We report 75 As nuclear magnetic resonance studies on an underdoped single-crystal Ba 0.77 K 0.23 Fe 2 As 2 with T c =16.5 K. Below T N =46 K, the NMR peaks for H c split and those for H a shift to higher frequencies, which indicates that an internal magnetic field along the c-axis develops below T N . The spin lattice relaxation rate 1/T 1 measured at the shifted peak with H a which experiences the internal field shows a distinct decrease below T c (µ 0 H=12 T) = 16 K, following a T 3 relation at low temperatures. Our results show unambiguously that antiferromagnetic order and superconductivity coexist microscopically. The unusual superconducting state with the coexisting magnetism is highlighted.

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