Metallic multilayers with strong Dzyaloshinskii-Moriya interaction to stabilize small individual skyrmions at room temperature
Magnetic skyrmions are arguably the smallest topologically non-trivial magnetic configurations and promising candidates for novel spintronic devices [1-3]. Numerical simulations have shown that the interfacial Dzyaloshinskii-Moriya interaction (DMI) can stabilise isolated chiral skyrmions in nanoscale disks and in tracks for a wide range of DMI strength [4,5].
For the present study, we sputtered metallic multilayers made of stacks of trilayers composed of 0.6 nm-thick Co layers sandwiched between heavy 5d metal (HM) layers, namely Pt, Ir and W. Asymmetric sandwiches are designed to introduce DMI by summing the HM contributions on the top and bottom interfaces of Co [2,5] while obtaining also a perpendicular magnetic anisotropy . We performed scanning transmission X-ray microscopy (STXM), which uses the x-ray magnetic circular dichroism (XMCD) effect to observe magnetic bubbles or skyrmions in our micro-fabricated structures with a resolution of 30 nm and at room temperature . We acquired XMCD images of single magnetic bubbles stabilised by the DMI at different perpendicular magnetic fields in different Pt/Co/HM multilayers. Using the variation of the bubble diameter with magnetic field, we estimate the DMI to reach up to 2.4 mJ/m2. This value is confirmed by domain size studies at remanence. We compare our experimental observations with micromagnetic simulations of skyrmions for different types of multilayer stacks to confirm the estimated DMI value in order to determine eventually the topological charge of the observed magnetic bubbles [3,4]. Images were also acquired with standard MFM techniques. Our room temperature imaging of individual magnetic skyrmions stabilised by chiral interaction represents an important breakthrough for future fundamental studies on single skyrmions and towards the development of skyrmion-based devices .
Finally, we will discuss about prospective devices, illustrated with micromagnetic simulations. In particular we will show how the spin Hall effect, also originating from a spin-orbit-related phenomenon, can very effectively be used in “skyrmion racetracks” .
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