Cationic behavior and the related magnetic and magnetotransport properties of manganese ferrite thin films

Manganese ferrite (MnxFe3−xO4) thin films have been prepared by a sol–gel method. The samples (x≤1.25) are polycrystalline containing well-defined grains and maintain cubic spinel structure with increasing lattice constant with x. The substituting Mn ions were found to have multi-valence, +2 and +3, by X-ray photoelectron spectroscopy. The saturation magnetization of the Mn-substituted films measured by vibrating sample magnetometry was found to increase from that of Fe3O4 at low x and then gradually decreases as x increases further. Such magnetic behavior can be explained in terms of the dominance of Mn2+ species over Mn3+ at low x and the gradual decrease in the population ratio Mn2+/Mn3+ as x increases. The observed decrease in the coercivity with increasing x implies the increase in octahedral Mn2+ population. The Mn-substituted samples exhibit magnetoresistance (MR) effect the maximum intensity of which is gradually reduced with x from that of Fe3O4. All samples show increasing MR with increasing external field (H), while their magnetization curves start to saturate near H=2 kOe. Such increasing MR with H can be explained in terms of the tunneling of spin-polarized carriers across grain boundaries. The reduction in the MR intensity with x can be partly explained in terms of the decrease in spin-polarized carrier density associated with octahedral occupation of Mn2+ ions.