On the hydrodynamic theory of a magnetic liquid II. Hydrodynamic modes in the Heisenberg fluid

On the basis of our previous results (Physica A 220 (1995) 325) the collective mode spectrum of the Heisenberg ferrofluid model is studied in the hydrodynamic region. We start from the rigorous microscopic approach taking into account all conserved quantities and calculate the explicit expressions for the dispersion and damping coefficient of sound modes. It is shown that the sound velocity of Heisenberg ferrofluid at constant external magnetic field is isotropic and can be expressed by the adiabatic compressibility at constant magnetization m. We also calculate the heat and spin modes which are purely diffusive. Explicit expressions for the viscosity, thermal conductivity, spin diffusion and thermomagnetic diffusion coefficients containing time correlation functions are derived. Our results are compared with previous ones obtained mainly within phenomenological theories.