Abstract
Ultrasonic attenuation in an insulating simple cubic Heisenberg antiferromagnet near the magnetic phase transition point is investigated on the basis of Mori's theory of irreversible processes. The sound wave is assumed to couple mainly to the spin-energy-density fluctuation which decays via a fast non-diffusive relaxation process arising from the spin-lattice interaction. Firstly, the relaxation time of the spin-energy-density fluctuation, τ, and then the sound attenuation coefficient, α q , is calculated. The calculations of τ and α q applied to RbMnF 3 agree quantitatively with the experimental results. The contribution of the longitudinal thermal-phonons to 1/τ is found to be 1/7 of that of the transverse thermal-phonons. The present calculations for RbMnF 3 involve none of adjustable parameters. The sound attenuation coefficient of MnF 2 also is discussed.
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