Postprint version. Published in Physics Letters A, Volume 25, Issue 2, July 31, 1967, pages 149-150.
NOTE: At the time of publication, the author Richard B. Frankel was not yet affiliated with Cal Poly.
The definitive version is available at https://doi.org/10.1016/0375-9601(67)90394-5.
Hyperfine interactions have been measured for Fe1+ and Fe2+ in MgO and CaO. The core polarization fields per spin moment are −127 kOeμB for both Fe1+ and Fe2+.