Self-Injection Locking of a Vortex Spin Torque Oscillator by Delayed Feedback.

A major scientific breakthrough in spintronics was the introduction of spin transfer forces as a new means to generate high frequency nonlinear dynamics in nanoscale magnetic devices. The wealth of physics in spin transfer phenomena paves the way to a new generation of multi-functional spintronic devices1. Recent trends range from nanoscale radiofrequency (rf) devices for an efficient microwave source2 to highly sensitive microwave detection3,4, magnonic devices5 and more recently neuro-inspired memory devices6. For the purpose of realizing these applications, it becomes of paramount importance to not only identify and control the sources of noise7 but also to achieve a fine control of the phase of these spin torque devices8. Indeed, it is known and widely used in other types of oscillators such as conventional optical lasers9 or voltage control oscillators10, that the control of the oscillator phase can be achieved by a self-delayed feedback. In these systems, the spectral linewidth strongly depends on the delay time or phase difference between the oscillator and the re-injected signal, the effect of which can be observed in the forced synchronization of a spin torque oscillator (STO) with an rf current source. Here, the STO phase is determined by the phase of injected rf current11,12. V. Tiberkevich et al.13 proposed a similar implementation for an STO circuit based on the delayed self-injection of the output rf current. It should be noticed that the large nonlinear behavior, which is specific to STOs might detrimentally impact the self-locking process of the device14,15. However, more recently, Khalsa et al. reported in a theoretical study that the control of linewidth reduction could be expected in a STO circuit based on the delayed self-injection of the output rf current16. To our knowledge, this approach has not yet been addressed experimentally. We believe that the demonstration of the tuning of the rf properties through a controlled delay represents an important step for mastering the properties of STOs (frequency, spectral coherence and power consumption), which is crucial for the targeted rf applications2,17 as well for neuro inspired STO based memory devices1,6.