We report the ${}^{23}\mathrm{Na}$ and ${}^{75}\mathrm{As}$ 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 $T_{\mathrm{SDW}}$ ($40.5$ K) are determined by the NMR line 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 $T_{\mathrm{SDW}}$. We further performed high-pressure NMR studies on NaFeAs, and found that the $T_{\mathrm{SDW}}$ increases by $~$7 K and the magnetic moment increases by $~$$30\%$ under 2.5 GPa pressure.

• Received 6 December 2010

DOI:https://doi.org/10.1103/PhysRevB.83.132501