First order magnetic transition in single crystal CaFe$_2$As$_2$ detected by $^{75}$As NMR

Abstract

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.

In Fig. 2, we incorrectly assigned AFM lines and quadrupole satellites. Fig.3 and the numbers in the text were corrected accordingly. This mistake has no affect on our main conclusion however