Antiferromagnetic coupling between Mn3+ and Mn2+ cations in Mn-doped spinel ferrites

Ferrite powder samples of MnxNi1-xFe2O4 (0.0 <= x <= 1.0) with single phase (A)[B](2)O-4 spinel structure were prepared using a sol-gel method. Following the successful proposal by our group, that the magnetic moment directions of Cr2+ (3d(4)) cations are antiparallel to those of Fe3+ (3d(5)) and Fe2+ (3d(6)) cations in a given sublattice of the Cr-doped spinel ferrites due to the constraints imposed by Hund's rules, we extend here the same idea and assume that the magnetic moment directions of Mn3+ (3d(4)) cations are also antiparallel to those of Mn2+ (3d(5)) and divalent and trivalent Fe (Ni) cations in a given sublattice of Mn-doped spinel ferrites. We have thereby obtained cation distributions for the samples by fitting the magnetic moments of the samples at 10 K. The results indicate that 72% of the Mn cations occupy the [B] sites in MnFe2O4, which is close to the results for Ni (82%) in NiFe2O4, but is different from the result obtained using the conventional view which yields 80% of the Mn cations in the (A) sites of MnFe2O4. On the basis of the present analyses of the magnetic structure of MnxNi1-xFe2O4 (0.0 <= x <= 1.0), we propose here a new model for spinel ferrites that is distinctly different from both the super-and the double-exchange model and that we refer to as the O2p itinerant electron model. Using this model, not only can the magnetic structure of the spinel ferrites MFe2O4 (M = Fe, Co, Ni, and Cu) be explained better than by using the super- and double-exchange interaction models, but also the magnetic structure and the cation distributions of Cr-, Mn-, and Ti-doped spinel ferrites can be explained. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim