In spin-triplet superconductors, exotic properties such as multiple superconducting (SC) phases are expected due to the additional spin degrees of freedom of Cooper pairs. The uranium-based superconductor UTe₂, discovered in 2018, is considered a strong candidate for spin-triplet superconductivity because it exhibits multiple SC phases under magnetic fields and pressure. Hence, both theoretical and experimental studies have been intensively performed in recent years.

Among them, nuclear magnetic resonance (NMR) measurement is one of the powerful techniques that can microscopically probe the spin susceptibility in the SC state.

From our previous NMR studies, it is revealed that spin susceptibility decreases below Tc down to 1.5 K along all crystallographic directions. As the reduction along the magnetic easy axis (a axis) is more than an order of magnitude larger than those along other directions, it is crucial to clarify the behavior of Knight shift along the a axis down to well below T_c to understand the SC spin state of UTe₂. Furthermore, spins are expected to polarize along the applied field in spin-triplet superconductivity. Therefore, the spin response to the field along magnetic easy a axis is also of great importance.