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

TL;DR

This study uses $^{75}$As NMR to investigate CaFe$_2$As$_2$, revealing a first-order coupled structural and magnetic transition at 167 K with significant changes in electronic properties.

Contribution

It provides direct NMR evidence that the structural and magnetic transitions in CaFe$_2$As$_2$ are first order and strongly coupled, highlighting the sensitivity of electron density to structural changes.

Findings

Discontinuous changes in NMR parameters at 167 K

Presence of internal magnetic field below transition

Structural and magnetic transitions are first order

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.