Effective sound attenuation exponents in magnets

We try to explain how some magnets belonging to the Heisenberg universality class can be characterised by positive sound attenuation exponents near the critical point. In magnets being also insulators the sound attenuation exponent should be equal to 2α where α is the usual specific-heat exponent. This exponent is however negative in the Heisenberg universality class. In this paper we interpret the positive attenuation exponents measured in isotropic magnets such as RbMnF₃, Y₃Fe₅O₁₂ and Gd₃Fe₅O₁₂ in terms of the effective critical exponents. The high value of the sound attenuation exponent in anisotropic FeF₂ can also be interpreted in this way. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)