Itinerant Antiferromagnetism in RuO$_{2}$

TL;DR

This study reveals that bulk RuO$_2$, previously considered a paramagnet, exhibits lattice distortion and antiferromagnetic order at high temperatures, driven by Fermi surface instability, challenging prior assumptions about its magnetic properties.

Contribution

The paper uncovers hidden antiferromagnetism in RuO$_2$ associated with a lattice distortion, supported by experimental neutron diffraction and DFT+$U$ calculations, revealing a new magnetic state.

Findings

RuO$_2$ shows lattice distortion below 900 K.

Antiferromagnetic order persists up to at least 300 K.

Small magnetic moments (~0.05 μ_B) are observed at room temperature.

Abstract

Bulk rutile RuO$_2$ has long been considered a Pauli paramagnet. Here we report that RuO$_2$ exhibits a hitherto undetected lattice distortion below approximately 900 K. The distortion is accompanied by antiferromagnetic order up to at least 300 K with a small room temperature magnetic moment of approximately 0.05 $\mu_B$ as evidenced by polarized neutron diffraction. Density functional theory plus $U$ (DFT+$U$) calculations indicate that antiferromagnetism is favored even for small values of the Hubbard $U$ of the order of 1 eV. The antiferromagnetism may be traced to a Fermi surface instability, lifting the band degeneracy imposed by the rutile crystal field. The combination of high N\'eel temperature and small itinerant moments make RuO$_2$ unique among ruthenate compounds and among oxide materials in general.