Hfs of F19 in the Electron Paramagnetic Resonance of MgF2: Co++

Abstract

The complex hyperfine structure of the $X$-band EPR spectra of ${\mathrm{Co}}^{++}$ as a dilute substitutional impurity in a single crystal of Mg${\mathrm{F}}_2$ has been assigned. Where the $x$ and $z$ axes are taken along the bond direction of the two equivalent fluorine ions and along the crystalline $c$ axis, respectively, the values of the spin-Hamiltonian tensors are: for the gyromagnetic ratio (6.0327, 2.2970, 4.2391) Mc/sec; for the cobalt hyperfine interaction, (637.1, 123.3, 210.0) Mc/sec; for the hfs of the two equivalent ${\mathrm{F}}^{19}$ nuclei along the [110] direction, (308, 59, 76) Mc/sec; for the four equivalent ${\mathrm{F}}^{19}$ nuclei in the [110] plane, (92, 76, 162, 49) Mc/sec. In the last case the final component is the off-diagonal $\mathrm{yz}$ contribution. First-order perturbation theory for the ${\mathrm{F}}^{19}$ superhyperfine structure predicts splittings independent of the cobalt nuclear-spin number. However, even though the ${\mathrm{F}}^{19}$ interactions are two orders of magnitude smaller than the Zeeman effect, for some magnetic-field orientations the observed splittings vary as much as 50% across the spectrum. The effect may be traced to fourth-order perturbations (squared in both the Co hyperfine and ${\mathrm{F}}^{19}$ hyperfine interaction) between nearly degenerate states. A set of computer programs to relate EPR spectra to spin-Hamiltonian parameters was essential to the analysis reported.