  The dispersion relations for surface plasmon-polaritons propagating in the
Voigt geometry in a metal-insulator-metal waveguide with a magneto-optically
active dielectric medium are derived. The symmetry between the upper and lower
interfaces is broken by the introduction of the magnetic field; the balance
between the field distributions on the two interfaces can be controlled by the
applied field. This control is illustrated by finite-element method numerical
simulations of the field distributions around a point dipole placed in the
centre of the short waveguide; it is shown that both the total emission of
radiation from the cavity and the distribution of the far-field radiation can
be strongly modified by tuning the magnetisation of the waveguide. This raises
the novel possibility of using magnetic fields to control light propagation in
nanostructures.
