¹⁰¹Ru Knight-shift measurement of Sr₂ RuO₄ in low fields applied parallel to the RuO₂ plane

We have measured ¹⁰¹Ru Knight shift (¹⁰¹K) in the spin triplet superconductor Sr₂ RuO₄ in various magnetic fields applied parallel and perpendicular to the RuO₂ plane to determine the spin direction of the Cooper pair. We measured temperature dependence of ¹⁰¹K in 0.15 and 0.055 T parallel to the RuO₂ plane using nuclear-quadrupole-resonance (NQR) technique. We found that ¹⁰¹K is invariant with temperature in both fields on passing through the superconducting (SC) transition temperature. These results indicate that the spin susceptibility parallel to the RuO₂ plane does not change in the field greater than 0.055 T, suggesting that the SC d-vector is along the c-axis when the magnetic field greater than 0.055 T is applied along to the RuO₂ plane. The possible spin state in magnetic fields is discussed on the basis of the Knight-shift measurements so far.