Effect of Uniaxial Stresses on the Paramagnetic Spectra ofMn3+andFe3+in MgO

We have investigated the lowest energy levels of the paramagnetic ions ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ in the host crystal MgO, as a function of externally applied uniaxial stress. The effect of the stress on the energy levels of the paramagnetic ion can be described by introducing into the spin Hamiltonian an additional term of the form $\mathrm{S}·D·\mathrm{S}$. When the components of $D$ are expressed as linear functions of the applied stress components, there are only two independent constants of proportionality in our simple case of a cubic crystalline lattice. We have experimentally determined these two constants, i.e., spin-lattice coefficients $C_{11}$ and $C_{44}$, for ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ in MgO, and find them to be, in units of ${10}^{-13\mathrm{}}$ cm/dyne: for ${\mathrm{Mn}}^{2+}$, $C_{11}=(+7.1\mathrm{}\pm 3)\%$ and $C_{44}=(-2.1\mathrm{}\pm 3)\%$; for ${\mathrm{Fe}}^{3+}$, $C_{11}=(+26\mathrm{}\pm 3)\%$ and $C_{44}=(-5.5\mathrm{}\pm 5)\%$. We have also investigated the absolute value and the angular dependence of the linewidth of the fine structure of ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ in MgO. We can account for these widths in terms of the spin-lattice coefficients, by assuming a random distribution of internal stresses in the host sample. We also indicate the relevance of the spin-lattice coefficients in the determination of the direct spin-lattice relaxation times.