NMR evidence of a high field phase transition in $\kappa $-(ET)Cu(NCS)$_{2}$

The anisotropic electronic properties of the organic superconductor $\kappa $-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$ make it an excellent candidate for inhomogeneous superconductivity (FFLO state) when the magnetic field is applied in-plane, so as to avoid orbital suppression of the superconductivity. Results of various previous measurements have been interpreted as supporting the existence of an FFLO state for fields B$\sim $21-22T. In this work, $^{13}$C NMR measurements were used to probe the hyperfine fields in the range of B=14-27T and T=0.5-1.6K. A sharp peak in the relaxation rate 1/T1(B) at B$\sim $22T signals a phase transition associated with a turn-on of spin susceptibility, while evolution the spectrum with field indicate the system remains superconducting at higher fields. Additionally, only small variations of the hyperfine fields are observed for external fields B=14-20T, and we discuss this observation in relation to the superconducting order parameter symmetry.