The magnetic structure of bixbyite a-Mn2O3: a combined density functional theory DFT+U and neutron diffraction study

TL;DR

This study combines density functional theory and neutron diffraction to elucidate the low-temperature magnetic structure of bixbyite Mn2O3, revealing a consistent antiferromagnetic ground state through computational and experimental methods.

Contribution

It introduces a Bayesian cluster-expansion model for predicting magnetic states and demonstrates the use of neutron diffraction with group theory constraints for structure refinement.

Findings

Computational and experimental results agree on the antiferromagnetic ground state.

Optimized synthesis enabled clear neutron diffraction peak splitting.

The magnetic structure involves antiferromagnetic ordering with frustration between MnO6 octahedra.

Abstract

First principles density functional theory DFT+U calculations and experimental neutron diffraction structure analyses were used to determine the low-temperature crystallographic and magnetic structure of bixbyite Mn2O3. The energies of various magnetic arrangements, calculated from first principles, were fit to a cluster-expansion model using a Bayesian method that overcomes a problem of underfitting caused by the limited number of input magnetic configurations. The model was used to predict the lowest-energy magnetic states. Experimental determination of magnetic structure benefited from optimized sample synthesis, which produced crystallite sizes large enough to yield a clear splitting of peaks in the neutron powder diffraction patterns, thereby enabling magnetic-structure refinements under the correct orthorhombic symmetry. The refinements employed group theory to constrain magnetic models. Computational and experimental analyses independently converged to similar ground states, with identical antiferromagnetic ordering along a principal magnetic axis and secondary ordering along a single orthogonal axis, differing only by a phase factor in the modulation patterns. The lowest-energy magnetic states are compromise solutions to frustrated antiferromagnetic interactions between certain corner-sharing MnO6 octahedra.