First-order magnetic transition in single-crystalline CaFe2As2 detected by 75As nuclear magnetic resonance

We report As-75 nuclear magnetic resonance (NMR) data in a single crystal of CaFe2As2. 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 CaFe2As2 are first order and strongly coupled, and that the electron density in the FeAs plane is highly sensitive to the out-of-plane structure.