Experimental Evidence for a Large Collinear Electric Field Gradient of Ir in hcp-Co

Measurements of quadrupole-interaction-resolved NMR on oriented nuclei were performed on ${}^{188}\mathrm{Ir}$ in a hcp-Co single crystal. The magnetic and electric hyperfine interaction frequencies were determined as function of the angle $\theta$ between the electronic magnetization $\overrightarrow{M}$ and the single crystal $c$ axis. A large collinear electric field gradient (CEFG) (principal axis parallel $\overrightarrow{M}$) was found, ${\mathrm{eq}}^{\left(c\right)}=-0.378\mathrm{}\left(7\right)\times {10}^{17}\mathrm{V}/{\mathrm{cm}}^2$. Within the series of the $5d$ elements Ir, Pt, Au, and Hg this is exceptional: The experimental CEFG's of Pt, Au, and Hg in hcp-Co are at least $\sim 2$ orders of magnitude smaller and consistent with ${\mathrm{eq}}^{\left(c\right)}=0\mathrm{}$.