First- and Second-Shell Hyperfine Interactions in Iron-Group-Doped Perovskite Fluorides

An electron-spin-resonance (ESR)/ electron-nuclear double-resonance (ENDOR) investigation of KZn${\mathrm{F}}_3$: $M (M={\mathrm{Mn}}^{2+}, {\mathrm{Fe}}^{3+}, {\mathrm{Co}}^{2+}, {\mathrm{Ni}}^{2+})$ has been carried out with a view toward obtaining an improved understanding of the second-neighbor fluorine (F2) hyperfine interaction in perovskite hosts. The isotropic part of the F2 hyperfine interaction is calculated using the "overlap" model of Davies and improved fluorine-ion wave functions. The results are compared with both the present experiment and recently published data for ${\mathrm{V}}^{2+}$, ${\mathrm{Cr}}^{3+}$, ${\mathrm{Mn}}^{2+}$, and ${\mathrm{Cr}}^{1+}$ in KMg${\mathrm{F}}_3$. This comparison indicates that the overlap model by itself is inadequate and that covalency effects are of comparable importance for the F2 ions. In addition to the F2 results for ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Fe}}^{3+}$, we report ESR and precise ENDOR measurements of the nearest-neighbor fluorine, the ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}$, and the ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}$ hyperfine interactions. The modified nuclear $g$ factor found for ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}_{}^{2+}{}_{}{}^{}$ is shown to be consistent with theory.