Paramagnetic Resonance of TrivalentFe57in Zinc Oxide

Tetrahedrally-coordinated trivalent iron has been found to be a common impurity in flux-grown crystals of zinc oxide. The room temperature paramagnetic resonance spectrum of isolated ions may be described by a conventional axial spin Hamiltonian with parameters: $g=2.0060\mathrm{}\pm 0.0005$, $D=-(594\mathrm{}\pm 1)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $F=(4\mathrm{}\pm 5)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $a=(39\mathrm{}\pm 5)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. Lines due to iron ions with nearby charge compensation (${\mathrm{Li}}^{+}$ ions) have also been observed. The magnitude of the cubic field parameter ($a$) is discussed. The hyperfine coupling constant of ${\mathrm{Fe}}^{57}$ in an enriched sample is $\mathrm{}\left|A\right|=(9.02\mathrm{}\pm 0.2)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ which is of interest in connection with Robert's nuclear resonance measurements of sublattice magnetizations in yttrium-iron garnet. It appears likely that both sublattices are aligned within 1% at very low temperatures.