We have measured the attenuation of ultrasonic waves with frequencies between 30 and 150 MHz in Zn1-xMnxSe, with x=0.37 and 0.53, and Cd1-xMnxTe with x=0.45 and 0.60 down to 1.5 K. A wide, shear-wave attenuation peak occurred almost entirely below the spin-glass transition temperature. The peak was higher the larger the value of x or measuring frequency. It was comprised of three non-Debye components whose amplitudes and relaxation rate activation energies, Ei, were larger the higher the temperature of the components summit. The Eis were simple multiples of the Mn-Mn exchange integral. The highest-T component was increased by an applied magnetic field. We attribute most of the attenuation to relaxation of the ultrasonic strain-induced changes in the orientation of spins frozen into clusters. The spin-phonon coupling seems to be anisotropic since no attenuation peak was observed with longitudinal waves.

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© 1988 The American Physical Society

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Physical Review B