Experimental observation of magnetic hopfions in multilayer disks*

Topological solitons are currently of high interest to condensed matter physics. Among those, magnetic skyrmions (Sk) are two-dimensional objects with a continuous winding of the magnetization, and magnetic Hopfions are 3D topological solitons that can be formed from a closed loop of a twisted skyrmion string. Whereas intense research with magnetic Sks has already led to a deep understanding and potential applications in advanced storage and logic devices^1,2, the experimental validation of magnetic Hopfions has been elusive so far. Theoretical models suggest that Hopfions can be stabilized in frustrated or chiral magnetic systems, and kπ skyrmions, aka target skymions (TSk) can be transformed into Hopfions by adapting their perpendicular magnetic anisotropy^3,4.
We present experimental evidence of magnetic Hopfions in magnetic Ir/Co/Pt multilayers shaped into nanoscale disks, which are known to host TSks⁵. Their distinct 3D spin texture was observed by combining two magnetic soft x-ray microscopies, namely surface sensitive X-PEEM, and bulk sensitive full field MTXM.
¹ A. Fert,et al, Nat. Nanotech. 8 152 (2013)
² A. Soumyanarayanan, et al. Nat. Mat. 16 898 (2017)
³ P. Sutcliffe, JPhysA (2018)
⁴ Y. Liu et al PRB 98 174437 (2018)
⁵ N. Kent et al. APL 115 112404 (2019)