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

An electron-spin-resonance (ESR)/ electron-nuclear double-resonance (ENDOR) investigation of KZnF₃: M (M=Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺) 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 V²⁺, Cr³⁺, Mn²⁺, and Cr¹⁺ in KMgF₃. 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 Mn²⁺ and Fe³⁺, we report ESR and precise ENDOR measurements of the nearest-neighbor fluorine, the ⁵⁵Mn, and the ⁵⁹Co hyperfine interactions. The modified nuclear g factor found for ⁵⁹Co²⁺ is shown to be consistent with theory.