  We study dynamical and thermal effects that are induced in nanoparticle
systems by a rotating magnetic field. Using the deterministic Landau-Lifshitz
equation and appropriate rotating coordinate systems, we derive the equations
that characterize the steady-state precession of the nanoparticle magnetic
moments and study a stability criterion for this type of motion. On this basis,
we describe (i) the influence of the rotating field on the stability of the
small-angle precession, (ii) the dynamical magnetization of nanoparticle
systems, and (iii) the switching of the magnetic moments under the action of
the rotating field. Using the backward Fokker-Planck equation, which
corresponds to the stochastic Landau-Lifshitz equation, we develop a method for
calculating the mean residence times that the driven magnetic moments dwell in
the up and down states. Within this framework, the features of the induced
magnetization and magnetic relaxation are elucidated.
