Microwave power generated by a spin-torque oscillator in the presence of noise

An expression for the microwave power generated by a spin-torque oscillator in the presence of thermal noise is derived. This expression, when used in a subcritical (I < I-th) regime, demonstrates that generated power is determined by the noise level and provides a simple way to experimentally determine the threshold current I-th for microwave generation. The same expression gives a good quantitative description of the experimentally measured dependence of the generated power on the bias current p(I) in a moderate supercritical (I-th < I < 1.5I(th)) range of bias current variation.