Abstract
A paramagnetic resonance absorption spectrum has been observed in single crystals of $\ensuremath{\alpha}$-${\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\ifmmode^\circ\else\textdegree\fi{}K to 300\ifmmode^\circ\else\textdegree\fi{}K. The spectroscopic splitting factors are found to be nearly constant over this temperature range with average values given by ${g}_{\mathrm{II}}=(2.1957\ifmmode\pm\else\textpm\fi{}0.0013)$ and ${g}_{\ensuremath{\perp}}=(2.1859\ifmmode\pm\else\textpm\fi{}0.0013)$. The crystal field splitting energy is, on the other hand, found to be temperature dependent following the approximate law $D=\ensuremath{-}(1.3287+\ensuremath{\alpha}{T}^{2})$ ${\mathrm{cm}}^{\ensuremath{-}1}$, where $\ensuremath{\alpha}=5.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ ${(\mathrm{\ifmmode^\circ\else\textdegree\fi{}}\mathrm{K})}^{\ensuremath{-}2}$. The average spin-orbit coupling constant over this temperature range is found to be (-285\ifmmode\pm\else\textpm\fi{}30) ${\mathrm{cm}}^{\ensuremath{-}1}$.
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