Spin susceptibility in the superconducting state of the ferromagnetic superconductor UCoGe

Abstract

In order to determine the superconducting paring state in the ferromagnetic superconductor UCoGe, the ${}^{59}$Co 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 ${}^{59}{K}^{a,b}$. ${}^{59}{K}^{a,b}$ 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 [Phys. Rev. B 81, 180504 (2010)].