Control of Spin-Orbit Torques by Interface Engineering in Topological Insulator Heterostructures

Area of Science:

• Condensed Matter Physics
• Materials Science
• Spintronics

Background:

• Topological insulators (TIs) like bismuth antimony telluride (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ show promise for spintronics due to efficient charge-to-spin conversion.
• Spin-orbit torques (SOTs) generated by TIs can manipulate ferromagnet (FM) magnetization, but their origin at TI/FM interfaces is not fully understood.
• Material intermixing and hybridized states at TI/FM interfaces are significant unexplored factors influencing SOT generation.

Purpose of the Study:

• To investigate the role of material intermixing in the generation of SOTs at (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$/FM interfaces.
• To explore how interface engineering, specifically reducing intermixing, affects SOT properties.
• To elucidate the mechanisms behind enhanced SOTs, including field-like torques and their relation to interface electronic states.

Main Methods:

• Interface chemical analysis techniques to quantify material intermixing.
• Spin-transfer ferromagnetic resonance (ST-FMR) measurements to probe SOT generation and dynamics.
• Fabrication of TI/FM heterostructures with and without a normal metal spacer layer.

Main Results:

• Material intermixing at the TI/FM interface significantly influences the generation and nature of SOTs.
• Insertion of a normal metal spacer reduces intermixing, improving TI properties at the interface.
• A dramatic enhancement of the field-like torque, surpassing the Oersted-field torque, was observed after interface engineering.

Conclusions:

• Intermixing is a critical factor in SOT generation at TI/FM interfaces.
• Interface engineering by reducing intermixing can significantly enhance SOT efficiency.
• The enhanced field-like torque is attributed to non-equilibrium spin density in Rashba-split surface bands and suppressed spin memory loss, with implications for other advanced heterostructures.