Spin susceptibility in the superconducting state of the ferromagnetic superconductor UCoGe

In order to determine the superconducting paring state in the ferromagnetic superconductor UCoGe, the 59Co NMR Knight shift, which is directly related to the microscopic spin susceptibility, was measured in the superconducting state under magnetic fields perpendicular to the spontaneous magnetization axis 59Ka^,b. 59Ka^,b shows to be almost constant, but does not decrease below a superconducting transition. These behaviors as well as the invariance of the internal field at the Co site in the superconducting state exclude the spin-singlet pairing, and can be interpreted with the equal-spin pairing state with a large exchange field along the c axis, which was studied by Mineev.