Critical sound propagation in MNF2

We analyse the critical sound attenuation in the antiferromagnet MnF2 above its Neel temperature. A general formula for the acoustic self-energy, derived in the model in which a sound mode is coupled to both the order-parameter fluctuations as well as to the energy mode, is applied to interpret the experimental data in MnF2. It has been shown that a very interesting competition between three asymptotic singularities is responsible for the complex behaviour of the sound attenuation coefficient in the high-temperature phase. The relative strength of these terms depends also on the ultrasonic frequency. For high frequency sound propagation also a background attenuation starts playing an important role.