Ultrasonic Measurements in MnF2near the Néel Temperature

The ultrasonic velocity has been measured for both longitudinal and transverse waves in the [110] and [001] directions in Mn${\mathrm{F}}_2$ near the critical point. A sharp minimum was observed for longitudinal waves. This minimum coincided with an attenuation maximum and occurred at the Néel temperature. The velocity anomaly was frequency-independent for the two frequencies used. No effect was observed for transverse waves. The attenuation was measured over a wide temperature range. The temperature dependence of both critical attenuation and velocity is given by ${(T-T_N)}^{-0.33\mathrm{}}$ for $T>\mathrm{}{T}_N$.