Nonmagnetic Ground State in ${\mathrm{RuO}}_2$ Revealed by Muon Spin Rotation

The magnetic ground state of single crystalline ${\mathrm{RuO}}_2$ was investigated by the muon spin rotation and relaxation ($\mu \mathrm{SR}$) experiment. The spin precession signal due to the spontaneous internal magnetic field $B_{\mathrm{loc}}$, which is expected in the magnetically ordered phase, was not observed in the temperature range 5–400 K. Muon sites were evaluated by first-principles calculations using dilute hydrogen simulating muon as pseudohydrogen, and $B_{\mathrm{loc}}$ was simulated for the antiferromagnetic structures with a Ru magnetic moment $\left|m_{\mathrm{Ru}}\right|\approx 0.05{\mu }_B$ suggested from diffraction experiments. As a result, the possibility was ruled out that muons are localized at sites where $B_{\mathrm{loc}}$ accidentally cancels. Conversely, assuming that the slow relaxation observed in $\mu \mathrm{SR}$ spectra was part of the precession signal, the upper limit for the magnitude of $\left|m_{\mathrm{Ru}}\right|$ was estimated to be $4.8\left(2\right)\times {10}^{-4}{\mu }_B$, which is significantly less than $0.05{\mu }_B$. These results indicate that the antiferromagnetic order, as reported, is unlikely to exist in the bulk crystal.