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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%.

Current-induced spin-orbit torques (SOTs) in heavy metal/ferromagnet (HM/FM) systems have attracted considerable attention due to the efficient manipulation of the magnetization [1] , To accelerate the application of the SOT spintronic devices into the next generation logic computing and beyond, intensive efforts have been made to explore new materials and mechanisms of SOT. Of particular technological promises are the HMs with a large spin Hall angle θ SH . Pt/Co bilayers/alloys present ultralow resistivity at ~30 μΩ cm [2] , and an enhanced interfacial spin transparency, and thereby strengthen efficiency of the damping-like spin-orbit torque arising from the spin Hall effect in Pt [3] , Researchers concluded that Pt/Co multilayers show great potential for improving the thermal stability and efficient writability. However, it remains undetermined as to how the ferromagnetic thin layer Co modulates the SOTs from HM Pt layers.

We investigated temperature dependent current driven spin-orbit torques in magnetron sputtered ${\mathrm{Ru}}_{2}{\mathrm{Sn}}_{3}$ (4 and 10 nm)/${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ (5 nm) layered structures with in-plane magnetic anisotropy. The room temperature dampinglike and fieldlike spin torque efficiencies of the amorphous ${\mathrm{Ru}}_{2}{\mathrm{Sn}}_{3}$ films were measured to be $0.14\ifmmode\pm\else\textpm\fi{}0.008\phantom{\rule{0.16em}{0ex}}(0.07\ifmmode\pm\else\textpm\fi{}0.012)$ and $--0.03\ifmmode\pm\else\textpm\fi{}0.006\phantom{\rule{0.16em}{0ex}}(--0.20\ifmmode\pm\else\textpm\fi{}0.009)$, for the four (10 nm) films, respectively, by utilizing the second-harmonic Hall technique. The large fieldlike torque in the relatively thicker ${\mathrm{Ru}}_{2}{\mathrm{Sn}}_{3}$ (10 nm) thin film is unique compared to the traditional spin Hall materials interfaced with thick magnetic layers with in-plane magnetic anisotropy which typically have dominant dampinglike and negligible fieldlike torques. Additionally, the observed room temperature fieldlike torque efficiency in ${\mathrm{Ru}}_{2}{\mathrm{Sn}}_{3}$ (10 nm)/CoFeB (5 nm) is up to three times larger than the dampinglike torque ($--0.20\ifmmode\pm\else\textpm\fi{}0.009$ and $0.07\ifmmode\pm\else\textpm\fi{}0.012$, respectively) and 30 times larger at 50 K ($--0.29\ifmmode\pm\else\textpm\fi{}0.014$ and $0.009\ifmmode\pm\else\textpm\fi{}0.017$, respectively). The temperature dependence of the fieldlike torques shows dominant contributions from the intrinsic spin Hall effect while the dampinglike torques show dominate contributions from the extrinsic spin Hall effects, skew scattering, and side jump. Through macrospin calculations, we found that including fieldlike torques on the order of or larger than the dampinglike torque can reduce the switching critical current and decrease magnetization procession for a perpendicular ferromagnetic layer.

High spin-orbit torques (SOTs) generated by topological materials and heavy metals interfaced with a ferromagnetic layer show promise for next generation magnetic memory and logic devices. SOTs generated from the in-plane spin polarization along y-axis originated by the spin Hall and Edelstein effects can switch magnetization collinear with the spin polarization in the absence of external magnetic fields. However, an external magnetic field is required to switch the magnetization along x and z-axes via SOT generated by y-spin polarization. Here, we present that the above limitation can be circumvented by unconventional SOT in magnetron-sputtered thin film MnPd3. In addition to the conventional in-plane anti-damping-like torque due to the y-spin polarization, out-of-plane and in-plane anti-damping-like torques originating from z-spin and x-spin polarizations, respectively have been observed at room temperature. The spin torque efficiency (θ_y) corresponding to the y-spin polarization from MnPd3 thin films grown on thermally oxidized silicon substrate and post annealed at 400 ℃ is 0.34 - 0.44 while the spin conductivity (σ_zx^y) is ~ 5.70 – 7.30× 105 ℏ⁄2e Ω-1m-1. Remarkably, we have demonstrated complete external magnetic field-free switching of perpendicular Co layer via unconventional out-of-plane anti-damping-like torque from z-spin polarization. Based on the density functional theory calculations, we determine that the observed x- and z- spin polarizations with the in-plane charge current are due to the low symmetry of the (114) oriented MnPd3 thin films. Taken together, the new material reported here provides a path to realize a practical spin channel in ultrafast magnetic memory and logic devices.

A diverse set of SOT materials with vastly different values of spin efficiency, conductivity, and thickness are being explored to achieve the lowest write energy. Research on SOT-assisted STT-MRAM and novel materials for the switching of magnets with perpendicular magnetic anisotropy (PMA) is also ongoing. This paper presents a comprehensive study on the impact of material parameters on array-level read and write operations for both in-plane and PMA MRAM cells. The results offer important guidelines for material development for this technology.

We measure spin-orbit torques (SOTs) in a model system of all-epitaxial ferrite/Pt bilayers to gain insights into charge-spin interconversion in Pt. With negligible electronic conduction in the insulating ferrite, the crystalline Pt film acts as the sole source of charge-to-spin conversion. A small fieldlike SOT independent of Pt thickness suggests a weak Rashba-Edelstein effect at the ferrite/Pt interface. By contrast, we observe a sizable dampinglike SOT that depends on the Pt thickness, from which we deduce the dominance of an extrinsic spin-Hall effect (skew scattering) and Dyakonov-Perel spin relaxation in the crystalline Pt film. Furthermore, our results point to a large internal spin-Hall ratio of $\ensuremath{\approx}0.8$ in epitaxial Pt. Our experimental work takes an essential step towards understanding the mechanisms of charge-spin interconversion and SOTs in Pt-based heterostructures, which are crucial for power-efficient spintronic devices.

In this paper, we studied the annealing temperature effects on the spin Hall magnetoresistance (SMR) in W/CoFeB metallic bilayer systems. The bilayers were deposited using dc magnetron sputtering. The angle-dependent magnetoresistance was performed in a field of 8 T. The sample was rotated in the yz plane to isolate the SMR signal from the anisotropic magnetoresistance present in metallic systems. For annealing temperatures above 350 °C, we found the SMR signal reduces with an increase in annealing temperature. By measuring the SMR for samples with a varying W layer thickness we were able to find that the spin Hall angle decreases as annealing temperature increases and the spin diffusion length for W remains almost constant with annealing temperature.

The spin-orbit torque (SOT) arising from materials with large spin-orbit coupling promises a path for ultra-low power and fast magnetic-based storage and computational devices. We investigated the SOT from magnetron-sputtered BixSe(1-x) thin films in BixSe(1-x)/CoFeB heterostructures by using a dc planar Hall method. Remarkably, the spin Hall angle (SHA) was found to be as large as 18.83, which is the largest ever reported at room temperature (RT). Moreover, switching of a perpendicular CoFeB multilayer using SOT from the BixSe(1-x) has been observed with the lowest-ever switching current density reported in a bilayer system: 2.3 * 105 A/cm2 at RT. The giant SHA, smooth surface, ease of growth of the films on silicon substrate, successful growth and switching of a perpendicular CoFeB multilayer on BixSe(1-x) film opens a path for use of BixSe(1-x) topological insulator (TI) material as a spin-current generator in SOT-based memory and logic devices.

Topological insulator (TI) materials are exciting candidates for integration into next-generation memory and logic devices because of their potential for efficient, low-energy-consumption switching of magnetization. Specifically, the family of bismuth chalcogenides offers efficient spin-to-charge conversion because of its large spin-orbit coupling and spin-momentum locking of surface states. However, a major obstacle to realizing the promise of TIs is the thin-film materials' quality, which lags behind that of epitaxially grown semiconductors. In contrast to the latter systems, the Bi-chalcogenides form by van der Waals epitaxy, which allows them to successfully grow onto substrates with various degrees of lattice mismatch. This flexibility enables the integration of TIs into heterostructures with emerging materials, including two-dimensional materials. However, understanding and controlling local features and defects within the TI films is critical to achieving breakthrough device performance. Here, we report observations and modeling of large-scale structural defects in (Bi,Sb)$_2$Te$_3$ films grown onto hexagonal BN, highlighting unexpected symmetry-breaking rotations within the films and the coexistence of a second phase along grain boundaries. Using first-principles calculations, we show that these defects could have consequential impacts on the devices that rely on these TI films, and therefore they cannot be ignored.

Topological materials with large spin-orbit coupling and immunity to disorder-induced symmetry breaking show great promise for efficiently converting charge to spin. Here, we report that long-range disordered sputtered WTex thin films exhibit local chemical and structural order as those of Weyl semimetal WTe2 and conduction behavior that is consistent with semi-metallic Weyl fermion. We find large charge-to-spin conversion properties and electrical conductivity in thermally annealed sputtered WTex films that are comparable with those in crystalline WTe2 flakes. Besides, the strength of unidirectional spin Hall magnetoresistance in annealed WTex/Mo/CoFeB heterostructure is 5 to 20 times larger than typical SOT layer/ferromagnet heterostructures reported at room temperature. We further demonstrate room temperature damping-like SOT-driven magnetization switching of in-plane magnetized CoFeB. These large charge-to-spin conversion properties that are robust in the presence of long-range disorder and thermal annealing pave the way for industrial application of a new class of sputtered semimetals.

Topological insulators have recently shown great promise for ultralow-power spin-orbit torque (SOT) devices thanks to their large charge-to-spin conversion efficiency originating from the spin-momentum-locked surface states. Recently, Weyl semimetals emerge as an alternative SOT source with spin-polarized surface as well as bulk states, robustness against magnetic and structural disorder, and higher electrical conductivity for integration in metallic magnetic tunnel junctions. Here, we report that long-range disordered sputtered WTex thin films exhibit local chemical and structural order as those of Weyl semimetal WTe2 and conduction behavior that is consistent with semi-metallic Weyl fermion. We find large charge-to-spin conversion properties in thermally annealed sputtered WTex films that are comparable with those in crystalline WTe2 flakes. Besides, the strength of unidirectional spin Hall magnetoresistance in annealed WTex/Mo/CoFeB heterostructure is 5 to 20 times larger than typical SOT/ferromagnet bilayers reported at room temperature. We further demonstrate room temperature field-free magnetization switching at a current density of 1 – 2 MA/cm2. These large charge-to-spin conversion properties that are robust in the presence of long-range disorder and thermal annealing pave the way for industrial integration of such topological materials. Our results open a new class of sputtered semimetals for memory and computing based on magnetic tunnel junctions as well as broader planar heterostructures consisting of SOT layer/ferromagnet interfaces.

600 mV magneto-electric switching in 30 nm La-doped BiFeO 3 multiferroic oxide and a proof of concept 7 μV spin-orbit signal output in Pt / CoFe local spin injection device with 100 μA supply current were experimentally demonstrated at room temperature for the 1st time. Also demonstrated was a path towards 70 mV spin orbit output using Bi 2 Se 3 in a local spin injection device. These are key accomplishments for WRITE and READ building blocks, respectively, toward realization of a magneto-electric spin-orbit (MESO) energy-efficient logic device. Moreover, the 1st generation of a MESO logic device with a functional READ unit is demonstrated.

As spin-orbit-torque magnetic random-access memory (SOT-MRAM) is gathering great interest as the next-generation 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, β-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 voltage-controlled magnetic anisotropy (VCMA)-assisted SOT-MRAM. This work will provide essential guidelines for SOT-MRAM materials, devices, and circuits research in the future.

Author(s): Everhardt, AS; Dc, M; Huang, X; Sayed, S; Gosavi, TA; Tang, Y; Lin, CC; Manipatruni, S; Young, IA; Datta, S; Wang, JP; Ramesh, R | Abstract: 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 semiclassical 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 anticorrelation between resistivity and carrier concentration paves a pathway to lower power losses due to resistance while keeping large spin Hall angles.

We present the Spin Hall Effect (SHE) Computational Random Access Memory (CRAM) for in-memory computation, incorporating considerations at the device, gate, and functional levels. For two specific applications (2-D convolution and neuromorphic digit recognition), we show that SHE-CRAM is 3x faster and has over 4x lower energy than a prior STT-based CRAM implementation, and is over 2000x faster and at least 130x more energy-efficient than state-of-the-art near-memory processing.

Despite the great promise for very efficient and fast switching of magnetization in embedded memory and computing applications, the performance of spin–orbit torque (SOT) lags behind conventional technologies due to the low spin-Hall conductivity of the spin Hall materials. This work reports an advantageous spin Hall material, periodic [Pt/Co]n multilayer, which combines a low resistivity with a widely tunable spin Hall effect along with magnetization as evidenced with an in-plane CoFeB ferromagnetic detector. Three detection methods have been employed to illustrate the trends of magnetic orientation, interlayer exchange coupling, spin transport, and SOT efficiency as a function of Co thickness, which casts insight into the mechanisms of the SOTs in the [Pt/Co]n multilayer. With the varying Co thickness in the [Pt/Co]n multilayer, it is found that the damping-like torque efficiency is negative and the field-like torque efficiency is 8.2–31.5 times larger. The [Pt/Co]n multilayers have two spin reorientation transition states where the spin Hall angle θSH is maximized with a low resistivity of ∼ 40 μΩ cm, at tCo = 0.507 nm and 0.159 nm. We simulated the magnetization trajectories and time-domain responses of SOT switching with a current pulse and demonstrated a much faster switching in the spin reorientation transition states based on the coupled Landau–Lifshitz–Gilbert equation.

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.

Several emerging spintronic devices have recently been proposed, performing computation by (a) generating spin currents based on input magnet states to switch an output magnet state using Spin-Transfer Torque (STT) [1,2], (b) using multiple nanopillars to drive a domain wall (DW) that switches an output nanopillar using STT [7], and (c) using magnetoelectric (ME) switching at the input, combined with DW automotion, to switch an output state [3]. All of these devices have delays of several nanoseconds. The energy for (a) and (b) is in the range of femtoJoules, while the ME mechanism in (c) facilitates greater energy-efficiency, in the aJ range. These numbers fall some distance away from CMOS, where gate delays and switching energies are in the range of picoseconds (ps) and attoJoules (aJ), respectively.

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.

The spin orbital torques in Ta/CoFeB/MgO structures are experimentally investigated utilizing the planar Hall effect and magnetoresistance measurement. By angular field characterization of the planar Hall resistance at ±current, the differential resistance which is directly related to the spin orbital torques is derived. Upon curve fitting of the analytical formulas over the experimental results, it is found that the anti-damping torque, also known as spin Hall effect, is sizable while a negligible field-like torque is observed. A spin Hall angle of about 18 ± 0.6% is obtained for the Ta layer. Temperature dependent study of the spin orbital torques is also performed. It is found that temperature does not significantly modify the spin Hall angle. By cooling down the sample down to 100 K, the obtained spin Hall angle has a maximum value of about 20.5 ± 0.43%.