Investigation of spin structure in $Mn_3O_4$ by NMR

Abstract

Spinel oxide are a good playground to study strong correlations among four degrees of freedoms, i.e. spin, charge, orbital and lattice because their various charge and orbital states. However, the observation of exact orbital state was one of difficult problems although the orbital states are an important factor to understand these materials. Previous NMR study on $Mn_3O_4$[7] revealed its orbital state and proved that NMR could be a useful tool to study the orbital states. But, for its zero-field NMR spectrum, it has not been revealed why there are three peaks. First, one possible reason is the site disorder of the cations of $Mn_3O_4$. In normal $Mn_3O_4$, the $Mn^{3+}$ ions should be at the center of octahedral of oxygens. However, in certain conditions, the migration of these cations into tetrahedral sites can occur. In order to investigate this possiblity, we annealed $Mn_3O_4$ under vacuum at $1000\degC$ for 12 hours and compared its NMR spectrum with un-annealed one. Additionally, we quenched $Mn_3O_4$ and compared its spectrum with the spectrum of the annealed $Mn_3O_4$ to see whether the replaced cations can return back to their equilibrium positions or not when it is cooled down. Second, it is reported that $Mn_3O_4$ has two different environments around $Mn^{3+}$. Thus, it is possible that at least two peaks appear on the NMR spectrum. We investigated how these environments are different. From thermal variation of NMR spectrum and anisotropy field of each peak, we investigated what peak corresponds to two different sites.