Itinerant antiferromagnetism in RuO$_{2}$*

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\'{e}el temperature and small itinerant moments make RuO$_{2}$ unique among ruthenate compounds and among oxide materials in general.