NMR Determination of an Incommensurate Helical Antiferromagnetic Structure in EuCo2As2

Abstract

We report ${}^{153}$Eu, ${}^{75}$As and ${}^{59}$Co nuclear magnetic resonance (NMR) results on EuCo${}_2$As${}_2$ single crystal. Observations of ${}^{153}$Eu and ${}^{75}$As NMR spectra in zero magnetic field at 4.3 K below an antiferromagnetic (AFM) ordering temperature $T_{\mathrm{N}}$ = 45 K and its external magnetic field dependence clearly evidence an incommensurate helical AFM structure in EuCo${}_2$As${}_2$. Furthermore, based on ${}^{59}$Co NMR data in both the paramagnetic and the incommensurate AFM states, we have determined the model-independent value of the AFM propagation vector $\mathbf{k}$ = (0, 0, 0.73 $\pm$ 0.07)2$\pi$/$c$ where $c$ is the $c$ lattice parameter. Thus the incommensurate helical AFM state was characterized by only NMR data with model-independent analyses, showing NMR to be a unique tool for determination of the spin structure in incommensurate helical AFMs.

6 pages, 4 figures, accepted for publication in Phys.Rev. B

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