Abnormal spin structure of manganese ferrite investigated by 57Fe NMR

The spin structure of partially inverted MnFe2O4 is investigated using Fe-57 nuclear magnetic resonance (NMR). The experimental results are inconsistent with a phenomenologically deduced theoretical model predicting that Fe spins have a ferrimagnetic order where the spins in the tetrahedral (A) and the octahedral (B) sites form magnetic sublattices as in Fe3O4. The results show that there is an antiferromagnetic order among the Fe spins at B sites in addition to the antiferromagnetic order between the Fe spins in A and B sites. Moreover, the directions of Fe and Mn spins in A sites are antiparallel to each other, contrary to the commonly accepted assumption. We propose a model of spin structure that is also consistent with the previous Mn-55 NMR results. Comparison with other spinel ferrites implies that the role of Jahn-Teller ions in determining exchange interactions should be emphasized in inverted MnFe2O4.