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 ${\nu }_q$ are measured in an NMR study of a fully magnetized single-crystal sphere of ferromagnetic hcp Co. For $\overrightarrow{\mathrm{M}}$ rotation in a ($\overline{1}2\overline{1}0$) plane it is found that $H_{\mathrm{hf}}=H^0+H^{\prime }\left[\frac{(3\mathrm{}{cos}^2\psi -1)}{2}\right]$, ${\nu }_q={\nu }_q^{\mathrm{rel}}+{{\nu }^{\prime }}_q\left[\frac{(3\mathrm{}{cos}^2\psi -1)}{2}\right]$, where $\psi$ is the angle between $\overrightarrow{\mathrm{M}}$ and the $c$ axis. The numerical values are $H^0=-226\mathrm{}$ kG, $H^{\prime }=5.73\mathrm{}$ kG, ${\nu }_q^{\mathrm{rel}}=24.5\mathrm{}$ kHz and ${{\nu }^{\prime }}_q=196.1\mathrm{}$ 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 ${\nu }_q$, i.e. ${\nu }_q^{\mathrm{rel}}$, 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.