Topological, non topological and instanton droplets driven by spin-transfer torque in materials with perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya Interaction

TL;DR

This paper explores how Dzyaloshinskii-Moriya Interaction influences droplet excitations driven by spin-transfer torque in magnetic materials, revealing new non-stationary instanton modes with potential applications in nanoscale microwave oscillators.

Contribution

It introduces the concept of instanton droplets exhibiting non-stationary skyrmion number transitions, expanding understanding of topological dynamics in current-driven magnetic systems.

Findings

Existence of non-stationary instanton droplet modes with skyrmion number transitions

Coupling of skyrmion number transitions with incoherent spin-wave emissions

Potential for designing nanoscale microwave oscillators based on these phenomena

Abstract

The interfacial Dzyaloshinskii-Moriya Interaction can modify the topology of droplets excited by a localized spin-polarized current. Here, we show that, in addition to the stationary droplet excitations with skyrmion number either one (topological) or zero (non-topological), there exists, for a fixed current, an excited mode with a non-stationary time behavior. We call this mode "instanton droplet", which is characterized by time domain transitions of the skyrmion number. These transitions are coupled to an emission of incoherent spin-waves that can be observed in the frequency domain as a source of noise. Our results are interesting from a fundamental point of view to study spin-wave emissions due to a topological transition in current-driven systems, and could open the route for experiments based on magnetoresistance effect for the design of a further generation of nanoscale microwave oscillators.