Structural and Mössbauer studies of nanocrystalline Mn2+-doped Fe3O4 particles

Abstract

Nanocrystalline Mn2+-doped magnetite (Fe 3O4) particles of the composition Mn x Fe 3−y O4 \(\left (x = 0.0, 0.1, 0.2, 0.3, 0.4 \text { and } 0.5; y = \frac {2x}{3}\right )\), prepared using chemical precipitation under reflux with the Mn2+ ions substituting for Fe3+ ions rather than Fe2+ ones, are characterized mainly with XRD and 57Fe Mossbauer spectroscopy. All samples were found to have spinel-related structures with average lattice parameters that increase linearly with the Mn2+ concentration, x. The particle size for the samples varied from ∼8 nm to 23 nm. The oxidation of Fe2+ to Fe3+ at surface layers of the Fe 3O4 nanoparticles leading to the formation of maghemite (γ-Fe 2O3) was found to considerably weaken with increasing Mn2+ concentration. The percentage of the nanoparticles that exhibit short range magnetic ordering due to cationic clustering and/or superparamagnetism increases from 17% to 32% with increasing x. The dependence of isomer shifts of the 57Fe nuclei at the tetrahedral and octahedral sites on dopant Mn2+ concentration is emphasized. The electric quadrupole shifts indicate that the Mn x Fe 3−y O4 particles undergo Verwey transition. The effective hyperfine magnetic fields at both crystallographic sites decrease with increasing Mn2+ concentration reflecting a size effect as well as a weakening in the magnetic super-exchange interaction. The Mossbauer data indicate that for x ≤ 0.2, the dopant Mn2+ ions substitute solely for octahedral Fe3+ ions whereas for x > 0.2 they substitute for Fe3+ at both tetrahedral and octahedral sites.