Creation of magnetic skyrmions by surface acoustic waves

The controlled creation of magnetic skyrmions is a prerequisite for their application in future spintronic devices. While charge currents can induce skyrmions via spin torque, surface acoustic waves can do the same through magnetoelastic coupling of inhomogeneous strain paired with thermal fluctuations. Non-collinear and non-coplanar spin textures, such as chiral domain walls(1) and helical or triangular spin structures(2,3), bring about diverse functionalities. Among them, magnetic skyrmions, particle-like non-coplanar topological spin structures characterized by a non-zero integer topological charge called the skyrmion number (N-sk), have great potential for various spintronic applications, such as energy-saving, non-volatile memory and non-von Neumann devices(4-7). Current pulses can initiate skyrmion creation in thin-film samples(8-10) but require relatively large current densities, which probably causes Joule heating. Moreover, skyrmion creation is localized at a specific position in the film depending on the sample design. Here, we experimentally demonstrate an approach to skyrmion creation employing surface acoustic waves (SAWs); in asymmetric multilayers of Pt/Co/Ir, propagating SAWs induce skyrmions in a wide area of the magnetic film. Micromagnetic simulations reveal that inhomogeneous torque arising from both SAWs and thermal fluctuations creates magnetic textures, with pair structures consisting of a Neel skyrmion-like and an antiskyrmion-like structure. Subsequently, such pairs transform to a Neel skyrmion due to the instability of the antiskyrmion-like structure in a system with interfacial Dzyaloshinskii-Moriya interaction. Our findings provide a tool for efficient manipulation of topological spin objects without heat dissipation and over large areas, given that the propagation length of SAWs is of the order of millimetres.