Neutron powder diffraction study of NaMn$_2$O$_4$ and Li$_{0.92}$Mn$_2$O$_4$: New insights on spin-charge-orbital ordering

TL;DR

This neutron diffraction study reveals detailed atomic and magnetic structures of NaMn₂O₄ and Li₀.92Mn₂O₄, uncovering charge, spin, and orbital ordering phenomena and their temperature-dependent transitions.

Contribution

The paper provides new insights into the low-temperature magnetic and charge ordering in NaMn₂O₄ and Li₀.92Mn₂O₄ using neutron powder diffraction, highlighting distinct magnetic behaviors.

Findings

NaMn₂O₄ exhibits Mn3+/Mn4+ charge ordering at low temperature.

NaMn₂O₄ shows two magnetic transitions: a commensurate order below 39 K and an incommensurate order below 10 K.

Li₀.92Mn₂O₄ lacks long-range magnetic order down to 1.5 K.

Abstract

The high-pressure synthesized quasi-one-dimensional compounds NaMn$_2$O$_4$ and Li$_{0.92}$Mn$_2$O$_4$ are both antiferromagnetic insulators, and here their atomic and magnetic structures were investigated using neutron powder diffraction. The present crystal structural analyses of NaMn2O4 reveal that Mn3+/Mn4+ charge-ordering state exist even at low temperature (down to 1.5 K). It is evident from one of the Mn sites shows a strongly distorted Mn3+ octahedra due to the Jahn-Teller effect. Above TN = 39 K, a two-dimensional short-range correlation is observed, as indicated by an asymmetric diffuse scattering. Below TN, two antiferromagnetic transitions are observed (i) a commensurate long-range Mn3+ spin ordering below 39 K, and (ii) an incommensurate Mn4+ spin ordering below 10 K. The commensurate magnetic structure (kC = 0.5, -0.5, 0.5) follows the magnetic anisotropy of the local easy axes of Mn3+, while the incommensurate one shows a spin-density-wave order with kIC = (0,0,0.216). For Li$_{0.92}$Mn$_2$O$_4$, on the other hand, absence of a long-range spin ordered state down to 1.5 K is confirmed.