  We report an approach to improving the performance of spin torque
nano-oscillators (STNOs) that utilizes power-dependent negative feedback to
achieve a significantly enhanced dynamic damping. In combination with a
sufficiently slow variation of frequency with power this can result in a
quasi-linear STNO, with very weak non-linear coupling of power and phase
fluctuations over a range of bias current and field. An implementation of this
approach that utilizes a non-uniform spin-torque demonstrates that highly
coherent room temperature STNOs can be achieved while retaining a significant
tunability.
