Sound propagation in magnetic fluids

In magnetic fluids, sound waves are coupled to the magnetic variables since the magnetization oscillates with the fluid density. Here, explicit formulas for the velocity and the damping of sound are determined from the hydrodynamic equations. Thermal expansion and heat diffusion are also included. The magnetic field is not restricted to be homogeneous; arbitrary magnetic forces are allowed in the analysis. The ambiguities inherent in the notion of pressure are shown to have no effect on the sound propagation. The sound velocity and the heat diffusion constant are derived to be anisotropic. This is a truly macroscopic result. The underlying mechanism is not any model assumption on the magnetic fluid particles, but demagnetization, i.e., the coupling of the wave geometry to the Maxwell equations.