Orbital degeneracy removed by charge order in triangular antiferromagnet AgNiO2

TL;DR

This study reveals how charge order lifts orbital degeneracy in AgNiO2, leading to a unique magnetic structure and providing insights into charge-driven mechanisms in triangular antiferromagnets.

Contribution

It demonstrates charge order as an alternative to Jahn-Teller distortions in lifting orbital degeneracy in AgNiO2, supported by neutron diffraction and first-principles calculations.

Findings

Charge order induces structural transition and lifts orbital degeneracy.

Unique magnetic ground state with ferromagnetic rows on a triangular lattice.

First-principles calculations explain the origin of charge and magnetic phenomena.

Abstract

We report a high-resolution neutron diffraction study on the orbitally-degenerate spin-1/2 hexagonal antiferromagnet AgNiO2. A structural transition to a tripled unit cell with expanded and contracted NiO6 octahedra indicates root(3) x root(3) charge order on the Ni triangular lattice. This suggests charge order as a possible mechanism of lifting the orbital degeneracy in the presence of charge fluctuations, as an alternative to Jahn-Teller distortions. A novel magnetic ground state is observed at base temperatures with the electron-rich S = 1 Ni sites arranged in alternating ferromagnetic rows on a triangular lattice, surrounded by a honeycomb network of non-magnetic and metallic Ni ions. We also report first-principles band-structure calculations that explain microscopically the origin of these phenomena.