  We present a mesoscopic approach to analyze the dynamics of a single magnetic
dipole under the influence of an oscillating magnetic field, based on the
formulation of a Fokker-Planck equation. The dissipated power and the viscosity
of a suspension of such magnetic dipoles are calculated from non-equilibrium
thermodynamics of magnetized systems. By means of this method we have found a
non-monotonous behaviour of the viscosity as a function of the frequency of the
field which has been referred to as the ``negative'' viscosity effect.
Moreover, we have shown that the viscosity depends on the vorticity field thus
exhibiting non-Newtonian behaviour. Our analysis is complemented with numerical
simulations which reproduce the behaviour of the viscosity we have found and
extend the scope of our analytical approach to higher values of the magnetic
field.
