Hfs of F19 in the electron paramagnetic resonance of MgF2: Co++

The complex hyperfine structure of the X-band EPR spectra of Co++ as a dilute substitutional impurity in a single crystal of MgF2 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 F19 nuclei along the [110] direction, (308, 59, 76) Mc/sec; for the four equivalent F19 nuclei in the [110] plane, (92, 76, 162, 49) Mc/sec. In the last case the final component is the off-diagonal yz contribution. First-order perturbation theory for the F19 superhyperfine structure predicts splittings independent of the cobalt nuclear-spin number. However, even though the F19 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 F19 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. © 1966 The American Physical Society.