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

Nanocrystalline Mn²⁺-doped magnetite (Fe₃O₄) particles of the composition Mn_xFe_3−yO₄$(x=0.0,0.1,0.2,0.3,0.4\text{ and }0.5;y=\frac{2x}{3})$, 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 Mössbauer 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 Fe₃O₄ nanoparticles leading to the formation of maghemite (#gamma#-Fe₂O₃) 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_xFe_3−yO₄ 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 Mössbauer 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.