Coupled vortex dynamics in spin-torque oscillators:
Dr. Romain Lebrun (Microelectronic group, Cavendish Laboratory, University of Cambridge, UK)

The discovery of the giant magnetoresistance in 1988 is considered as the birth date of a new and dynamic research field called spintronics. The rich physics associated with spin transport has created a breakthrough for the future of nano-electronics. In the magnetism roadmap, spin-torque oscillators (STOs) are candidates for future generation of spintronic based rf-devices, ranging from classical rf-sources/detectors to bio-inspired beyond CMOS applications [1]. However, one major issue of spin-torque oscillators is the difficulty to control with precision their phase dynamics. To enhance this control, we have investigated different approaches ranging from the study of collective mode dynamics in hybridized magnetic systems [2] to the synchronization of multiple STOs.
By a careful analysis of the spin transfer forces (Slonczewski and Field-like) involved in the locking process of a vortex based STO, we succeeded in demonstrating that a vortex based STO can phase lock with an external rf-current without any phase slips, i.e desynchronization events, when the locking process is driven by a Field-like in-plane torque [3]. Taking advantages of the large Field-like torque in our magnetic tunnel junction, we proposed an innovative concept of rf-threshold detector [4] based on the controlled nucleation and expulsion of the vortex core out of our junctions.
More recently, our understanding of the locking process of such vortex based STOs has permitted us to electrically synchronize two STOs directly connected in parallel or in series [5]. Contrary to classical magnetic coupling (like spin-wave of dipolar coupling), the electrical coupling using the rf current emitted by each STO has the intrinsic advantages of merging local and global couplings. Thus a full control of the synchronized state can be obtained by tuning the phase shift between the two oscillators, either externally with an electrical delay or internally through the different spin-transfer torques. These first promising results marks an important milestone towards the observation of a large variety of nanoscale collective dynamics of nonlinear oscillators, and opens among others the perspective of STO networks mimicking some of the basic brain functionalities.