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 ¹⁸⁸Ir in a hcp-Co single crystal. The magnetic and electric hyperfine interaction frequencies were determined as function of the angle θ between the electronic magnetization M⃗ and the single crystal c axis. A large collinear electric field gradient (CEFG) (principal axis parallel M⃗) was found, eq^(c) = -0.378(7)×10¹⁷V/cm². 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 ∼2 orders of magnitude smaller and consistent with eq^(c) = 0.