Multiple Charge States, Hyperfine Interactions, and Relaxation Processes of Fe in ${}_{}{}^{57}{}_{}{}^{}\mathrm{C}\mathrm{o}-\mathrm{D}\mathrm{o}\mathrm{p}\mathrm{e}\mathrm{d}$ MgO

The Mössbauer spectra of cobalt-57-doped magnesium oxide have been studied between 1.3 and 550°K in external magnetic fields of up to 135 kOe for both single-crystal and polycrystalline samples. At room temperature and zero external magnetic field, the velocity spectrum is composed of three single lines, attributed to monovalent, divalent, and trivalent iron. In certain samples, for $T<\mathrm{}14\mathrm{}$°K, the ${\mathrm{Fe}}^{2+}$ line undergoes a transition to a quadrupole doublet with a splitting $\Delta E_Q=0.30\mathrm{}\pm 0.02$ mm/sec, while in certain other samples, annealed differently, no doublet is observed down to 1.5°K. The first case is in quantitative agreement with a random-strain model proposed by Ham. The application of an external magnetic field induces a magnetic hyperfine interaction for the three charge states with saturation hyperfine fields of approximately +20, -120, and -560 kOe for ${\mathrm{Fe}}^{1+}$, ${\mathrm{Fe}}^{2+}$, and ${\mathrm{Fe}}^{3+}$, respectively. From these values the core polarization contribution per spin is estimated to be - 127 kOe/μB. Effects of slow paramagnetic relaxation for the ${\mathrm{Fe}}^{2+}$ spectrum in the presence of an external magnetic field have been observed. In addition to the magnetic hyperfine interaction, an external magnetic field induces an ${\mathrm{Fe}}^{2+}$ quadrupole interaction, as predicted by crystal-field theory. From the value of this interaction, $\Delta E_Q^{}{}_{}{}^{\prime }=0.32\mathrm{}$ mm/sec, the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ nuclear quadrupole moment is estimated to be +0.21 b.