We have observed acoustically a spin-resonance line that we believe is due to the ground state of ${\mathrm{Al}}_2$${\mathrm{O}}_3$: ${\mathrm{Fe}}^{2+}$. This spin system has not previously been detected by resonance techniques. The transition has a $g_{\mathrm{II}}$ of 3.40±0.04, and its variation with magnetic field direction suggests that a singlet level is also present, at an energy, when the field is parallel to the $c$ axis, of not less than 6 ${\mathrm{cm}}^{-1\mathrm{}}$ above the ground doublet. From the intensity of the line, it was estimated that the magneto-elastic coefficient ${[\frac{1}{4}{(G_{11}-G_{12})}^2+{G_{16}}^2]}^{\frac{1}{2}}$ has a value of not less than 15 ${\mathrm{cm}}^{-1\mathrm{}}$ ${\mathrm{}\left(\mathrm{unit}\mathrm{strain}\right)\mathrm{}}^{-1\mathrm{}}$. These results are consistent with recent calculations by Stevens and Walsh.

• Received 25 March 1969

DOI:https://doi.org/10.1103/PhysRev.185.546