  Building on a previous study that analyses surface waves in magnetic slabs
embedded in a non-magnetic external environment, in this study the model is
generalised and external magnetic fields are added. The slab is assumed to be
thin, with weak magnetic asymmetry. The frequencies of the standing harmonic
modes are derived to leading-order in the small quantities representing the
thin slab width and the weak asymmetry. It is found that the frequencies are
more prone to changes to the width of the slab than changes in the magnetic
asymmetry. The frequency ratio of the first harmonic to the fundamental mode is
derived, along with the amplitude difference between the two sides of the slab,
as these may be observable quantities that can be compared with observational
results and applied to carry out solar magneto-seismology.
