Microwave Emitting Nanomagnet Oscillator: Strongly coupled spin-photon modes*

We describe a microwave emitting nanomagnet oscillator confined in a high Q-cavity. The precession of the magnetization of a typical Fe nanomagnet of 100 nm in radius, possessing roughly $10^9$ spins and described as a macrospin, is tuned to be in resonance with a microwave cavity of 1 mm$^3$ in volume with an applied magnetic field. The Hamiltonian of the spin-cavity system is quantized in a fully quantum treatment. Due to the very large number of coherently-oriented spins, the interaction Hamiltonian of the spin-cavity system contains magnet-microwave mode coupling terms that exceed several GHz. Coherent coupling of the microwave field with nanomagnet spins is analyzed in terms of the coherent states of the spin-cavity system, which are characterized by large oscillations in the nanomagnet spin and cavity photon number, as well as by exceptionally long dephasing times. Therefore, the nanomagnet-cavity system is predicted to have distinguishable large total spin, long coherence times, and high power output. This may serve as an efficient means of transferring information between a magnetic and a photonic system.