Antiferroacoustic Resonances and Magnetoelastic Coupling in GdAlO3

Two resonances between spin waves and phonons are reported in the antiferromagnetic phase of GdAl${\mathrm{O}}_3$. The stronger of these resonances is the analog to the antiferromagnetic resonance observed by Rohrer and Blazey, and is consistent with free-energy parameters determining their phase diagram of GdAl${\mathrm{O}}_3$. The weaker line is attributed to a two-magnon, one-phonon process. These two resonances are observed with the magnetic field and the acoustic propagation vector along several symmetry axes. From their intensity the magnetoelastic coupling constants $\left|G_{11}\right|=0.7\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and $\left|G_{44}\right|=0.3\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ are deduced. The diagonal elements of the elastic tensor and the corresponding sound velocities are also given.