Interplay of spin-torque and nonlinearity:
Disentangling and controlling magnon processes in nanomagnets

Nanoscale magnets are the building blocks of various spintronics technologies. However, many aspects of spin dynamics in the nanoscale geometrical confinement have remained elusive. For instance, untangling damping contributions and engineering response of nanomagnets to external drives is a challenging task. 
Using magnetic tunnel junctions – the flagship technology of spintronics industry – as a sample platform, we investigate the discrete magnon spectrum of individual zero-dimensional magnets and identify the inherent inter-magnon processes. While manipulation of magnetization by spin-torque, i.e. by influx of angular momentum, is a key functionality of today’s spintronics, we find that resonant magnon processes redefine and invert nanomagnet’s response to spin-torque [1]. We discuss the mechanisms of this counter-intuitive interplay of nonlinearity and angular momentum flows, which has far-reaching implications for the performance of magnetic memory and oscillators.
Controlling magnon processes and thus forging the nonlinearity of a nanomagnet would add decisive functionality to existing and emerging technologies, in particular to magnetic neuromorphic networks where tunability of the nonlinear response is instrumental. We develop an approach for engineering magnon interaction by means of symmetry-breaking fields with nanoscale nonuniformity [2]. In a proof-of-concept, we employ a nanoscale auxiliary element (a synthetic antiferromagnet) as a switchable source of such fields and achieve tunability of magnon coupling by at least one order of magnitude. The results open up avenues for controlling magnon processes by external stimuli at nanoscale and show prospects for spin-torque applications, magnetic neuromorphic systems, and emergent hybrid quantum information technologies.

This work was supported by the National Science Foundation through Grant No. ECCS-1810541 and by NVidia Corp. 

References
1. Giant nonlinear damping in nanoscale ferromagnets, I. Barsukov et al., Sci. Adv. 5, eaav6943 (2019),   http://dx.doi.org/10.1126/sciadv.aav6943
2. Controlling magnon interaction by a nanoscale switch, A. Etesamirad et al., ACS Appl. Mater. Interfaces 13, 20288 (2021), https://doi.org/10.1021/acsami.1c01562