Magnetization switching by asymmetric topological surfaces

Recently, a study on spin-orbit torque switching in the magnetic topological material MnSb₂Te₄ was published in National Science Review (NSR). The research has pioneered new pathways for applying intrinsic magnetic topological materials in spintronic devices. The paper features Dr. Zihan Li (Department of Physics, Fudan University) as the first author, with Sheng Pan (Ph.D. candidate from the School of Physical Science and Technology, Soochow University) serving as co-first author. The corresponding authors are Professor Faxian Xiu from Fudan University's Department of Physics and Professor Jiexiang Yu from Soochow University's School of Physical Science and Technology.

With the continuous advancement of storage technologies, there is growing demand for enhanced storage capacity and read/write speeds. Spin-orbit torques (SOTs), as an effective electric switching approach, have great potential for breakthroughs both in the spintronic theory and in device designs. SOT systems usually need to have large spin orbit coupling, such as ferromagnetic/heavy metal, inversion-symmetry-broken interface, etc. It seems that the high spin-to-charge conversion efficiency and simplified structures with good stability and repeatability cannot be satisfied simultaneously.

The intrinsic magnetic topological insulator MnSb₂Te₄ is a magnetic material with a special electronic band structure, which shows high spin-charge conversion efficiency due to the spin-momentum locking. The research team conducted first principles calculations to get the Hall conductivity. Notably, the magnitude of the spin Hall conductivity is invariant across different spin configurations. In contrast, the anomalous Hall conductivity, derived from Berry curvature, is zero for all configurations within the gap. This behavior originates from the weakly asymmetric topological surface states which generate persistent spin currents. In experiments, high-quality MnSb₂Te₄ thin films have been synthesized by molecular beam epitaxy (MBE). The ferromagnetism in MnSb₂Te₄ with an out-of-plane anisotropy has been confirmed by magnetization measurements and transport measurements. When the dc bias current changes under the external field of -0.2 T, the anomalous Hall measurement shows a clockwise loop with two distinct states that remain well-preserved at zero current. The hysteresis behavior confirms the current-induced magnetization switching in a single MnSb₂Te₄ layer. (Figure. 1)

To fully understand the characteristics of SOT in MnSb₂Te₄, the research team tried to test the samples at different temperatures and magnetic fields. In the comparison of switching behaviors under positive and negative fields at 10 K, the anomalous Hall resistances show the opposite polarities. With increasing temperature, the anomalous Hall resistances and switching currents decrease, attributable to the simultaneous decrease in magnetization. These observations indicate that in-plane fields and dc currents can independently modulate the different stable magnetization states in MnSb₂Te₄. (Figure. 2)

To quantify the efficiency, the harmonic Hall effect measurements were used to characterize the effective field of SOT. The ac current can generate an alternating effective field, driving the magnetization to oscillate around its equilibrium position, and then give rise to a second-harmonic resistance. The spin Hall angle, which characterizes the SOT efficiency, can be obtained quantitatively by analyzing the second-harmonic signal. The spin Hall angle of the sample at 6 K is about 41, which is much larger than that in the traditional systems. (Figure. 3)

Furthermore, the research team prepared MnSb₂Te₄/FeTe_0.9 heterostructures, utilizing ferromagnetic/antiferromagnetic coupling to generate exchange bias. The exchange bias can generate an equivalent effective magnetic field, achieving the field-free switching. They successfully observed the exchange bias in the heterostructure and realized the SOT induced magnetization switching under zero magnetic field. (Figure. 4)

This study successfully predicted feasible magnetization switching in MnSb₂Te₄ utilizing SOT without the necessity of external spin generator layers through theoretical calculation and observed the switching in experiments. The spin Hall angle is about ~ 41 at 6 K, much larger than that in the traditional systems. The research team also achieved the field-free switching by constructing MnSb₂Te₄/FeTe_0.9 heterostructures. These results reveal a new mechanism of magnetic modulation in a single MnSb₂Te₄ layer, and show its application potential in spintronic devices.

 

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