  We study the motion of a ferromagnetic helical nanostructure under the action
of a rotating magnetic field. A variety of dynamical configurations were
observed that depended strongly on the direction of magnetization and the
geometrical parameters, which were also confirmed by a theoretical model, based
on the dynamics of a rigid body under Stokes flow. Although motion at low
Reynolds numbers is typically deterministic, under certain experimental
conditions, the nanostructures showed a surprising bistable behavior, such that
the dynamics switched randomly between two configurations, possibly induced by
thermal fluctuations. The experimental observations and the theoretical results
presented in this letter are general enough to be applicable to any system of
ellipsoidal symmetry under external force or torque.
