral B-sites of the spinel-related structure. The Mössbauer spectra of the milled ferrites indicate that more particles turn superparamagnetic with increasing milling time. The Mössbauer data collected at 78 K suggest that while in the non-milled sample the Mn
4+ ions substitute for Fe
3+ at the octahedral B-sites, this is reversed as milling proceeds with doped Mn
4+ ions, balancing Fe
3+ 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.
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