Temperature-Dependent Lifetimes of NonequilibriumFe3+Ions in CoO from the Mössbauer Effect

The lifetime of the nonequilibrium ${\mathrm{Fe}}^{3+}$ state in a CoO:${\mathrm{Co}}^{57}$ source has been determined in the temperature range 78 to 1000°K. The intensity of the ferrous state is highly temperature-dependent and was hardly detectable above 800°K. The width of the nuclear excited level of the nonequilibrium state is determined by the nuclear decay time $\tau$ and the atomic decay time $\theta \mathrm{}\left(3\right)\mathrm{}$. The expected changes in the linewidth of the nonequilibrium ${\mathrm{Fe}}^{3+}$ state in the Mössbauer spectra have been observed. The observed deviation of the temperature variation of the hf magnetic field at the nuclei of both ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ions from that expected by the molecular-field theory may be due to a possible biquadratic exchange interaction in CoO, with $\frac{j}{J_2}\simeq 0.022$.