Skyrmion Density Modulation via Current-induced Skyrmion-to-stripe Transformation

Abstract

Electrical current remains one of the simplest and most accessible tools for the control of magnetic skyrmions. However, it can be challenging to only perform the intended operation among all the possible current-induced effects such as deformation, nucleation, and propagation [1-3]. While current-induced skyrmion nucleation has been demonstrated [1,2], its equally important complementary operation of current-induced skyrmion annihilation remains elusive. Our investigation on the current-induced skyrmion effects revealed an additional regime at low current densities where skyrmion annihilation can be performed via skyrmion-stripe transformation [4].In this work, an experimental study of current-induced skyrmion-stripe transformation in a Pt/Co/Fe/Ir magnetic bilayer was performed. Using a Hall cross device shown in Fig. 1(a), a slow evolution of the magnetization after current injection was observed and showed distinct features in the high and low current density regimes. High current density injection induced a sharp negative change in magnetization followed by a continuous decay back to equilibrium while low current density induced a gradual positive change in magnetization from equilibrium before decaying back to equilibrium forming a peak as shown in Fig. 1(b). In situ observation under magneto-optical Kerr (MOKE) imaging showed that at high current densities, additional magnetic skyrmions were nucleated as shown in Fig. 2(e-h). In contrast, at low current densities, skyrmions expanded into stripes. The resulting competition for space annihilates skyrmions. Eventually the stripes contract into skyrmions with a lower overall density as shown in Fig. 2(a-d).The mechanism for the skyrmion-to-stripe transformation was further investigated by analyzing the MOKE images of the stripe domains over many runs. The stripe domains were found to nucleate at predictable spots on the wire which can be attributed to pinning sites. Together with the additional requirement of a long current pulse duration of several milliseconds for the transformation to occur, the transformation was concluded to be occurring as the result of a skyrmion-pinning site interaction and creep motion. A skyrmion at a pinning site undergoes slow creep elongation into a stripe due to the weak propagative being insufficient to fully depin the skyrmion.By utilizing the combination of skyrmion nucleation at high current density and skyrmion annihilation at low current density, skyrmion density control was demonstrated purely by current density modulation. In addition, the volatile nature of these current-induced stripes offers interesting possibilities for neuromorphic computing applications such as artificial leaky-integrate fire neurons which requires a continuous decaying property after an excitation, and artificial synapses with short-term plasticity which requires a temporary retention of state after excitation.In conclusion, our work revealed additional current-induced magnetic texture transformations which can be exploited for skyrmion density control using solely current modulation. In addition, our findings establish an additional requirement of a minimum operating current density in the design of skyrmionic devices to avoid unintended skyrmion deletion. **