Spin Relaxation ofFe1+in MgO via Excited Electronic States

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 [$\Delta H\left(T\right)-\Delta H\left(0\right)\mathrm{}$] of the Lorentzian EPR linewidth at 23 K, the absolute scale for $\frac{1}{T_1}$ from linewidth was determined to be (4.7±0.6)×${10}^7$ ${\sec }^{-1\mathrm{}}$/Oe. The data from 10 to 37 K are accurately described over seven decades of time by $\frac{1}{T_1}=B{T}^9+\frac{C^{\prime }}{(e^{\frac{{\Delta }^{\prime }}{T}}-1)}+\frac{C^{\prime \prime }}{(e^{\frac{{\Delta }^{\prime \prime }}{T}}-1)}$, where $B=(1.2\mathrm{}\pm 0.1)\times {10}^{-7\mathrm{}}/sec{\mathrm{K}}^9$, $C^{\prime }=(2.2\mathrm{}\pm 0.4)\times {10}^{10}/sec$, $C^{\prime \prime }=(2.7\mathrm{}\pm 2.3)\times {10}^{13}/sec$, ${\Delta }^{\prime }=213\mathrm{}\pm 4$ K, and ${\Delta }^{\prime \prime }=450\mathrm{}\pm 30$ 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 ∼150 and ∼310 ${\mathrm{cm}}^{-1\mathrm{}}$. Comparison of the excited-state energies with predictions from the crystal-field model for $3d^7$ ions in cubic MgO indicates a large crystal-field strength parameter or covalent bonding with $\frac{\lambda }{{\lambda }_0}=0.57\mathrm{}$, the orbital reduction factor as low as 0.65, and some evidence for inadequacies in the model.