Abnormal spin structure of manganese ferrite investigated byFe57NMR

Abstract

The spin structure of partially inverted ${\mathrm{MnFe}}_{2}{\mathrm{O}}_{4}$ is investigated using $^{57}\mathrm{Fe}$ 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 ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$. 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 $^{55}\mathrm{Mn}$ NMR results. Comparison with other spinel ferrites implies that the role of Jahn-Teller ions in determining exchange interactions should be emphasized in inverted ${\mathrm{MnFe}}_{2}{\mathrm{O}}_{4}$.