First order magnetic transition in single crystal CaFe$_2$As$_2$   detected by $^{75}$As NMR
  We report $^{75}$As Nuclear Magnetic Resonance data in a single crystal of CaFe$_2$As$_2$. The Knight shift, electric field gradient, and spin-lattice relaxation rate are strongly temperature dependent in the paramagnetic state, and change discontinuously at the structural transition temperature, $T_S=T_N=167$ K. Immediately below, the NMR spectra reveal an internal field at the As site associated with the presence of a commensurate magnetic order. These results indicate that the structural and magnetic transitions in CaFe$_2$As$_2$ are first order and strongly coupled, and that the electron density in the FeAs plane is highly sensitive to the out-of-plane structure. 
S.-H. Baek, N. J. Curro, T. Klimczuk, E. D. Bauer, F. Ronning, and J.   D. Thompson