  When periodically driven by an external magnetic field, a spin system can
enter a phase of steady entrained oscillations with nonequilibrium probability
distribution function. We consider an arbitrary magnetic field switching its
direction with frequency comparable with the spin-flip rate and show that the
resulting nonequilibrium probability distribution can be related to the system
equilibrium distribution in the presence of a constant magnetic field of the
same magnitude. We derive convenient approximate expressions for this exact
relation and discuss their implications.
