EuTe. III. Ultrasonic Behavior

Ultrasonic attenuation and velocity measurements were carried out on EuTe single crystals at $1.4\le T\le 14$ K and in magnetic fields up to 100 kOe. At $H=0\mathrm{}$, the attenuation for shear waves with $\overrightarrow{\mathrm{q}}\parallel \mathrm{}\left[100\right]\mathrm{}$ increased abruptly as the sample entered the antiferromagnetic phase at $T_N=(9.\mathrm{}6\pm 0.2)$ K. At $T<\mathrm{}{T}_N$ the attenuation of some modes of propagation was higher in the canted phase than in the paramagnetic phase. The canted-to-paramagnetic transition was accompanied by an abrupt decrease and/or by a sharp peak in the attenuation of some modes of propagation. The canted-to-paramagnetic transition field $H_c\mathrm{}\left(T\right)\mathrm{}$ was determined as a function of temperature. At $1.4\le T\le 4.2$ K, $H_c\mathrm{}\left(T\right)\mathrm{}$ followed the theoretical $T^{\frac{3}{2}}$ law. At $T=0\mathrm{}$, $H_c\mathrm{}\left(0\right)\mathrm{}\cong 74$ kOe, which leads to an intersublattice exchange field $H_E\mathrm{}\left(0\right)\mathrm{}\cong 37$ kOe. At liquid-helium temperatures, large changes in the ultrasonic attenuation and velocity occurred in fields of order ${10}^2$ Oe. The elastic constants at 77.6 K, in cgs units, are $c_{11}=(9.\mathrm{}36\pm 0.4)\times {10}^{11}$, $c_{44}=(1.\mathrm{}63\pm 0.07)\times {10}^{11}$, and $c_{12}=(0.\mathrm{}67\pm 0.6)\times {10}^{11}$. These lead to an adiabatic compressibility ${\kappa }_s=(2.\mathrm{}8\pm 0.4)\times {10}^{-12\mathrm{}}$ cgs units, and to a Debye temperature $\Theta =189\mathrm{}$ K.