Nonresonance spin-lattice absorption of ultrasound in Ising magnets

A theory of nonresonant acoustic absorption in Ising magnets is constructed by the nonequilibrium statistical operator method. It is assumed that the spins are coupled with the sound via modulation of the Ising exchange integral, and the nonresonant absorption is due to the interaction of the spin system with the thermal vibrations of the lattice. The frequency dependence of the absorption χ″(ω), which has an analytical form different from that of the well-known Debye function, is determined. The temperature dependence of χ″(ω), which is made up of 1) the averages of the vibrational variables and 2) the temperature dependence of the spin correlation functions of the linear Ising model, is analyzed. It is shown that the cross-coupling of the two subsystems (Ising and Zeeman) produces a second absorption peak at low frequencies together with the peak characteristic of the Debye curve. Fiz. Tverd. Tela (St. Petersburg) 39, 516–521 (March 1997)