Modulation bandwidth of spin torque oscillators under current modulation

For practical applications of spin torque nano-oscillators (STNO), one of the most critical characteristics is the speed at which an STNO responds to variations of external control parameters, such as current or/and field. Theory predicts that this speed is limited by the amplitude relaxation rate Γ_p that determines the timescale over which the amplitude fluctuations are damped out. In this study, this limit is verified experimentally by analyzing the amplitude and frequency noise spectra of the output voltage signal when modulating an STNO by a microwave current. In particular, it is shown that due to the non-isochronous nature of the STNO the amplitude relaxation rate Γ_p determines not only the bandwidth of an amplitude modulation, but also the bandwidth of a frequency modulation. The presented experimental technique will be important for the optimisation of the STNO characteristics for applications in telecommunications or/and data storage and is applicable even in the case when the STNO output signal is only several times higher than noise.