Abstract
Significant deviations from the behavior usual for dielectrics are observed in the ultrasonic attenuation at gigahertz frequencies in Nd, Gd, and Tb garnets and are attributed to the rare-earth ions. Relaxation of low-lying electron states by Orbach processes satisfactorily explains attenuation peaks in neodymium gallium garnet and terbium scandium aluminum garnet near 60 K. Below 20 K the attenuation in all these garnets, irrespective of electronic structure, increases as the inverse of temperature and the square of frequency. Mechanisms that could explain these results based on the relaxation of low-lying states are proposed. The lack of knowledge about the parameters entering the theoretical models prevents a detailed comparison with experiment at this time.
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