Nonmagnetic Ground State in RuO₂ Revealed by Muon Spin Rotation

The magnetic ground state of single crystalline RuO₂ was investigated by the muon spin rotation/relaxation (μSR) experiment. The spin precession signal due to the spontaneous internal magnetic field Bloc, 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 pseudo-hydrogen, and Bloc was simulated for the antiferromagnetic (AFM) structures with a Ru magnetic moment | mRu|≈0.05μB suggested from diffraction experiments. As a result, the possibility was ruled out that muons are localized at sites where Bloc accidentally cancels. Conversely, assuming that the slow relaxation observed in μSR spectra was part of the precession signal, the upper limit for the magnitude of | mRu| was estimated to be 4.8(2)×10⁻⁴μB, which is significantly less than 0.05μB. These results indicate that the AFM order, as reported, is unlikely to exist in the bulk crystal.