  We present a full analysis of the mode spectrum in a cavity formed by two
parallel plates, one of which is a magneto-dielectric, e.g. a metamaterial,
while the other one is metallic, and obtain dispersion relations in closed
form. The optical properties of the cavity walls are described in terms of
realistic models for the effective permittivity and the permeability. Surface
polaritons, i.e. electromagnetic modes that have at least partly an evanescent
character, are shown to dominate the Casimir interaction at small separations.
We analyze in detail the s-polarized polaritons, which are a characteristic
feature of a magneto-dielectric configuration, and discuss their role in the
repulsive Casimir force.
