Fully Localized3d-Shell Behavior of Fe Ions in Alkali-Metal Hosts

The local susceptibility and $3d$-spin dynamics of isolated Fe ions recoil implanted into alkali-metal hosts is measured by the perturbed-$\gamma$-ray-distribution method. The observations of extremely large values for the hyperfine fields and magnetic moments and of an extremely small spin linewidth are quantitatively consistent with a stable $3d^6$ configuration of ${\mathrm{Fe}}^{2+}$ ions in spin-orbit coupling along with a negligible crystal-field splitting. Fe in Li strongly deviates from the magnetic behavior of a purely ionic configuration. The results permit a comparison of local-moment formation of $3d$ with $4f$ systems.