Hydrodynamic Theory of Spinwaves

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Abstract
We have developed a hydrodynamic theory of spin fluctuations in an isotropic antiferromagnet, which is similar to the theory of two-fluid hydrodynamics for helium.1 In the long-wavelength limit, we find spin waves at any temperature below TN, with real frequency proportional to the wavevector k, and a dampling rate proportional to k2. The real frequency is given in terms of thermodynamic quantities and agrees with earlier predictions; the wavevector dependence of he damping, however, disagrees with earlier (microscopic) calculations.2 The results may be tested by neutron-diffraction experiments on RbMnF3. A similar hydrodynamic theory fo the isotropic ferromagnet predicts spinwaves with a frequency proportional to k2 and a damping proportional to k4.

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