Paramagnetic Resonance Absorption of Divalent Nickel inα-Al2O3Single Crystal

A paramagnetic resonance absorption spectrum has been observed in single crystals of $\alpha$-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ having nickel oxide added as an impurity. The spectrum is interpreted as being due to transitions between fine structure states of the divalent nickel ion. Best-fit constants to an axially symmetric spin Hamiltonian are obtained from 4.2°K to 300°K. The spectroscopic splitting factors are found to be nearly constant over this temperature range with average values given by $g_{\mathrm{II}}=(2.1957\mathrm{}\pm 0.0013)$ and $g_{\perp }=(2.1859\mathrm{}\pm 0.0013)$. The crystal field splitting energy is, on the other hand, found to be temperature dependent following the approximate law $D=-(1.3287+\alpha T^2)$ ${\mathrm{cm}}^{-1\mathrm{}}$, where $\alpha =5.2\mathrm{}\times {10}^{-7\mathrm{}}$ ${\left(\mathrm{°}\mathrm{K}\right)}^{-2\mathrm{}}$. The average spin-orbit coupling constant over this temperature range is found to be (-285±30) ${\mathrm{cm}}^{-1\mathrm{}}$.