  We study an ensemble of strongly coupled electrons under continuous microwave
irradiation interacting with a dissipative environment, a problem of relevance
to the creation of highly polarized non-equilibrium states in nuclear magnetic
resonance. We analyse the stationary states of the dynamics, described within a
Lindblad master equation framework, at the mean-field approximation level. This
approach allows us to identify steady state phase transitions between phases of
high and low polarization controlled by the distribution of electronic
interactions. We compare the mean-field predictions to numerically exact
simulations of small systems and find good agreement. Our study highlights the
possibility of observing collective phenomena, such as metastable states, phase
transitions and critical behaviour in appropriately designed paramagnetic
systems. These phenomena occur in a low-temperature regime which is not
theoretically tractable by conventional methods, e.g., the spin-temperature
approach.
