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1. Topological insulators for efficient spin–orbit torques

   https://doi.org/10.1063/5.0048619  

   Han, Jiahao; Liu, Luqiao (May 2021, APL Materials)

   Current-induced magnetic switching via spin–orbit torques has been extensively pursued for memory and logic applications with promising energy efficiency. Topological insulators are a group of materials with spin-momentum locked electronic states at the surface due to spin–orbit coupling, which can be harnessed to reach strong spin–orbit torques. In this paper, we summarize and compare the methods for calibrating the charge-spin conversion efficiency in topological insulators, with which topological insulators are identified as outstanding spin–orbit torque generators compared with the well-studied heavy metals. We then review the results of magnetic switching under reduced current density in topological insulator/ferromagnet heterostructures. Finally, we provide insights on current challenges as well as possible exploration directions in the emerging field of topological spintronics. 

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2. Nonlinear optical diode effect in a magnetic Weyl semimetal

   https://doi.org/10.1038/s41467-024-47291-8  

   Tzschaschel, Christian; Qiu, Jian-Xiang; Gao, Xue-Jian; Li, Hou-Chen; Guo, Chunyu; Yang, Hung-Yu; Zhang, Cheng-Ping; Xie, Ying-Ming; Liu, Yu-Fei; Gao, Anyuan; et al (December 2024, Nature Communications)

   Abstract Diode effects are of great interest for both fundamental physics and modern technologies. Electrical diode effects (nonreciprocal transport) have been observed in Weyl systems. Optical diode effects arising from the Weyl fermions have been theoretically considered but not probed experimentally. Here, we report the observation of a nonlinear optical diode effect (NODE) in the magnetic Weyl semimetal CeAlSi, where the magnetization introduces a pronounced directionality in the nonlinear optical second-harmonic generation (SHG). We demonstrate a six-fold change of the measured SHG intensity between opposite propagation directions over a bandwidth exceeding 250 meV. Supported by density-functional theory, we establish the linearly dispersive bands emerging from Weyl nodes as the origin of this broadband effect. We further demonstrate current-induced magnetization switching and thus electrical control of the NODE. Our results advance ongoing research to identify novel nonlinear optical/transport phenomena in magnetic topological materials and further opens new pathways for the unidirectional manipulation of light. 

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3. High‐Efficiency Magnon‐Mediated Magnetization Switching in All‐Oxide Heterostructures with Perpendicular Magnetic Anisotropy

   https://doi.org/10.1002/adma.202203038  

   Zheng, Dongxing; Lan, Jin; Fang, Bin; Li, Yan; Liu, Chen; Ledesma‐Martin, J. Omar; Wen, Yan; Li, Peng; Zhang, Chenhui; Ma, Yinchang; et al (July 2022, Advanced Materials)

   Abstract The search for efficient approaches to realize local switching of magnetic moments in spintronic devices has attracted extensive attention. One of the most promising approaches is the electrical manipulation of magnetization through electron‐mediated spin torque. However, the Joule heat generated via electron motion unavoidably causes substantial energy dissipation and potential damage to spintronic devices. Here, all‐oxide heterostructures of SrRuO₃/NiO/SrIrO₃are epitaxially grown on SrTiO₃single‐crystal substrates following the order of the ferromagnetic transition metal oxide SrRuO₃with perpendicular magnetic anisotropy, insulating and antiferromagnetic NiO, and metallic transition metal oxide SrIrO₃with strong spin–orbit coupling. It is demonstrated that instead of the electron spin torques, the magnon torques present in the antiferromagnetic NiO layer can directly manipulate the perpendicular magnetization of the ferromagnetic layer. This magnon mechanism may significantly reduce the electron motion‐related energy dissipation from electron‐mediated spin currents. Interestingly, the threshold current density to generate a sufficient magnon current to manipulate the magnetization is one order of magnitude smaller than that in conventional metallic systems. These findings suggest a route for developing highly efficient all‐oxide spintronic devices operated by magnon current. 

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4. Field-free spin-orbit switching of perpendicular magnetization enabled by dislocation-induced in-plane symmetry breaking

   https://doi.org/10.1038/s41467-023-41163-3  

   Liang, Yuhan; Yi, Di; Nan, Tianxiang; Liu, Shengsheng; Zhao, Le; Zhang, Yujun; Chen, Hetian; Xu, Teng; Dai, Minyi; Hu, Jia-Mian; et al (December 2023, Nature Communications)

   Abstract Current induced spin-orbit torque (SOT) holds great promise for next generation magnetic-memory technology. Field-free SOT switching of perpendicular magnetization requires the breaking of in-plane symmetry, which can be artificially introduced by external magnetic field, exchange coupling or device asymmetry. Recently it has been shown that the exploitation of inherent crystal symmetry offers a simple and potentially efficient route towards field-free switching. However, applying this approach to the benchmark SOT materials such as ferromagnets and heavy metals is challenging. Here, we present a strategy to break the in-plane symmetry of Pt/Co heterostructures by designing the orientation of Burgers vectors of dislocations. We show that the lattice of Pt/Co is tilted by about 1.2° when the Burgers vector has an out-of-plane component. Consequently, a tilted magnetic easy axis is induced and can be tuned from nearly in-plane to out-of-plane, enabling the field-free SOT switching of perpendicular magnetization components at room temperature with a relatively low current density (~10¹¹A/m²) and excellent stability (> 10⁴cycles). This strategy is expected to be applicable to engineer a wide range of symmetry-related functionalities for future electronic and magnetic devices. 

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5. Angular dependence of spin-orbit torque in monolayer ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$

   https://doi.org/10.1103/PhysRevB.108.144422  

   Xue, Fei; Stiles, Mark D.; Haney, Paul M. (October 2023, Physical Review B)

   In ferromagnetic systems lacking inversion symmetry, an applied electric field can control the ferromagnetic order parameters through the spin-orbit torque. The prototypical example is a bilayer heterostructure composed of a ferromagnet and a heavy metal that acts as a spin current source. In addition to such bilayers, spin-orbit coupling can mediate spin-orbit torques in ferromagnets that lack bulk inversion symmetry. A recently discovered example is the two-dimensional monolayer ferromagnet Fe3GeTe2. In this paper, we use first-principles calculations to study the spin-orbit torque and ensuing magnetic dynamics in this material. By expanding the torque versus magnetization direction as a series of vector spherical harmonics, we find that higher order terms (up to ℓ=4) are significant and play important roles in the magnetic dynamics. They give rise to deterministic, magnetic field-free electrical switching of perpendicular magnetization. 

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