  We report the first study on the optical force exerted by an evanescent wave
on a small sphere with both electric and magnetic response to the incident
field, immersed in an arbitrary nondissipative medium. New expressions and
effects from their gradient, radiation pressure, and curl components are
obtained due to the particle induced electric and magnetic dipoles, as well as
to their mutual interaction. We predict possible dramatic changes in the force
depending on either the host medium, the polarization and the nature of the
surface wave.
