Mössbauer Effect of DivalentFe57in NiO and MnO

Doppler spectra of ${\mathrm{Fe}}^{++}$ in NiO and MnO have been observed over the temperature range from 4.2°K to above the Néel temperature $T_N$. A single-valued magnetic hyperfine interaction is observed over the full range when the ${\mathrm{Co}}^{57}$ is diffused into these materials under controlled atmospheres. An electric-field gradient which appears at the ${\mathrm{Fe}}^{57}$ nucleus below $T_N$ grows as $T$ decreases. The internal magnetic field of ${\mathrm{Fe}}^{++}$ in MnO peaks at 90°K and 170 kG, then decreases to 97 kG at 4.2°K. The internal magnetic field of ${\mathrm{Fe}}^{++}$ in NiO peaks at about 300°K and 216 kG, then decreases to about 55 kG at 4.2°K. The temperature dependences of the fields are interpreted in terms of the crystal-field effects on a $3d^6$ ion. The direction of the spins of ${\mathrm{Ni}}^{++}$ in NiO was found to be close to a $〈112〉$ direction. Fixed-absorber measurements gave $T_N=524\mathrm{}$°K for NiO and $T_N=116.9\mathrm{}$°K for MnO. The magnetic transition of MnO appears to be near first order, but no temperature hysteresis was observed in the transition.