Spin Relaxation ofFe1+in MgO via Excited Electronic States

Abstract

The spin-lattice relaxation time ${T}_{1}$ in the ground-state Kramers doublet of ${\mathrm{Fe}}^{1+}$ in MgO has been measured in a fast travelling-wave-tube X-band spectrometer by pulsed saturation from 10 to 23 K and by line broadening from 23 to 37 K. By simultaneously measuring recovery from pulsed saturation and the relaxation-induced component [$\ensuremath{\Delta}H(T)\ensuremath{-}\ensuremath{\Delta}H(0)$] of the Lorentzian EPR linewidth at 23 K, the absolute scale for $\frac{1}{{T}_{1}}$ from linewidth was determined to be (4.7\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}${10}^{7}$ ${\mathrm{sec}}^{\ensuremath{-}1}$/Oe. The data from 10 to 37 K are accurately described over seven decades of time by $\frac{1}{{T}_{1}}=B{T}^{9}+\frac{{C}^{\ensuremath{'}}}{({e}^{\frac{{\ensuremath{\Delta}}^{\ensuremath{'}}}{T}}\ensuremath{-}1)}+\frac{{C}^{\ensuremath{'}\ensuremath{'}}}{({e}^{\frac{{\ensuremath{\Delta}}^{\ensuremath{'}\ensuremath{'}}}{T}}\ensuremath{-}1)}$, where $B=(1.2\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}/sec{\mathrm{K}}^{9}$, ${C}^{\ensuremath{'}}=(2.2\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{10}/sec$, ${C}^{\ensuremath{'}\ensuremath{'}}=(2.7\ifmmode\pm\else\textpm\fi{}2.3)\ifmmode\times\else\texttimes\fi{}{10}^{13}/sec$, ${\ensuremath{\Delta}}^{\ensuremath{'}}=213\ifmmode\pm\else\textpm\fi{}4$ K, and ${\ensuremath{\Delta}}^{\ensuremath{'}\ensuremath{'}}=450\ifmmode\pm\else\textpm\fi{}30$ K. The temperature dependence is consistent with the interpretation that spin-lattice relaxation in the ground-state Kramers doublet of MgO: ${\mathrm{Fe}}^{1+}$ occurs predominantly by resonant phonon scattering involving excited electronic states at \ensuremath{\sim}150 and \ensuremath{\sim}310 ${\mathrm{cm}}^{\ensuremath{-}1}$. Comparison of the excited-state energies with predictions from the crystal-field model for $3{d}^{7}$ ions in cubic MgO indicates a large crystal-field strength parameter or covalent bonding with $\frac{\ensuremath{\lambda}}{{\ensuremath{\lambda}}_{0}}=0.57$, the orbital reduction factor as low as 0.65, and some evidence for inadequacies in the model.