Low-temperature magnetic ordering and structural distortions in Vanadium Sesquioxide (V$_2$O$_3$)

TL;DR

This paper investigates the magnetic and structural properties of V$_2$O$_3$, demonstrating that electronic structure calculations can accurately describe its magnetic order, structural distortions, and the role of specific orbitals in its insulating phase.

Contribution

The study provides a comprehensive explanation of V$_2$O$_3$'s magnetic order and structural distortions using GGA and GGA+U calculations, aligning with DMFT results and highlighting the role of the $a_{1g}$ orbital.

Findings

Magnetic order involves one ferromagnetic and two antiferromagnetic bonds.

Structural distortion is monoclinic and accompanies the magnetic transition.

The $a_{1g}$ orbital causes magnetic frustration, influencing the magnetic structure.

Abstract

Vanadium Sesquioxide (V$_2$O$_3$) is an antiferromagnetic insulator below $T_{\mathrm N}\approx$ 155 K. The magnetic order is not of C- or G-type as one would infer from the bipartite character of the hexagonal basal plane in the high-temperature corundum structure. In fact, the N\'eel transition is accompanied by a monoclinic distortion that makes one bond of the honeycomb plane inequivalent from the other two, thus justifying a magnetic structure with one ferromagnetic bond and two antiferromagnetic ones. We show here that the magnetic ordering, the accompanying monoclinic structural distortion, the magnetic anisotropy and also the recently discovered high-pressure monoclinic phase, can all be accurately described by conventional electronic structure calculations within GGA and GGA+U. Our results are in line with DMFT calculations for the paramagnetic phase, which predict that the insulating character is driven by a correlation-enhanced crystal field splitting between $e^\pi_g$ and $a_{1g}$ orbitals that pushes the latter above the chemical potential. We find that the $a_{1g}$ orbital, although almost empty in the insulating phase, is actually responsible for the unusual magnetic order as it leads to magnetic frustration whose effect is similar to a next-nearest-neighbor exchange in a Heisenberg model on a honeycomb lattice.