Electron-Nuclear Double Resonance of ${\mathrm{Mn}}^{2+}$ in KMg${\mathrm{F}}_3$ and ${\mathrm{K}}_2$Mg${\mathrm{F}}_4$

The ENDOR spectra of ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}_{}^{2+}{}_{}{}^{}$ in KMg${\mathrm{F}}_3$ and ${\mathrm{K}}_2$Mg${\mathrm{F}}_4$ have been measured at 4.2 °K. Whereas the spectrum in KMg${\mathrm{F}}_3$ could be analyzed in terms of a spin Hamiltonian of cubic symmetry, the spectrum in ${\mathrm{K}}_2$Mg${\mathrm{F}}_4$ shows axial anisotropy and appropriate axial terms had to be added. The principal results are for KMg${\mathrm{F}}_3$, $\frac{A}{\mathrm{hc}}=(-91.\mathrm{}347\pm 0.007)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$; and, for ${\mathrm{K}}_2$Mg${\mathrm{F}}_4$, $\frac{D}{\mathrm{hc}}=(+108\mathrm{}\pm 5)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\frac{A}{\mathrm{hc}}=(-90.\mathrm{}809\pm 0.018)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\frac{B}{\mathrm{hc}}=(-90.\mathrm{}676\pm 0.014)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $\frac{Q^{\prime }}{h}=\frac{3e^2\mathrm{qQ}}{40h}=(+0.\mathrm{}19\pm 0.04)$ MHz. A comparison with other compounds indicates that the anisotropic part of the hyperfine structure mainly results from a second-order process, linear in the axial component of the crystalline field and linear in the dipolar hyperfine coupling. Finally, the importance of the $D$ term for the anisotropy field in antiferromagnetic ${\mathrm{K}}_2$Mn${\mathrm{F}}_4$ is pointed out.