Topological spin textures in synthetic antiferromagnets: stabilization, nucleation and dynamics of (Bi)merons, Skyrmions and Skyrmion–Bimeron Pairs (Skymerons)

Abstract

In the emerging landscape of post-CMOS technologies, spintronics-based devices have garnered growing interest for their potential to enable non-volatile, energy-efficient, and densely integrated logic and memory technologies. Among the various strategies explored, the utilization of topologically non-trivial magnetic spin textures, such as skyrmions and bimerons, has emerged as a strong candidate for several spintronic devices owing to their nanoscale dimensions, topologically enhanced stability, and susceptibility to manipulation by electrical currents. While skyrmions in ferromagnets have enabled seminal advances in racetrack memories, logic gates, and unconventional computing concepts, their practical use is hampered by stray fields, limited thermal stability, and the intrinsic skyrmion Hall effect during dynamics. These challenges motivate the exploration of alternative topological spin textures beyond skyrmions and their realization in compensated magnetic systems, where antiparallel sublattice magnetizations cancel the net moment, suppress the skyrmion Hall effect, and enable the stabilization of spin textures over a broad range of magnetic fields suitable for device applications.

This thesis addresses these challenges by establishing an experimental framework to investigate the stabilization, nucleation, and dynamics of various topological spin textures stabilized by the interfacial Dzyaloshinskii–Moriya interaction (iDMI) in synthetic antiferromagnets (SyAFMs). SyAFMs, consisting of ferromagnetic layers antiferromagnetically coupled through metallic spacers, exhibit compensated spin textures with negligible net dipolar fields. Their magnetic properties can be finely tuned via the thickness of the constituent sublattices, enabling independent control over key material parameters such as the effective perpendicular magnetic anisotropy, iDMI, and saturation magnetization. This work demonstrates the stabilization of chiral in-plane antiferromagnetic topological spin textures, namely merons, antimerons, and bimerons, by primarily tuning the net effective anisotropy of the system to achieve a nearly vanishing condition [1, 2]. Perpendicular synthetic antiferromagnetic multilayers are engineered to host skyrmions with radii of 50–100 nm that exhibit distinct static and dynamic characteristics. Element-specific pump–probe X-ray microscopy enables the sublatticeresolved nanosecond dynamic imaging of skyrmion–skyrmion interactions such as scattering, recoil, and collective flow dynamics [3]. Beyond two-dimensional spin textures, by tuning the saturation magnetization of each ferromagnetic sublattice, this thesis demonstrates a three-dimensional extension of skyrmions, referred to as hybrid skyrmion tubes [4]. Finally, this thesis introduces and experimentally realizes a new topological spin texture, termed the skymeron, which emerges from the near-orthogonal alignment of skyrmions and bimerons. Overall, these findings establish a quantitative framework for the stabilization and nucleation of various topological spin textures in synthetic antiferromagnets and demonstrate their fully reproducible collective dynamics over billions of cycles, even in the incoherent flow regime, thereby paving the way for their utilization in future spintronic devices.