Hyperfine Structure of Divalent and TrivalentFe57in Cobalt Oxide

The hyperfine structure of divalent and trivalent iron in CoO has been obtained from the Mössbauer effect of ${\mathrm{Fe}}^{57}$ produced by the electron-capture decay of ${\mathrm{Co}}^{57}$. The two valence states are obtained respectively by x ray and Auger effect de-excitation of the $K$-shell hole resulting from electron capture in divalent ${\mathrm{Co}}^{57}$. Higher valence states which could be produced by multiple Auger de-excitation are not observed, indicating that their lifetimes are short. The magnetic field at a divalent iron nucleus at low temperature is 2.0×${10}^5$ oe while that at a trivalent one is 5.6×${10}^5$ oe. The quadrupole coupling in this almost cubic environment is less than 1.0 Mc/sec.