Linewidth of higher harmonics in a nonisochronous auto-oscillator: Application to spin-torque nano-oscillators

In an isochronous auto-oscillator, where the auto-oscillation frequency f does not depend on the oscillation amplitude, the linewidth of the nth harmonic is n² times larger than the linewidth of the main auto-oscillation mode. Here we present a theoretical description that predicts that the increase in the linewidth of higher harmonics with harmonic number n is substantially slower in the case of a nonisoschronous oscillator, where f depends strongly on amplitude. Using spin-torque nano-oscillators as an example of a nonisochronous oscillator, this description is confirmed numerically and experimentally. The presented model allows one to extract important nonlinear parameters of an auto-oscillator of any physical nature from the measurement of higher harmonic linewidths.