23Na and 75As NMR study of antiferromagnetism and spin fluctuations in NaFeAs single crystals

We report the Na-23 and As-75 nuclear magnetic resonance (NMR) studies on NaFeAs single crystals. The structure transition temperature T-S (55 K) and the spin density wave (SDW) transition temperature T-SDW (40.5 K) are determined by the NMRline splits. The spin-lattice relaxation rates indicate that the spin fluctuations are strongly enhanced just below T-S and drive a second order SDW transition. A fluctuating feature of the SDW ordering is also seen below the T-SDW. We further performed high-pressure NMR studies on NaFeAs, and found that the T-SDW increases by similar to 7 K and the magnetic moment increases by similar to 30% under 2.5 GPa pressure.