  A theoretical study of delayed feedback in a spin-torque nano-oscillator
model is presented. The feedback acts as a modulation of the supercriticality,
which results in changes in the oscillator frequency through a strong
nonlinearity, amplitude modulations, and a rich modulation sideband structure
in the power spectrum at long delays. Modulation sidebands persist at finite
temperatures but some of the complex structure is lost through the finite
coherence time of the oscillations.
