The spin-orbit torque (SOT) that arises from materials with large spin-orbit coupling promises a path for ultralow power and fast magnetic-based storage and computational devices. We investigated the SOT from magnetron-sputtered BiSe thin films in BiSe/CoFeB heterostructures by using d.c. planar Hall and spin-torque ferromagnetic resonance (ST-FMR) methods. Remarkably, the spin torque efficiency (θ) was determined to be as large as 18.62 ± 0.13 and 8.67 ± 1.08 using the d.c. planar Hall and ST-FMR methods, respectively. Moreover, switching of the perpendicular CoFeB multilayers using the SOT from the BiSe was observed at room temperature with a low critical magnetization switching current density of 4.3 × 10 A cm. Quantum transport simulations using a realistic sp tight-binding model suggests that the high SOT in sputtered BiSe is due to the quantum confinement effect with a charge-to-spin conversion efficiency that enhances with reduced size and dimensionality. The demonstrated θ, ease of growth of the films on a silicon substrate and successful growth and switching of perpendicular CoFeB multilayers on BiSe films provide an avenue for the use of BiSe as a spin density generator in SOT-based memory and logic devices.

We investigated spin-to-charge conversion in sputtered Bi43Se57/Co20Fe60B20 heterostructures with in-plane magnetization at room temperature. High spin-to-charge conversion voltage signals have been observed at room temperature. The transmission electron microscope images show that the sputtered bismuth selenide thin films are nanogranular in structure. The spin-pumping voltage decreases with an increase in the size of the grains. The inverse Edelstein effect length (λIEE) is estimated to be as large as 0.32 nm. The large λIEE is due to the spin-momentum locking and is further enhanced by quantum confinement in the nanosized grains of the sputtered bismuth selenide films. We also investigated the effect on spin-pumping voltage due to the insertion of layers of MgO and Ag. The MgO insertion layer has almost completely suppressed the spin-pumping voltage, whereas the Ag insertion layer has enhanced the λIEE by 43%.

Efficient charge to spin conversion is important for low power spin logic devices. Spin and charge interconversion is commonly performed using heavy metals and topological insulators, while the field of oxides is not yet fully explored. Strontium iridate thin films were grown, where the different crystal structures form a perfect playground to understand the key factors in obtaining high charge to spin conversion efficiency (i.e., large spin Hall angle). It was found that the semiconducting Sr2IrO4 has a spin Hall angle of ~0.1 (depending on measurement technique), which is promising for a spin-orbit coupled electronic system and comparable to Pt. In contrast, the perovskite SrIrO3, reported to have a Dirac cone near the Fermi level, has a larger spin Hall angle of 0.3-0.4 degrees. The largest difference between the two materials is a large degree of spin-momentum locking in SrIrO3, comparable to known topological insulators. A simple semi-classical relationship is found where the spin Hall angle increases for higher degrees of spin-momentum locking and it also increases for lower Fermi wave vectors. This relationship is then able to explain the decreased spin Hall angle below 10 nm film thickness in SrIrO3, by relating it to the correspondingly higher carrier concentration (related to the higher Fermi wave vector). Breaking the commonly believed anti-correlation between resistivity and carrier concentration paves a pathway to lower power losses due to resistance while keeping large spin Hall angles.

Spin-orbit torque (SOT) induced magnetization switching has become a research focus in spintronics because it enables energy-efficient switching. There have been several experiments realizing field-free SOT-induced magnetization switching of materials with perpendicular magnetic anisotropy (PMA) in a bilayer system, either using thin Co(Fe) and CoFeB layers with interfacial PMA or using Co/Ni multilayers. All of these stacks are ferromagnets with large saturation magnetization (MS). Here, we demonstrate SOT switching in a multilayer stack of CoFeB/Gd/CoFeB. This stack shows a good PMA and a low MS (370 ± 20 emu/cm3), where CoFeB and Gd layers are antiferromagnetically exchange-coupled with each other. SOT induced magnetization switching has been demonstrated in this stack at zero magnetic field with a switching current density of ∼9.6 × 106 A/cm2 by using antiferromagnetic PtMn as the spin Hall channel material. The spin Hall angle of PtMn was also determined to be ∼0.084 ± 0.005 by performing a second harmonic Hall measurement. This layer structure is compatible with perpendicular magnetic tunnel junctions (p-MTJs), which could enable field-free three-terminal p-MTJs and lead to memory and logic devices based on SOT.

We investigated spin-to-charge current conversion in sputtered Y3Fe5O12 (YIG)/granular bismuth selenide (GBS) bi-layers at room temperature. The spin current is pumped to the GBS layer by the precession of magnetization at ferromagnetic resonance in the YIG layer. The spin-mixing conductance is determined to be as large as (13.64 ± 1.32) × 1018 m−2, which is larger than that of YIG/Pt and comparable or better than that of YIG/crystalline bismuth selenide indicating that GBS is a good spin-sink. The figure of merit of spin-to-charge conversion, the inverse Edelstein effect length (λIEE), is estimated to be as large as (0.11 ± 0.03) nm. λIEE shows GBS film thickness dependence, and its value is three times as large as in crystalline bismuth selenide. The λIEE value larger than that of crystalline bismuth selenide and other topological insulators indicates that the spin-to-charge conversion is due to the spin-momentum locking. As the thickness of GBS increases, λIEE decreases, which means the figure-of-merit of spin-to-charge conversion is influenced by grain size.

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We studied the spin-to-charge and charge-to-spin conversion at room temperature in sputteredWTe2-x (x=0.8)(t)/Co20Fe60B20(6 nm) heterostructures. Spin pumping measurements were used to characterize the spin-to-charge efficiency and the spin efficiency was calculated to be larger than ~0.035. Second harmonic Hall measurements were carried out to estimate the charge-to-spin conversion ratio. We found that the system exhibits a large field-like-torque (spin torque efficiency ~ 0.1) and small damping-like-torque (spin torque efficiency ~0.001) compared to that reported for heavy metals. High-resolution transmission electron microscopy images show that the WTe2-x layer is amorphous, which may enhance the spin swapping effect by inducing large interfacial spin orbit scattering, thus contributing to a large field like torque.

As spin-orbit-torque magnetic random-access memory (SOT-MRAM) is gathering great interest as the nextgeneration low-power and high-speed on-chip cache memory applications, it is critical to analyze the magnetic tunnel junction (MTJ) properties needed to achieve sub-ns, and ~fJ write operation when integrated with CMOS access transistors. In this paper, a 2T-1MTJ cell-level modeling framework for in-plane type Y SOT-MRAM suggests that high spin Hall conductivity and moderate SOT material sheet resistance are preferred. We benchmark write energy and speed performances of type Y SOT cells based on various SOT materials experimentally reported in the literature, including heavy metals, topological insulators and semimetals. We then carry out detailed benchmarking of SOT material Pt,-W, and BixSe(1-x) with different thickness and resistivity. We further discuss how our 2T-1MTJ model can be expanded to analyze other variations of SOT-MRAM, including perpendicular (type Z) and type X SOT-MRAM, two-terminal SOT-MRAM, as well as spin-transfer-torque (STT) and voltagecontrolled magnetic anisotropy (VCMA)-assisted SOT-MRAM. This work will provide essential guidelines for SOT-MRAM materials, devices, and circuits research in the future.