Magnetic Properties of Iron Ions in CoO(I) and CoO(II)

Abstract

Hyperfine M\"ossbauer spectra of ${\mathrm{Fe}}^{57}$ in cobaltous oxide in the pure and vacated forms are measured below the N\'eel temperatures. The hyperfine spectra of CoO(I) are of a pure ${\mathrm{Fe}}^{2+}$ type and give a pattern characteristic of magnetic and quadrupole hyperfine interactions. They are analyzed by diagonalizing a 4\ifmmode\times\else\texttimes\fi{}4 magnetic and quadrupole interaction matrix of the first excited state of an ${\mathrm{Fe}}^{57}$ nucleus and fitting eight Lorentzians to the M\"ossbauer spectra. In connection with this analysis we have developed a general formalism which can give $H$, $\frac{1}{2}({e}^{2}qQ)$, $\ensuremath{\varphi}$, $\ensuremath{\theta}$, and $\ensuremath{\eta}$ from M\"ossbauer data by using an IBM computer. Below 200\ifmmode^\circ\else\textdegree\fi{}K, the magnetic hyperfine field at ${\mathrm{Fe}}^{57}$ nuclei in CoO(I) decreases with decreasing temperature, and has an internal field of magnitude 120 kG as its zero-point value. The quadrupole interaction increases with decreasing temperature, and the zero-point value of $\frac{1}{2}{e}^{2}\mathrm{qQ}$ is - 1.32 mm/sec for CoO(I). The tendency of the magnetic hyperfine field to decrease and the tendency of the quadrupole interaction to increase with decreasing temperature, as well as the temperature dependence of $\ensuremath{\theta}$, $\ensuremath{\varphi}$, and $\ensuremath{\eta}$, can be explained by crystal-field theory in terms of temperature-dependent variations in the statistical population of electronic levels. The hyperfine M\"ossbauer spectra of CoO(II) below its N\'eel temperature consist of six lines, which are purely magnetic except for line broadening due to possible quadrupole interactions arising from defects which destroy long-range translational symmetry in this material. The zero-point value of the magnetic hyperfine field at ${\mathrm{Fe}}^{57}$ nuclei in CoO(II) is measured to be 553 kG. The temperature dependence of the magnetic hyperfine field for CoO(II) follows a Brillouin function for $S=\frac{3}{2}$ approximately, and its N\'eel temperature is measured to be 270\ifmmode\pm\else\textpm\fi{}10\ifmmode^\circ\else\textdegree\fi{}K. The M\"ossbauer hyperfine spectra of the mixed form CoO(I, II) was also determined, and it is discussed in the framework of the model proposed in the previous paper. The magnetic hyperfine field and quadrupole coupling constant for the ${\mathrm{Fe}}^{2+}$ component of the M\"ossbauer spectra are found to be 150 kG and -1.01 mm/sec at 77\ifmmode^\circ\else\textdegree\fi{}K, respectively, which are not in accord with the values previously reported by Wertheim.