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1. Monolayer WSe2 induced giant enhancement in the spin Hall efficiency of Tantalum

   https://doi.org/10.1038/s41699-020-0153-z  

   Debashis, Punyashloka ; Hung, Terry Y. T. ; Chen, Zhihong ( June 2020 , npj 2D Materials and Applications)

   Spin Orbit Torque Magnetic RAM (SOT-MRAM) is emerging as a promising memory technology owing to its high endurance, reliability and speed. A critical factor for its success is the development of materials that exhibit efficient conversion of charge current to spin current, characterized by their spin Hall efficiency. In this work, it is experimentally demonstrated that the spin Hall efficiency of the industrially relevant ultra-thin Ta can be enhanced by more than 25× when a monolayer (ML) WSe₂is inserted as an underlayer. The enhancement is attributed to spin absorption at the Ta/WSe₂interface, suggested by harmonic Hall measurements. The presentedmore »hybrid spin Hall stack with a 2D WSe₂underlayer has a total body thickness of less than 2 nm and exhibits greatly enhanced spin Hall efficiency, which makes this hybrid a promising candidate for energy efficient SOT-MRAM.

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2. Field-free spin-orbit torque-induced switching of perpendicular magnetization in a ferrimagnetic layer with a vertical composition gradient

   https://doi.org/10.1038/s41467-021-24854-7  

   Zheng, Zhenyi ; Zhang, Yue ; Lopez-Dominguez, Victor ; Sánchez-Tejerina, Luis ; Shi, Jiacheng ; Feng, Xueqiang ; Chen, Lei ; Wang, Zilu ; Zhang, Zhizhong ; Zhang, Kun ; et al ( July 2021 , Nature Communications)

   Current-induced spin-orbit torques (SOTs) are of interest for fast and energy-efficient manipulation of magnetic order in spintronic devices. To be deterministic, however, switching of perpendicularly magnetized materials by SOT requires a mechanism for in-plane symmetry breaking. Existing methods to do so involve the application of an in-plane bias magnetic field, or incorporation of in-plane structural asymmetry in the device, both of which can be difficult to implement in practical applications. Here, we report bias-field-free SOT switching in a single perpendicular CoTb layer with an engineered vertical composition gradient. The vertical structural inversion asymmetry induces strong intrinsic SOTs and amore »gradient-driven Dzyaloshinskii–Moriya interaction (g-DMI), which breaks the in-plane symmetry during the switching process. Micromagnetic simulations are in agreement with experimental results, and elucidate the role of g-DMI in the deterministic switching processes. This bias-field-free switching scheme for perpendicular ferrimagnets with g-DMI provides a strategy for efficient and compact SOT device design.

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3. Millimetre-long transport of photogenerated carriers in topological insulators

   https://doi.org/10.1038/s41467-019-13711-3  

   Hou, Yasen ; Wang, Rui ; Xiao, Rui ; McClintock, Luke ; Clark Travaglini, Henry ; Paulus Francia, John ; Fetsch, Harry ; Erten, Onur ; Savrasov, Sergey Y. ; Wang, Baigeng ; et al ( December 2019 , Nature Communications)

   Excitons are spin integer particles that are predicted to condense into a coherent quantum state at sufficiently low temperature. Here by using photocurrent imaging we report experimental evidence of formation and efficient transport of non-equilibrium excitons in Bi_2-xSb_xSe₃nanoribbons. The photocurrent distributions are independent of electric field, indicating that photoexcited electrons and holes form excitons. Remarkably, these excitons can transport over hundreds of micrometers along the topological insulator (TI) nanoribbons before recombination at up to 40 K. The macroscopic transport distance, combined with short carrier lifetime obtained from transient photocurrent measurements, indicates an exciton diffusion coefficient at least 36 m² s⁻¹, which correspondsmore »to a mobility of 6 × 10⁴ m² V⁻¹ s⁻¹at 7 K and is four order of magnitude higher than the value reported for free carriers in TIs. The observation of highly dissipationless exciton transport implies the formation of superfluid-like exciton condensate at the surface of TIs.

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4. Giant field-like torque by the out-of-plane magnetic spin Hall effect in a topological antiferromagnet

   https://doi.org/10.1038/s41467-021-26453-y  

   Kondou, Kouta ; Chen, Hua ; Tomita, Takahiro ; Ikhlas, Muhammad ; Higo, Tomoya ; MacDonald, Allan H. ; Nakatsuji, Satoru ; Otani, YoshiChika ( November 2021 , Nature Communications)

   Spin-orbit torques (SOT) enable efficient electrical control of the magnetic state of ferromagnets, ferrimagnets and antiferromagnets. However, the conventional SOT has severe limitation that only in-plane spins accumulate near the surface, whether interpreted as a spin Hall effect (SHE) or as an Edelstein effect. Such a SOT is not suitable for controlling perpendicular magnetization, which would be more beneficial for realizing low-power-consumption memory devices. Here we report the observation of a giant magnetic-field-like SOT in a topological antiferromagnet Mn₃Sn, whose direction and size can be tuned by changing the order parameter direction of the antiferromagnet. To understand the magneticmore »SHE (MSHE)- and the conventional SHE-induced SOTs on an equal footing, we formulate them as interface spin-electric-field responses and analyzed using a macroscopic symmetry analysis and a complementary microscopic quantum kinetic theory. In this framework, the large out-of-plane spin accumulation due to the MSHE has an inter-band origin and is likely to be caused by the large momentum-dependent spin splitting in Mn₃Sn. Our work demonstrates the unique potential of antiferromagnetic Weyl semimetals in overcoming the limitations of conventional SOTs and in realizing low-power spintronics devices with new functionalities.

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5. Spin memory of the topological material under strong disorder

   https://doi.org/10.1038/s41535-020-0241-5  

   Korzhovska, Inna ; Deng, Haiming ; Zhao, Lukas ; Deshko, Yury ; Chen, Zhiyi ; Konczykowski, Marcin ; Zhao, Shihua ; Raoux, Simone ; Krusin-Elbaum, Lia ( June 2020 , npj Quantum Materials)

   Robustness to disorder is the defining property of any topological state. The ultimate disorder limits to topological protection are still unknown, although a number of theories predict that even in the amorphous state a quantized conductance might yet reemerge. Here we report that in strongly disordered thin films of the topological material Sb₂Te₃disorder-induced spin correlationsdominate transport of charge—they engender a spin memory phenomenon, generated by the nonequilibrium charge currents controlled by localized spins. We directly detect a glassy yet robust disorder-induced magnetic signal in filmsfree of extrinsic magnetic dopants, which becomes null in a lower-disorder crystalline state. This ismore »where large isotropic negative magnetoresistance (MR)—a hallmark of spin memory—crosses over to positive MR, first with only one e²/hquantum conduction channel, in a weakly antilocalized diffusive transport regime with a 2D scaling characteristic of the topological state. A fresh perspective revealed by our findings is that spin memory effect sets a disorder threshold to the protected topological state. It also points to new possibilities of tuning spin-dependent charge transport by disorder engineering of topological materials.

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