Itinerant Antiferromagnetism in ${\mathrm{RuO}}_2$

Bulk rutile ${\mathrm{RuO}}_2$ has long been considered a Pauli paramagnet. Here we report that ${\mathrm{RuO}}_2$ 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 $U$ ($\mathrm{DFT}+U$) 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 Néel temperature and small itinerant moments make ${\mathrm{RuO}}_2$ unique among ruthenate compounds and among oxide materials in general.

Figure 1

(a) Total energy per Ru in meV, Ru magnetic moments within the rutile unit cell in ${\mu }_B$, band gap in eV, and density of states at the Fermi energy per eV per Ru, of the antiferromagnetic (AFM), the nonmagnetic (NM), and the ferromagnetic (FM) configurations calculated with $\mathrm{PBE}+U$. (b) Atomic and antiferromagnetic structure of bulk rutile ${\mathrm{RuO}}_2$ as predicted by DFT and confirmed by neutron diffraction.

Figure 2

(a) X-ray and (b) unpolarized neutron diffraction data taken at 295 K at the HB-3A instrument at the High Flux Isotope Reactor in Oak Ridge National Laboratory (HFIR/ORNL). (c) Temperature evolution of the integrated intensity of the nuclear (200) (left) and magnetic (100) (right) peak measured at the HB-3A and HB-3 instruments at HFIR/ORNL. (d) Magnetic susceptibility of different multigrain ${\mathrm{RuO}}_2$ samples as a function of temperature.

Figure 3

(a) Lindhard response function ${\chi }_0[q]$ in ${\mathrm{eV}}^{-1}$. (b) Fermi surface. (c) Momentum resolved contribution to ${\chi }_0[q=(2\pi ,0,0)]$. (d) Band structure at $k_y=0$ and $k_z=0.5\pi$ along the $k_x$ direction. Black arrow in various panels indicates one of eight nested hot spots.