Recently, the research group of Prof. Xiu Faxian from the Department of Physics at Fudan University, in collaboration with other researchers, achieved magnetization switching based on asymmetric topological surface states in the magnetic topological material MnSb₂Te₄. On May 9, 2025, the related findings were published online in the journal National Science Review under the title "Magnetization Switching by Asymmetric Topological Surfaces" [Natl. Sci. Rev., nwaf178 (2025)]. This work was a collaborative effort involving Prof. Xiu Faxian's group at Fudan University's Department of Physics, Prof. Yu Jiexiang's group at Soochow University's School of Physical Science and Technology, and researchers Li Ang and Prof. Han Xiaodong at Beijing University of Technology.
With the continuous advancement of storage technologies, the demand for higher storage capacity and read/write speeds continues to grow. In magnetic storage, Spin-Orbit Torque (SOT), an effect that induces magnetization switching via electric current, holds significant potential for breakthroughs in spintronics theory and the optimization of device design. Conventional SOT systems typically require materials with strong spin-orbit coupling, such as interfaces between ferromagnetic/heavy metals or materials with broken inversion symmetry. However, these classic systems often struggle to achieve high spin-to-charge conversion efficiency while simplifying device structure. Consequently, the search for novel materials that combine both spin current generation capability and intrinsically tunable magnetism has become a major research focus.
The intrinsic magnetic topological insulator MnSb₂Te₄, possessing a unique band structure, exhibits magnetism while its topologically protected surface states, characterized by spin-momentum locking, offer extremely high spin-to-charge conversion efficiency. The research team conducted first-principles calculations on the material's band structure. As shown in Fig. 1(a), its spin Hall conductivity is unaffected by the direction of the Mn magnetic moment and remains constant within the band gap. This originates from its asymmetric topological surface states and enables the attainment of exceptionally high spin-to-charge conversion efficiency. The constant spin conductivity implies a stable generation of spin current, which can further facilitate SOT-induced magnetization switching.
The research team synthesized high-quality MnSb₂Te₄ thin-film single crystals using molecular beam epitaxy, with the X-ray diffraction pattern shown in Fig. 1(b). Utilizing micro/nano-fabrication processes, the samples were patterned into Hall bar structures suitable for transport measurements. By applying an in-plane magnetic field and injecting pulsed current into the device, the anomalous Hall resistance was measured to determine the magnetization switching behavior. Experimentally, it was observed that as the pulsed current amplitude increased, the anomalous Hall resistance changed correspondingly until saturation. When the current direction was reversed and the amplitude increased, the trend of resistance change reversed. Scanning the current forward and backward revealed a window similar to a hysteresis loop. Figures 1(c) and (d) show that as the applied magnetic field increases, the magnetization becomes increasingly pinned, and the window size of the current-resistance loop gradually decreases. When the direction of the applied field is reversed, the loop polarity also completely reverses, changing from counter-clockwise under a positive field to clockwise under a negative field. This phenomenon demonstrates that the magnetization in the MnSb₂Te₄ thin film can be directly controlled by electric current, achieving SOT-induced magnetization switching.
Subsequently, the team quantitatively characterized the SOT effect in the sample using second harmonic Hall measurements. When a small amplitude alternating current (AC) is passed through the sample, the resulting SOT effect causes the magnetization to oscillate around its equilibrium position, generating a second-order signal in the Hall measurement. Analysis of this signal allows for the quantitative determination of the spin Hall angle, a key parameter characterizing SOT efficiency. By measuring the second harmonic signal under different magnetic field angles and current amplitudes, and fitting the obtained SOT effective field (Fig. 1(e)), the spin Hall angle at 6 K was determined to be approximately 41, significantly larger than that in conventional systems.
Following the demonstration of SOT switching in the MnSb₂Te₄ film, the research team further fabricated a MnSb₂Te₄/FeTe₀.₉ heterostructure, leveraging ferromagnetic/antiferromagnetic coupling to induce exchange bias. Exchange bias creates an equivalent effective magnetic field without an external field, enabling SOT switching in the absence of an applied magnetic field. The team successfully observed exchange bias in the heterostructure and achieved SOT-induced magnetization switching at zero magnetic field (Fig. 1(f)).
Fig. 1: (a) Spin Hall conductivity calculated for different Mn magnetization directions. (b) X-ray diffraction pattern of the MnSb₂Te₄ thin film. (c) and (d) Switching behavior under different magnetic fields at 14 K. (e) Fitting of the SOT effective field to obtain the spin Hall angle value. (f) Current-induced magnetization switching at zero field in the MnSb₂Te₄/FeTe₀.₉ heterostructure.
This work is the first to theoretically predict the possibility of intrinsic spin-current-induced magnetization switching in MnSb₂Te₄ and experimentally observe the phenomenon. Furthermore, it achieved zero-field magnetization switching by constructing a heterostructure. These results reveal a novel mechanism for magnetic manipulation within a single MnSb₂Te₄ layer and demonstrate its potential for applications in spintronic devices.
This research received substantial support and funding from the Department of Physics at Fudan University, the State Key Laboratory of Surface Physics, the National Natural Science Foundation of China (Young Scientists Fund A, Original Exploration Program Project), and the Shanghai Basic Research Special Zone Program. The Department of Physics at Fudan University is the first affiliation of the paper. Prof. Xiu Faxian from Fudan University and Prof. Yu Jiexiang from Soochow University are the corresponding authors. Dr. Li Zihan (now a postdoctoral fellow at Shanghai Jiao Tong University), a former Ph.D. student in Prof. Xiu's group, and Pan Sheng, a Ph.D. student in Prof. Yu's group, are the co-first authors.
Prof. Xiu Faxian's research group primarily focuses on the growth and quantum control of topological materials, as well as device research on novel low-dimensional atomic crystal materials. In the field of topological Dirac materials, the group is dedicated to the growth of novel quantum materials, property measurements, and the fabrication and characterization of quantum devices. For novel low-dimensional atomic crystal materials, research concentrates on their electrical, magnetic, and optoelectronic properties.
Paper Link:https://doi.org/10.1093/nsr/nwaf178
