Generation of magnetic skyrmion bubbles by inhomogeneous spin-Hall currents

Recent experiments have shown that magnetic skyrmion bubbles can be generated and injected at room temperature in thin films$^1$. Here, we demonstrate, using micromagnetic modeling, that such skyrmions can be generated by an inhomogeneous spin Hall torque in the presence of Dzyaloshinskii- Moriya interactions (DMIs). In the experimental Ta-Co$_{20}$Fe$_{60}$B$_{20}$ thin films, the DMI is rather small; nevertheless, the skyrmion bubbles are stable, or at least metastable on observational time scales. We identify two different mechanisms, one in a low-current regime and the other in a high current regime, that destabilize a domain wall injected from a narrow region into an expanding region with inhomogeneous spin-Hall torque. In the first, asymmetric torques on the domain wall lead to a cascade of bubble formation and subsequent fragmentation. In the second, an approximately steady-state texture is injected into the wide region. When the current is turned off, the inhomogeneous spin texture relaxes and regions can coalesce into bubbles that attain a definite chirality because of the DMI. [1] W. Jiang {\it et al.}, Science {\bf 349}, 283 (2015).