  Hydrodynamics of complex fuids with multiple order parameters is governed by
a set of dynamic equations with many material constants, of which only some are
easily measurable. We present a unique example of a dynamic magneto-optic
coupling in a ferromagnetic nematic liquid, in which long-range orientational
order of liquid crystalline molecules is accompanied by long-range magnetic
order of magnetic nanoplatelets. We investigate the dynamics of the
magneto-optic response experimentally and theoretically and find out that it is
significantly affected by the dissipative dynamic cross-coupling between the
nematic and magnetic order parameters. The cross-coupling coefficient
determined by fitting the experimental results with a macroscopic theory is of
the same order of magnitude as the dissipative coefficient (rotational
viscosity) that governs the reorientation of pure liquid crystals.
