Thin-Film Heterostructures Based on Co/Ni Synthetic Antiferromagnets on Polymer Tapes: Toward Sustainable Flexible Spintronics

Synthetic antiferromagnets with perpendicular magnetic anisotropy(PMA-SAFs) have gained growing attention for both conventional and nextgenerationspin-based technologies. While the progress of PMA-SAF spintronicdevices on rigid substrates has been remarkable, only few examples of flexible thin-filmheterostructures are reported in the literature, all containing platinum group metals(PGMs). Systems based on Co/Ni may offer additional advantages with respect todevices containing PGMs, i.e., low damping and high spin polarization. Moreover,limiting the use of PGMs may relieve the demand for critical raw materials and reducethe environmental impact of related technologies, thus contributing to the transitiontoward a more sustainable future. Here, we discuss for the first time the realization ofCo/Ni-based PMA-SAFs on polymer tapes and exploit it to obtain flexible giantmagneto-resistive spin valves (GMR-SVs) with perpendicular magnetic anisotropy.Several combinations of buffer and capping layers (i.e., Pt, Pd, and Cu/Ta) are alsoinvestigated. High-quality flexible SAFs with a fully compensated antiferromagnetic region and SVs with a sizable GMR ratio (up to4.4%), in line with the values reported in the literature for similar systems on rigid substrates, were obtained in all cases. However, wedemonstrate that PGMs allows achieving the best results when used as a buffer layer, while Cu is the best choice as a capping layer tooptimize the properties of the stacks. We justify the role of buffer and capping layers in terms of different interdiffusion mechanismsoccurring at the interface between the metallic layers. These results, along with the high robustness of the samples' properties againstbending (up to 180°), indicate that complex and bendable Co/Ni-based heterostructures with reduced content of PGMs can beobtained on flexible tapes, allowing for the development of novel flexible and sustainable spintronic devices for applications in manyfields including wearable electronics, soft robotics, and biomedicine.