Coupling of Elastic Waves to the "Nonactivated" Spin-Flop Mode in MnF2

The coupling of the nonactivated spin-flop mode in Mn${\mathrm{F}}_2$ to two elastic modes via the linear magnetoelastic interaction is shown to lead to discontinuities at the spin-flop transition in the elastic constants $c_{11}$ and $c^{\prime }=\frac{1}{2}(c_{11}-c_{12})$. The directions of easy magnetization in the basal plane are determined to be the equivalent [110] axes.