Neutron powder diffraction study of NaMn_{2}O_{4} and Li_{0.92}Mn_{2}O_{4}: Insights on spin-charge-orbital ordering

Abstract

High-pressure synthesized quasi-one-dimensional NaMn_{2}O_{4} and Li_{0.92}Mn_{2}O_{4} are both antiferromagnetic insulators. Here their atomic and magnetic structures are investigated using neutron powder diffraction. The present crystal structural analyses of NaMn_{2}O_{4} reveal that a Mn^{3+}/Mn^{4+} charge-ordering state exists even at low temperature (down to 1.5 K). It is evident that one of the Mn sites shows a strongly distorted Mn^{3+} octahedron due to the Jahn-Teller effect. Above T_{N}=35 K, a two-dimensional short-range correlation is observed, as indicated by asymmetric diffuse scattering. Below T_{N}, two antiferromagnetic transitions are observed: (i) a commensurate long-range Mn^{3+} spin ordering below T_{N1}=35 K and (ii) an incommensurate Mn^{4+} spin ordering below T_{N2}=11 K. Surprisingly, the two antiferromagnetic orders are found to be independent of each other. The commensurate magnetic structure (k_{C}=0.5,0.5,0.5) follows the magnetic anisotropy of the local easy axes of Mn^{3+}, while the incommensurate Mn^{4+} one shows a spin-density-wave or a cycloidal order with k_{IC}=(0,0,0.216). For Li_{0.92}Mn_{2}O_{4}, on the other hand, the absence of a long-range spin-ordered state is confirmed down to 1.5 K.