Anisotropic hyperfine interactions in ferromagnetic hcp Co

The anisotropy of the magnetic hyperfine field ${H}_{\mathrm{hf}}$, and the angular dependence of the quadrupole splitting ${\ensuremath{\nu}}_{q}$ are measured in an NMR study of a fully magnetized single-crystal sphere of ferromagnetic hcp Co. For $\stackrel{\ensuremath{\rightarrow}}{\mathrm{M}}$ rotation in a ($\overline{1}2\overline{1}0$) plane it is found that ${H}_{\mathrm{hf}}={H}^{0}+{H}^{\ensuremath{'}}[\frac{(3{cos}^{2}\ensuremath{\psi}\ensuremath{-}1)}{2}]$, ${\ensuremath{\nu}}_{q}={\ensuremath{\nu}}_{q}^{\mathrm{rel}}+{{\ensuremath{\nu}}^{\ensuremath{'}}}_{q}[\frac{(3{cos}^{2}\ensuremath{\psi}\ensuremath{-}1)}{2}]$, where $\ensuremath{\psi}$ is the angle between $\stackrel{\ensuremath{\rightarrow}}{\mathrm{M}}$ and the $c$ axis. The numerical values are ${H}^{0}=\ensuremath{-}226$ kG, ${H}^{\ensuremath{'}}=5.73$ kG, ${\ensuremath{\nu}}_{q}^{\mathrm{rel}}=24.5$ kHz and ${{\ensuremath{\nu}}^{\ensuremath{'}}}_{q}=196.1$ kHz. A phenomenological interpretation is given and the data are used to examine some aspects of the band model of hcp Co. It is shown that the isotropic part of ${\ensuremath{\nu}}_{q}$, i.e. ${\ensuremath{\nu}}_{q}^{\mathrm{rel}}$, can be ascribed to the relativistic contribution of the $d$ band to the electronic-field gradient at the $^{59}\mathrm{Co}$ site. A model in which the orthogonal-plane-wave band calculations concerning the magnetic hyperfine anisotropy can be confronted with the experimental observation is presented.