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1. Strong Damping‐Like Spin‐Orbit Torque and Tunable Dzyaloshinskii–Moriya Interaction Generated by Low‐Resistivity Pd 1− x Pt x Alloys

   https://doi.org/10.1002/adfm.201805822  

   Zhu, Lijun ; Sobotkiewich, Kemal ; Ma, Xin ; Li, Xiaoqin ; Ralph, Daniel C. ; Buhrman, Robert A. ( February 2019 , Advanced Functional Materials)

   Despite their great promise for providing a pathway for very efficient and fast manipulation of magnetization, spin‐orbit torque (SOT) operations are currently energy inefficient due to a low damping‐like SOT efficiency per unit current bias, and/or the very high resistivity of the spin Hall materials. This work reports an advantageous spin Hall material, Pd_1−xPt_x, which combines a low resistivity with a giant spin Hall effect as evidenced with three independent SOT ferromagnetic detectors. The optimal Pd_0.25Pt_0.75alloy has a giant internal spin Hall ratio of >0.60 (damping‐like SOT efficiency of ≈0.26 for all three ferromagnets) and a low resistivity of ≈57.5 µΩ cm at a 4 nm thickness. Moreover, it is found that the Dzyaloshinskii–Moriya interaction (DMI), the key ingredient for the manipulation of chiral spin arrangements (e.g., magnetic skyrmions and chiral domain walls), is considerably strong at the Pd_1−xPt_x/Fe_0.6Co_0.2B_0.2interface when compared to that at Ta/Fe_0.6Co_0.2B_0.2or W/Fe_0.6Co_0.2B_0.2interfaces and can be tuned by a factor of 5 through control of the interfacial spin‐orbital coupling via the heavy metal composition. This work establishes a very effective spin current generator that combines a notably high energy efficiency with a very strong and tunable DMI for advanced chiral spintronics and spin torque applications.

    

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2. Giant bulk spin–orbit torque and efficient electrical switching in single ferrimagnetic FeTb layers with strong perpendicular magnetic anisotropy

   https://doi.org/10.1063/5.0087260  

   Liu, Qianbiao ; Zhu, Lijun ; Zhang, Xiyue S. ; Muller, David A. ; Ralph, Daniel C. ( June 2022 , Applied Physics Reviews)

   Efficient manipulation of antiferromagnetically coupled materials that are integration-friendly and have strong perpendicular magnetic anisotropy (PMA) is of great interest for low-power, fast, dense magnetic storage and computing. Here, we report a distinct, giant bulk damping-like spin–orbit torque in strong-PMA ferrimagnetic Fe 100− x Tb x single layers that are integration-friendly (composition-uniform, amorphous, and sputter-deposited). For sufficiently thick layers, this bulk torque is constant in the efficiency per unit layer thickness, [Formula: see text]/ t, with a record-high value of 0.036 ± 0.008 nm −1 , and the damping-like torque efficiency [Formula: see text] achieves very large values for thick layers, up to 300% for 90 nm layers. This giant bulk torque by itself switches tens of nm thick Fe 100− x Tb x layers that have very strong PMA and high coercivity at current densities as low as a few MA/cm 2 . Surprisingly, for a given layer thickness, [Formula: see text] shows strong composition dependence and becomes negative for composition where the total angular momentum is oriented parallel to the magnetization rather than antiparallel. Our findings of giant bulk spin torque efficiency and intriguing torque-compensation correlation will stimulate study of such unique spin–orbit phenomena in a variety of ferrimagnetic hosts. This work paves a promising avenue for developing ultralow-power, fast, dense ferrimagnetic storage and computing devices. 

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3. Efficient Spin‐Orbit Torque Switching of the Semiconducting Van Der Waals Ferromagnet Cr 2 Ge 2 Te 6

   https://doi.org/10.1002/adma.201906021  

   Ostwal, Vaibhav ; Shen, Tingting ; Appenzeller, Joerg ( January 2020 , Advanced Materials)

   Being able to electrically manipulate the magnetic properties in recently discovered van der Waals ferromagnets is essential for their integration in future spintronics devices. Here, the magnetization of a semiconducting 2D ferromagnet, i.e., Cr₂Ge₂Te₆, is studied using the anomalous Hall effect in Cr₂Ge₂Te₆/tantalum heterostructures. The thinner the flakes, hysteresis and remanence in the magnetization loop with out‐of‐plane magnetic fields become more prominent. In order to manipulate the magnetization in such thin flakes, a combination of an in‐plane magnetic field and a charge current flowing through Ta—a heavy metal exhibiting giant spin Hall effect—is used. In the presence of in‐plane fields of 20 mT, charge current densities as low as 5 × 10⁵A cm^–2are sufficient to switch the out‐of‐plane magnetization of Cr₂Ge₂Te₆. This finding highlights that current densities required for spin‐orbit torque switching of Cr₂Ge₂Te₆are about two orders of magnitude lower than those required for switching nonlayered metallic ferromagnets such as CoFeB. The results presented here show the potential of 2D ferromagnets for low‐power memory and logic applications.

    

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4. Perspectives on field-free spin–orbit torque devices for memory and computing applications

   https://doi.org/10.1063/5.0135185  

   Lopez-Dominguez, Victor ; Shao, Yixin ; Khalili Amiri, Pedram ( January 2023 , Journal of Applied Physics)

   The emergence of embedded magnetic random-access memory (MRAM) and its integration in mainstream semiconductor manufacturing technology have created an unprecedented opportunity for engineering computing systems with improved performance, energy efficiency, lower cost, and unconventional computing capabilities. While the initial interest in the existing generation of MRAM—which is based on the spin-transfer torque (STT) effect in ferromagnetic tunnel junctions—was driven by its nonvolatile data retention and lower cost of integration compared to embedded Flash (eFlash), the focus of MRAM research and development efforts is increasingly shifting toward alternative write mechanisms (beyond STT) and new materials (beyond ferromagnets) in recent years. This has been driven by the need for better speed vs density and speed vs endurance trade-offs to make MRAM applicable to a wider range of memory markets, as well as to utilize the potential of MRAM in various unconventional computing architectures that utilize the physics of nanoscale magnets. In this Perspective, we offer an overview of spin–orbit torque (SOT) as one of these beyond-STT write mechanisms for the MRAM devices. We discuss, specifically, the progress in developing SOT-MRAM devices with perpendicular magnetization. Starting from basic symmetry considerations, we discuss the requirement for an in-plane bias magnetic field which has hindered progress in developing practical SOT-MRAM devices. We then discuss several approaches based on structural, magnetic, and chiral symmetry-breaking that have been explored to overcome this limitation and realize bias-field-free SOT-MRAM devices with perpendicular magnetization. We also review the corresponding material- and device-level challenges in each case. We then present a perspective of the potential of these devices for computing and security applications beyond their use in the conventional memory hierarchy. 

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5. Experimental Demonstration of an Extreme Subwavelength Nanomagnetic Acoustic Antenna Actuated by Spin–Orbit Torque from a Heavy Metal Nanostrip

   https://doi.org/10.1002/admt.201901076  

   Abeed, Md Ahsanul ; Bandyopadhyay, Supriyo ( February 2020 , Advanced Materials Technologies)

   A novel on‐chip extreme subwavelength “acoustic antenna” whose radiation efficiency is ≈50 times larger than the theoretical limit for a resonantly driven antenna is demonstrated. The antenna is composed of magnetostrictive nanomagnets deposited on a piezoelectric substrate. The nanomagnets are partially in contact with a heavy metal (Pt) nanostrip. Passage of alternating current through the nanostrip exerts alternating spin–orbit torque on the nanomagnets and periodically rotates their magnetizations. During the rotation, the magnetostrictive nanomagnets expand and contract, thereby setting up alternating tensile and compressive strain in the piezoelectric substrate underneath. This leads to the generation of a surface acoustic wave in the substrate and makes the nanomagnet assembly act as an acoustic antenna. The measured radiation efficiency of this acoustic antenna at the detected frequency is ≈1%, while the wavelength to antenna dimension ratio is ≈67:1. For a standard antenna driven at acoustic resonance, the efficiency would have been limited to ≈(1/67)²= 0.02%. It became possible to beat that limit (by ≈50 times) via actuating the antenna not at acoustic resonance, but by using a completely different mechanism involving spin–orbit torque originating from the giant spin Hall effect in Pt.

    

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