Electron-Spin-Resonance Spectrum of Mn 2+ in β-Ga 2 O 3

The electron-spin-resonance spectrum of ${\mathrm{Mn}}^{2+}$ has been investigated in monoclinic $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ at 24 kMc/sec. It was found that the spectrum consists of a set of lines corresponding to a single type of environment. This spectrum (including the fine and $\ensuremath{\Delta}m=0$ hyperfine structure) is satisfactorily described by the spin Hamiltonian for ${\mathrm{Mn}}^{2+}$ in a rhombic crystal field with the following derived constants: ${A}_{z}=\ensuremath{-}87.7\ifmmode\pm\else\textpm\fi{}0.2$ G; ${A}_{y}=\ensuremath{-}85.6\ifmmode\pm\else\textpm\fi{}0.2$ G; ${g}_{z}=2.002$; ${g}_{y}=2.007$; $D=545.0$ G; $E=124.3$ G. The $y$ crystal-field axis is along the monoclinic axis (b) and the $z$ crystal-field axis makes an angle of 18\ifmmode^\circ\else\textdegree\fi{} with the $c$ crystal axis ($agc$). Forbidden $\ensuremath{\Delta}m=\ifmmode\pm\else\textpm\fi{}1,\ifmmode\pm\else\textpm\fi{}2$ hyperfine transitions were also observed. From the separations between the $\ensuremath{\Delta}m=\ifmmode\pm\else\textpm\fi{}1$ doublets, a value of ${Q}^{\ensuremath{'}}=+0.9\ifmmode\pm\else\textpm\fi{}0.2$ G was obtained. On the basis of the axial crystal fields inferred from the measured values of ${Q}^{\ensuremath{'}}$ in $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ and in other oxide materials, it appears that the calculated crystal-field contribution to the $D$ parameter of ${\mathrm{Mn}}^{2+}$ does not explain the experimental results.