Abstract
High resolution TEM (HRTEM), Mössbauer spectroscopy and SQUID magnetometry were utilized to study the fine structure of equiaxed, Mn- and Co-doped γ-Fe2O3 nanocrystals with a particle size of 30–50 nm. The allocation of cation distribution, based on a redox mechanism and known cation site preferences is (Mn2+0.11Fe3+0.89) [Mn4+0.07Mn3+0.16) Co2+0.34Fe3+1.23⧠0.20]O4, in which ⧠ is the vacancy, (⋅⋅) is the A site, and [⋅⋅] is the B site. This leads to an Fe occupancy ratio of B sites to A sites of 1.38, as verified by Mössbauer spectroscopy. The calculated saturation magnetic moment of 2.99μB/MF is very close to the measured 2.84μB/MF. HRTEM micrographs of the air-cooled nanocrystals fits well with the simulated lattice images using the proposed cation distribution (without directional ordering). While the HRTEM micrograph of the core of an annealed nanocrystal are not in good agreement. This is thought to arise from directional ordering inside the annealed particles.
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