Mössbauer Effect in MgO:Fe+2; Low-Temperature Quadrupole Splitting

Abstract

Mössbauer spectra of absorbers of ${\mathrm{Fe}}^{+2\mathrm{}}$ in MgO have been measured from room temperature (RT) down to liquid-helium temperature. At the higher temperatures, the spectrum consists of a single narrow peak ($\Gamma =0.25\mathrm{}$ mm/sec at RT). Below 20°K, the absorption peak becomes broader and non-Lorentzian, and finally resolves into a close quadrupole doublet, $\Delta E=0.33\mathrm{}$ mm/sec. This splitting arises from the combined effect of random strains which lift the degeneracy of the ground state, and relaxation between the ground levels by an Orbach process through the next higher spin-orbit level. The exponential dependence of the relaxation frequencies derived from the spectra is characterized by a value of $\Delta =93\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. Measurements in a large applied field agree with predictions of the random-strain model.