Proposal for a Spin-Torque-Oscillator Maser Enabled by Microwave Photon-Spin Coupling

We study a direct-current-driven maser device enabled by spin-photon coupling, where coherent magnetic self-oscillation can be realized in a large-area ferromagnetic thin film under the excitation of spin-orbit torques. We show via both micromagnetic simulation and analytical derivation that above a critical value of the coupling strength between the spin-torque oscillator and the microwave resonator, magnetic oscillation develops macroscopic phase coherence, a narrow linewidth, and becomes phase-locked with the photon mode. The threshold coupling strength for synchronizing individual spins reduces as the sample dimension increases, suggesting that the spin-torque-oscillator maser can be readily realized using large-area thin-film ferromagnets without relying on dimension confinement. Moreover, the photon mode can directly provide microwave emission, which exhibits enhanced power and a reduced linewidth with an increasing number of spins, leading to a useful approach for developing highly coherent on-chip microwave sources.