  It is known that there are nonlinear wave equations with localized solitary
wave solutions. Some of these solitary waves are stable (with respect to a
small perturbation of initial data) and have nonzero spin (nonzero intrinsic
angular momentum in the center of momentum frame). In this paper we consider
vector-valued solitary wave solutions to a nonlinear Klein-Gordon equation and
investigate the behavior of these spinning solitary waves under the influence
of an externally imposed uniform magnetic field. We find that the only
stationary spinning solitary wave solutions have spin parallel or anti-parallel
to the magnetic field direction.
