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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.

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  This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Rohiteswar Mondal, Zhenchao Wen, Chandrasekhar Murapaka, Seiji Mitani, Hiroaki Sukegawa, Quang Le, Xiaoyong Liu, Brian York, Maki Maeda; Enhanced spin–orbit torque efficiency via interface engineering in BiSb/X/CoFeB (X = Ge, NiAl, NiFeGe) topological heterostructures. J. Appl. Phys. 14 May 2026; 139 (18): 183903 and may be found at https://doi.org/10.1063/5.0313856.

Keyword: interface engineering, topological heterostructures, BiSb/X/CoFeB (X = Ge, NiAl, NiFeGe)

Date published: 2026-05-14

Publisher: AIP Publishing

Journal:

• Journal of Applied Physics (ISSN: 00218979) vol. 139 issue. 18 183903

Funding:

• JICA Friendship Program 2.0
• the International Cooperative Graduate Program in NIMS

Manuscript type: Author's version (Accepted manuscript)

MDR DOI: https://doi.org/10.48505/nims.6281

First published URL: https://doi.org/10.1063/5.0313856

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Updated at: 2026-05-12 09:47:48 +0900

Published on MDR: 2026-05-12 12:27:01 +0900