Quantum coherence in spin-torque nano-oscillators

We study the quantum coherence properties of spin waves in a ferromagnetic thin film driven by a dc current in a spin-torque nano-oscillator. We show that if the current barely exceeds the threshold value the driven magnon states are not coherent and exhibit pronounced thermal noise. Only when the current exceeds the threshold value by a certain amount that depends on the material parameters and temperature of the nanostructure the microwave oscillation will exhibit quantum coherence.