Creation and confirmation of Hopfions in magnetic multilayer systems

Topological solitons have been studied for decades in classical field theories, and have started recently to impact condensed matter physics. Among those solitons, magnetic skyrmions are two-dimensional particle-like objects with a continuous winding of the magnetization, and magnetic Hopfions are three-dimensional topological solitons that can be formed from a closed loop of a twisted skyrmion string. Whereas intense research is underway with magnetic skyrmions towards a fundamental understanding and potential applications in advanced storage and logic devices, the experimental creation and confirmation of magnetic Hopfions has been elusive so far. Theoretical models suggest that Hopfions can be stabilized in frustrated or chiral magnetic systems, and that target skymions can be transformed into Hopfions by adapting their perpendicular magnetic anisotropy. Here, we present experimental evidence of magnetic Hopfions that were created in magnetic Ir/Co/Pt multilayers shaped into nanoscale disks, which are known to host target skyrmions. The three-dimensional spin texture, which distinguishes magnetic Hopfions from target skyrmions was confirmed by combining two advanced element-specific magnetic X-ray microscopy techniques with about 20-30nm lateral resolution, using X-ray magnetic circular dichroism effect as magnetic contrast mechanism in surface-sensitive X-ray photoemission electron microscopy and bulk-sensitive soft x-ray transmission microscopy. We anticipate that these results will stimulate further investigations of Hopfions with different topologies and their potential application in three-dimensional spintronics devices.