The current-induced magnetization switching by spin-orbit torque (SOT) is an important ingredient for modern non-volatile magnetic devices, like magnetic random access memories and logic devices that are required for high-performance data storage and computing. Because of this, researchers around the world are actively searching for novel ways to reduce the present high switching current density in order to achieve highly efficient SOT-driven magnetization switching. Researchers from the National University of Singapore (NUS) have recently made a significant breakthrough in this field of research.
Yang Hyunsoo, Associate Professor from the Department of Electrical and Computer Engineering and leader of the team, led this research to successfully demonstrate the first instance of room temperature magnetization switching driven by giant SOTs in topological insulator/conventional ferromagnetic (Bi2Se3/NiFe) heterostructures with a low current density. This can address the issue of scalability and high power consumption needed in modern spintronic devices.
Professor Yang said: “Our findings can solve the fundamental obstacle of a high switching current in present heavy metal based SOT applications, and this is a big step towards room temperature topological insulator based spintronic device applications with ultralow power dissipation and high integration density. We believe our work will greatly invigorate topological insulator-based global research activities from diverse disciplines."
Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator, but also support conducting states on their surface, possessing a strong spin-orbit coupling and spin-momentum-locked topological surface states (TSS), on which the electron momentum and spin polarization directions are strongly locked.
"Due to spin-momentum-locked properties, as charge current flows in the TSS, all electron spins will be fully polarised in a perpendicular direction to the direction of the moving electron. Therefore, a very high efficient spin current generation and thus a giant SOT efficiency is expected in topological insulators," explained Dr. Zhu Dapeng, who is a co-first author of the study and a Research Fellow at the Department.
Taking advantage of TSS is a major part of realizing high-performance topological insulator based SOT devices. But in typical topological insulators like Bi2Se3, the parasitic bulk states and 2D electron gas can contaminate and/or wash out the high SOT efficiently in TSS dominated SOT effect in ultrathin Bi2Se₃ films, exhibiting a large SOT efficiency up to 1.75 at room temperature, much larger than the values of ~0.01-0.3 in conventional used heavy metals.
In traditional heavy metal/ferromagnet SOT devices, the current density required for the magnetization switching is still high in the order of ~107-108 A/cm2 which hinders use in high-performance SOT applications.
The team demonstrated the high-efficient current-induced magnetization switching at room temperature using topological insulator Bi2Se3 with a conventional 3D ferromagnet NiFe (6nm), which is widely utilized in various industries.
"Our work successfully presents a significant reduction of switching current density for the magnetization switching by utilizing the giant SOT effect in Bi2Se3. The value is about 6×105A/cm2, which is almost two orders of magnitude smaller than that of heavy metals. This is a major milestone for the ultralow power consumption and high integration density SOT device applications. Moreover, our devices work robustly at room temperature, which breaks out the limit of ultralow working temperature in previous TI devices," said Dr. Wang Yi of the Department, who is the other co-first author of the study.
"Our magnetization switching scheme does not require an assistive magnetic field. This makes the topological insulator/ferromagnet material systems easy to integrate into the well-established industrial technology for magnetic devices," said Assoc Prof Yang.
From here on out, Yang and his team are conducting experiments to further decrease the switching current by further refining the materials and structures of the systems. They are also planning to incorporate and test the technology in core magnetic memory devices. The team hopes to work with industry partners to further explore various applications with the novel material systems.
The paper on this research was published in Nature Communications.