Critical sound propagation in MnF₂

Abstract

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 MnF 2. 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.

Acoustic Signatures of the Phases and Phase Transitions in the Blume Capel Model with Random Crystal Field

“…One can observe the usual 2 dependence of the attenuation in the low-frequency region, where << 1. This result is in accordance with the theoretical and experimental findings for CoF 2 [1] and MnF 2 [51,52]. The ultrasonic attenuation has different frequency dependence in the high-frequency regime at which order parameter can no longer follow sound wave motion: this part of the plot has zero slope; thus, ultrasonic attenuation behaves as ∝ 0 , while >> 1.…”

Effective sound attenuation exponents in magnets

“…For sufficiently low frequencies and very small reduced temperatures the stronger singularity of the exponent zν + α matters in the numerator of Eq. (1) [20]. On the other hand, for higher frequencies the denominator and the saturation effects are important.…”

“…For very low and very high frequencies we have to take into regard also the other terms in Eq. (1) [5,20].…”

Magnetic Field Effect on Sound Propagation in Antiferromagnets

“…Many experimental and theoretical studies have been made on anomalous ultrasonic behaviour near the transition temperature [1,2]. Much of the effort has been devoted to understanding of the effect of the external field on the critical sound propagation [3][4][5].…”