Sound Absorption in Magnetics
One of the important aspects of the dynamics of magnets is the sound propagation near the critical point. A number of theories have been developed for the case of ultrasonic attenuation at magnetic phase transitions (see for instance the paper by B. Luthi, T.J. Moran and R.J. Pollina  and references quoted there). The spin-phonon interaction, responsible for the critical effects, arises in most cases via the strain modulation of the exchange interaction (volume magnetostrictive coupling). As a result, the attenuation is proportional to the space-time Fourier Laplace transform of a four spin correlation function. This result is the starting point of all further calculations. Most of the present theories, however, consider temperatures above T c, and therefore come down to the calculation of the characteristic decay time, t e, of the spin fluctuations. Using the various expressions for t e, one gets the temperature dependence for the attenuation. As a result one finds for the attenuation coefficient γ ∿ ω2 τ-η (τ = (T - T c)/Tc). The critical changes in sound wave attenuation follow a quadratic frequency dependence, whilst the temperature dependence varies widely for different substances. The variation of the critical exponent can be characterized by the following parameters: spin structure (ferro- or antiferromagnets), degree of magnetic anisotropy of the spin interaction, and range of the exchange interaction.
KeywordsSpin Wave Magnetic Phase Transition Sound Absorption Average Spin Phonon Line
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