  We develop a theory for dynamic hysteresis in ferromagnetic thin films, on
the basis of the phenomenological principle of loss separation. We observe
that, remarkably, the theory of loss separation, originally derived for bulk
metallic materials, is applicable to disordered magnetic systems under fairly
general conditions regardless of the particular damping mechanism. We confirm
our theory both by numerical simulations of a driven random--field Ising model,
and by re--examining several experimental data reported in the literature on
dynamic hysteresis in thin films. All the experiments examined and the
simulations find a natural interpretation in terms of loss separation. The
power losses dependence on the driving field rate predicted by our theory fits
satisfactorily all the data in the entire frequency range, thus reconciling the
apparent lack of universality observed in different materials.
