  Based on theoretical models, the dynamics of spin-torque nano-oscillators can
be substantially modified by re-injecting the emitted signal to the input of
the oscillator after some delay. Numerical simulations for vortex magnetic
tunnel junctions show that with reasonable parameters this approach can
decrease critical currents as much as 25 % and linewidths by a factor of 4.
Analytical calculations, which agree well with simulations, demonstrate that
these results can be generalized to any kind of spin-torque oscillator.
