Unconventional skyrmions in synthetic antiferromagnets

Magnetic skyrmions are topologically protected spin textures that can act as reconfigurable nanoscale information carriers. In synthetic antiferromagnets, interlayer exchange coupling provides a control parameter beyond the interfacial Dzyaloshinskii-Moriya interaction and magnetic anisotropy. Here we engineer a synthetic antiferromagnet of two chemically distinct ferromagnets, CoB and CoFeB, in which the external field and interlayer exchange act asymmetrically on the sublattices. Their competition, acting as an effective field, gives rise to two skyrmion families in different field regimes: conventional-polarity skyrmions at large fields, and inverse-polarity skyrmions at smaller fields, where the effective field reverses sign. Using element-resolved X-ray magnetometry, correlative magnetic force and Lorentz transmission electron microscopies, and micromagnetic modelling, we show that all textures reside solely in the CoFeB layers, driven by a Ruderman-Kittel-Kasuya-Yosida exchange field from the CoB layers. This effective-field route enables programmable three-dimensional spin textures with layer-selected polarity for skyrmion-based computing.