Ferromagnetic quantum critical fluctuations and anomalous coexistence of ferromagnetism and superconductivity in UCoGe revealed by Co-NMR and NQR studies

Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies were carried out for the recently discovered UCoGe, in which the ferromagnetic and superconducting (SC) transitions are reported to occur at T-Curie similar to 3 K and T-S similar to 0.8 K [Huy et al.: Phys. Rev. Lett. 99 (2007) 067006], in order to investigate the coexistence of ferromagnetism and superconductivity as well as the normal-state and SC properties from a microscopic point of view. From the nuclear spin-lattice relaxation rate 1/T-1 and Knight-shift measurements, we confirm that ferromagnetic fluctuations that possess a quantum critical character are present above T-Curie and also the occurrence of a ferromagnetic transition at 2.5 K in our polycrystalline sample. The magnetic fluctuations in the normal state show that UCoGe is an itinerant ferromagnet similar to ZrZn2 and YCO2. The onset SC transition is identified at T-S similar to 0.7 K, below which 1/T-1 arising from 30% of the volume fraction starts to decrease due to the opening of the SC gap. This component of 1/T-1 which follows a T-3 dependence in the temperature range 0.3-0.1 K, coexists with the magnetic components of 1/T-1 showing root T dependence below T-S. From the NQR measurements in the SC state, we suggest that the self-induced vortex state is realized in UCoGe.