Metastable electronic populations and relaxation of Fe(I), Fe(II), and Fe(III) in MgO observed by Mössbauer emission spectroscopy.

Abstract

Magnetic-field M\"ossbauer emission spectra of $^{57}\mathrm{Co}$ in MgO single crystals covering a broad velocity range and measured up to high signal-to-noise ratios are presented. In accordance with a previous study, three charge states of $^{57}\mathrm{Fe}$ are found after $^{57}\mathrm{Co}$(EC${)}^{57}$Fe (EC stands for electron capture). The evaluation of the Fe(III) fraction indicates nonthermalized populations of the $^{6}$${\mathit{A}}_{1}$ ground-state Zeeman levels. The field, temperature, and angular dependences of these populations are evaluated and display qualitative differences to the findings in $^{57}\mathrm{Co}$/${\mathrm{LiNbO}}_{3}$. The implications of the cubic symmetry on the spin-selective ground-state population are considered. In addition, a completely analogous phenomenon is evidenced for the first time within an Fe(II) electronic manifold, namely, the ${\mathrm{\ensuremath{\Gamma}}}_{5\mathit{g}}$ ground state of Fe(II) in MgO, after the nuclear decay. In contrast to the Fe(III) case, these populations are not static within the M\"ossbauer time window. It turns out that the attainment of thermal equilibrium can be conveniently observed by changing the field value, evidencing a direct relaxation process at 4.2 K within ${\mathrm{\ensuremath{\Gamma}}}_{5\mathit{g}}$. The relaxation rates are compatible with static strain data; an initial alignment is observed. Finally, there is strong evidence that the Fe(I) fraction is also populated out of thermal equilibrium. In addition to these ground-state spectra, two features are present that may be attributed to metastable excited states of Fe(II) and Fe(III). It is described in detail how these various contributions can be disentangled.