Enhanced spin–orbit torque efficiency via interface engineering in BiSb/X/CoFeB (X = Ge, NiAl, NiFeGe) topological heterostructures

Interlayer engineering is crucial for achieving efficient spin–orbit torque (SOT) generation in topological insulator (TI)/ferromagnet-based heterostructures. In this work, we investigated the impact of semiconducting and intermetallic interlayers, i.e., Ge, NiAl, and NiFeGe on SOT efficiency in topological BiSb/CoFeB heterostructures. Through comprehensive structural and transport analyses, we demonstrate that the Ge interlayer significantly enhances the performance. The insertion of a 1 nm-thick Ge layer significantly improves the crystallinity and orientation of BiSb, suppresses interfacial interdiffusion, and achieves a very low magnetic damping constant (αeff ∼ 0.006). The optimized BiSb/Ge/CoFeB structure exhibits an enhanced SOT efficiency of ∼50%, surpassing the performance of heterostructures without an interlayer, as well as those with NiAl or NiFeGe interlayers, which suffer from higher damping or reduced efficiency. Furthermore, we revealed that Ge thickness critically influences interfacial transparency, with excessive thickness degrading SOT efficiency. These findings establish Ge as an optimum interlayer material and indicate the importance of precise interface engineering to maximize charge-spin conversion in TI-based spintronic devices, paving the way for next-generation energy-efficient technologies.