Spin Current Source Research Articles

Enhanced Spin-Orbit Torque Efficiency in Platinum-Gadolinium Oxide Nanocomposite Films.

Spin-orbit torque (SOT) has emerged as an effective means of manipulating magnetization. However, the current energy efficiency of SOT operation is inefficient due to low damping-like SOT efficiency per unit current bias. In this work, we dope conventional rare earth oxides, GdOy, into highly conductive platinum by magnetron sputtering to form a new group of spin Hall materials. A large damping-like spin-orbit torque (DL-SOT) efficiency of about 0.35 ± 0.013 is obtained in Pt0.70(GdOy)0.30 measured by the spin-torque ferromagnetic resonance (ST-FMR) technique, which is about five times that of pure Pt under the same conditions. The substantial enhancement of the spin Hall effect is revealed by theoretical analysis to be attributed to the strong side jump induced by the rare earth oxide GdOy impurities. Moreover, this large DL-SOT efficiency contributes to a low critical switching current density (8.0 × 106 A·cm-2 in the Pt0.70(GdOy)0.30 layer) in current-induced magnetization switching measurements. This systematic study on SOT switching properties suggests that Pt1-x(GdOy)x is an attractive spin current source with large DL-SOT efficiency for future SOT applications and provides another idea to regulate the spin Hall angle.

Current-Induced Magnetization Switching Behavior in Perpendicular Magnetized L10-MnAl/B2-CoGa Bilayer

Rare-earth-free Mn-based binary alloy L10-MnAl with bulk perpendicular magnetic anisotropy (PMA) holds promise for high-performance magnetic random access memory (MRAM) devices driven by spin-orbit torque (SOT). However, the lattice-mismatch issue makes it challenging to place conventional spin current sources, such as heavy metals, between L10-MnAl layers and substrates. In this work, we propose a solution by using the B2-CoGa alloy as the spin current source. The lattice-matching enables high-quality epitaxial growth of 2-nm-thick L10-MnAl on B2-CoGa, and the L10-MnAl exhibits a large PMA constant of 1.04 × 106 J/m3. Subsequently, the considerable spin Hall effect in B2-CoGa enables the achievement of SOT-induced deterministic magnetization switching. Moreover, we quantitatively determine the SOT efficiency in the bilayer. Furthermore, we design an L10-MnAl/B2-CoGa/Co2MnGa structure to achieve field-free magnetic switching. Our results provide valuable insights for achieving high-performance SOT-MRAM devices based on L10-MnAl alloy.

Integration of BiSb Topological Insulator and CoFeB/MgO With Perpendicular Magnetic Anisotropy Using an Oxide Interfacial Layer for Ultralow Power SOT-MRAM Cache Memory

BiSb topological insulator is attractive for the spin current source in spin-orbit torque (SOT) magnetoresistive random access memory (MRAM) thanks to its giant charge-to-spin conversion efficiency. However, the large spin Hall angles have been reported so far only in BiSb deposited on top of Co/Pt and Co/Tb multilayers. In realistic SOT-MRAM application, it is essential to integrate BiSb to CoFeB/MgO junction with perpendicular magnetic anisotropy (PMA). Here, we report a large spin Hall angle of 2.8 in junctions of bottom BiSb and CoFeB/MgO with perpendicular magnetic anisotropy using a CrOx interfacial layer, which is suitable for use in magnetic tunnel junctions (MTJ). We demonstrated SOT magnetization switching by a small current density of 3.1 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at a pulse width of 50 μs, which is an order of magnitude smaller than that in heavy metals. Our work demonstrates the capability of integrating BiSb to CoFeB/MgO-based MTJ.

Room temperature energy-efficient spin-orbit torque switching in two-dimensional van der Waals Fe3GeTe2 induced by topological insulators

Two-dimensional (2D) ferromagnetic materials with unique magnetic properties have great potential for next-generation spintronic devices with high flexibility, easy controllability, and high heretointegrability. However, realizing magnetic switching with low power consumption at room temperature is challenging. Here, we demonstrate the room-temperature spin-orbit torque (SOT) driven magnetization switching in an all-van der Waals (vdW) heterostructure using an optimized epitaxial growth approach. The topological insulator Bi2Te3 not only raises the Curie temperature of Fe3GeTe2 (FGT) through interfacial exchange coupling but also works as a spin current source allowing the FGT to switch at a low current density of ~2.2×106 A/cm2. The SOT efficiency is ~2.69, measured at room temperature. The temperature and thickness-dependent SOT efficiency prove that the larger SOT in our system mainly originates from the nontrivial topological origin of the heterostructure. Our experiments enable an all-vdW SOT structure and provides a solid foundation for the implementation of room-temperature all-vdW spintronic devices in the future.

Electrical detection of spin pumping in van der Waals ferromagnetic Cr2Ge2Te6 with low magnetic damping

The discovery of magnetic order in atomically-thin van der Waals materials has strengthened the alliance between spintronics and two-dimensional materials. An important use of magnetic two-dimensional materials in spintronic devices, which has not yet been demonstrated, would be for coherent spin injection via the spin-pumping effect. Here, we report spin pumping from Cr2Ge2Te6 into Pt or W and detection of the spin current by inverse spin Hall effect. The magnetization dynamics of the hybrid Cr2Ge2Te6/Pt system are measured, and a magnetic damping constant of ~ 4–10 × 10−4 is obtained for thick Cr2Ge2Te6 flakes, a record low for ferromagnetic van der Waals materials. Moreover, a high interface spin transmission efficiency (a spin mixing conductance of 2.4 × 1019/m2) is directly extracted, which is instrumental in delivering spin-related quantities such as spin angular momentum and spin-orbit torque across an interface of the van der Waals system. The low magnetic damping that promotes efficient spin current generation together with high interfacial spin transmission efficiency suggests promising applications for integrating Cr2Ge2Te6 into low-temperature two-dimensional spintronic devices as the source of coherent spin or magnon current.

Investigation of spin–orbit torque performance with W/Cu-multilayers as spin current source

We study the W/Cu multilayers as a spin current source and the coherent spin–orbit torques in a Fe layer using the spin-torque ferromagnetic resonance (STFMR) technique. With increasing numbers of layers, the line shape of the STFMR signals changes from predominantly antisymmetric to predominantly symmetric. When using [W(0.5)/Cu(0.5)]5 as a spin current source, the symmetric amplitude increases by a factor of 5 compared to a single W layer. Simultaneously, the effective damping parameter also increases slightly due to enhanced spin pumping. Along with an increasing trend in the damping-like torque efficiency, this suggests that the extrinsic spin Hall effect is enhanced. Concurrently, the antisymmetric amplitude decreases significantly by a factor of 27, which indicates an increase in the field-like torque when multilayers are used as a spin current source.

Interfacial Engineering Strategies for Efficient Spin–Orbit Torque Devices with Pt Alloys

To make spin–orbit torque magnetic random-access memories (SOT-MRAMs) become competitive enough to replace the contemporary memory architecture, it is of great importance to utilize conductive SOT materials that possess large damping-like spin–orbit torque (DL-SOT) efficiencies and back-end-of-line (BEOL) compatibility. In this work, we report a BEOL-compatible CoFeB/MgO-based heterostructure with the Pt–Cr alloy as the spin current source (SCS) and inserted a Pt/Co/Pt composite layer to achieve texture decoupling and ferromagnetic coupling. A thermally robust perpendicular magnetic anisotropy (PMA) of the hybrid heterostructure can be retained after annealing at 400 °C for 1 h, while maintaining large DL-SOT efficiencies of the annealed Pt–Cr alloys (ξDL ∼ 0.34 for Pt0.70Cr0.30). Additionally, by optimizing the current distribution with a resistive Pt spacer layer, it is possible to simultaneously enhance the current-to-effective field conversion ratio (Hzeff/I) and retain the apparent ξDL (ξDL ∼ 0.37 for Pt0.79Cr0.21). Moreover, the critical switching current density can be reduced from JcPt ∼ 2.0 × 107 A·cm–2 for Pt to JcPt0.70Cr0.30 ∼ 4.9 × 106 A·cm–2 for Pt0.70Cr0.30 without tampering the thermal stability of the annealed heterostructure. These interfacial engineering strategies provide directions to integrate Pt-based SCSs with large ξDL into a BEOL-compatible platform for future SOT-MRAM devices with PMA.

Room-temperature half-metallicity in rich Ti-alloyed [formula omitted] monolayer

A new family of MoSi2N4-type monolayers have been extensively studied since it was recently synthesized, but half-metallic behavior is observed infrequently and unstable in the new family. Here, based on antiferromagnetic semiconductor CrSi2N4 monolayer, we predict a ferromagnetic half-metallicity (FHM) in alloy compound TiCrSi4N8 monolayer with a minority-spin gap of 3eV and a considerable magnetic anisotropy energy by applying first-principles calculations. The FHM of TiCrSi4N8 are robust against biaxial strain from −3% to 3%. The TiCrSi4N8 belongs to XY ferromagnet with negligible in-plane magnetocrystalline anisotropy and the Curie temperature is estimated to be near room temperature by Monte Carlo simulation with the consideration of symmetric and antisymmetric exchange interactions. Such a FHM at room temperature may be permitted in the range of Ti concentrations from 0.5 to 0.75. Strong d−−d hybridization between Cr and Ti atoms and the larger exchange splitting of Cr-d orbitals with respect to Ti-d orbitals are origin of the HM and magnetic moment of Cr atom, and ferromagnetic coupling between Cr atoms is due to the Cr–N–Cr superexchange interaction. Besides, the applied potentials of TiCrSi4N8 are further expanded by the large Young’s modulus and strong bending resistance. Thus, the rich Ti-alloyed CrSi2N4 monolayers are noteworthy candidates for generating spin current source.

Room temperature spin-orbit torque efficiency in sputtered low-temperature superconductor δ -TaN

In the course of searching for promising topological materials for applications in future topological electronics, we evaluated spin-orbit torques (SOTs) in high-quality sputtered \ensuremath{\delta}-TaN/${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ devices through spin-torque ferromagnetic resonance (ST-FMR) and spin pumping measurements. From the ST-FMR characterization we observed a significant linewidth modulation in the magnetic ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ layer attributed to the charge-to-spin conversion generated from the \ensuremath{\delta}-TaN layer. Remarkably, the spin-torque efficiency determined from ST-FMR and spin pumping measurements is as large as $\mathrm{\ensuremath{\Theta}}=$ 0.034 and 0.031, respectively. These values are over two times larger than for \ensuremath{\alpha}-Ta, but almost five times lower than for \ensuremath{\beta}-Ta, which can be attributed to the low room temperature electrical resistivity $\ensuremath{\sim}74\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$ in \ensuremath{\delta}-TaN. A large spin diffusion length of at least $\ensuremath{\sim}8\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ is estimated, which is comparable to the spin diffusion length in pure Ta. Comprehensive experimental analysis, together with density functional theory calculations, indicates that the origin of the pronounced SOT effect in \ensuremath{\delta}-TaN can be mostly related to a significant contribution from the Berry curvature associated with the presence of a topically nontrivial electronic band structure in the vicinity of the Fermi level (${E}_{\mathrm{F}}$). Through additional detailed theoretical analysis, we also found that an isostructural allotrope of the superconducting \ensuremath{\delta}-TaN phase, the simple hexagonal structure \ensuremath{\theta}-TaN, has larger Berry curvature, and that, together with expected reasonable charge conductivity, it can also be a promising candidate for exploring a generation of spin-orbit torque magnetic random access memory as cheap, temperature stable, and highly efficient spin current source.

Type-Y magnetic tunnel junctions with CoFeB doped tungsten as spin current source

Spin–orbit torque magnetic tunnel junctions (SOT-MTJs) with high tunneling magnetoresistance (TMR) ratio and high energy-efficiency are crucial for the development of SOT-magnetic random-access memory and other SOT devices. Here, the SOT-MTJs doped with an ultrathin layer of 0.2 nm CoFeB in the W writing line are fabricated, and the TMR ratio of the updated MTJs is up to 179%. Meanwhile, the SOT efficiency of the W layer doped with magnetic atoms (∼0.149) is weakly dependent on the doping, manifesting the intrinsic mechanism of the W layer in generating the spin Hall effect. This study shows promise of the magnetic-atom doped W/CoFeB/MgO/synthetic antiferromagnetic stacks to achieve high TMR and efficient type-Y SOT devices.

Transparent spin thermoelectricity with enhanced energy conversion

Spin-thermoelectrics (STE) allows energy harvesting from the waste heat through the orthogonal relationship between the charge and heat flow, which offers large area scalability. Moreover, STE devices can be well adapted for transparent energy harvesting when a transparent magnetic insulator is exploited as a spin current source. Nevertheless, the transparent characteristics of STE devices have not been studied extensively so far. In this study, we show transparent characteristics of STE device based on thin films of Yttrium Iron Garnet (YIG)/Pt bilayers. Decreasing thickness of the Pt layer not only improves transmittance of visible light but simultaneously enhances the STE performance. The fabricated STE device of YIG (20 nm)/Pt (0.8 nm) exhibits a longitudinal spin Seebeck coefficient, SLSSE ≈ 7.04 μV/K, while sustaining over 80% transparency at λ = 700 nm. Our study shows the potential usage of STE as an alternative transparent energy harvesting, which could be adapted in various multi-purpose coatings.

Ultrahigh efficient spin orbit torque magnetization switching in fully sputtered topological insulator and ferromagnet multilayers

Spin orbit torque (SOT) magnetization switching of ferromagnets with large perpendicular magnetic anisotropy has a great potential for the next generation non-volatile magnetoresistive random-access memory (MRAM). It requires a high performance pure spin current source with a large spin Hall angle and high electrical conductivity, which can be fabricated by a mass production technique. In this work, we demonstrate ultrahigh efficient and robust SOT magnetization switching in fully sputtered BiSb topological insulator and perpendicularly magnetized Co/Pt multilayers. Despite fabricated by the magnetron sputtering instead of the laboratory molecular beam epitaxy, the topological insulator layer, BiSb, shows a large spin Hall angle of θSH = 10.7 and high electrical conductivity of σ = 1.5 × 105 Ω−1 m−1. Our results demonstrate the feasibility of BiSb topological insulator for implementation of ultralow power SOT-MRAM and other SOT-based spintronic devices.

Variation of spin-orbit torque and spin transport properties by V alloying in β-W-based magnetic heterostructures

Of transition metals, β-W exhibits the maximum charge-to-spin conversion efficiency. However, due to the lack of phase stability, the β-W-based alloy has rarely been investigated as a spin current generating layer. This study examines W–V alloy layers with various W100-xVx/CoFeB/MgO/Ta heterostructure compositions. X-ray diffraction confirms that the β-W matrix is conserved up to a V content of 20 at%. We attain a maximum damping-like torque efficiency of −0.45 ± 0.04 for the W80V20 alloy-based heterostructure through the harmonic response method. Furthermore, we determine the spin diffusion length of the W80V20 layer and specify the interfacial spin transparency of the W80V20/CoFeB structure by adopting the spin diffusion model. Consequently, we suggest an intrinsic spin-Hall angle of −0.86. In addition, we observe in-plane current-induced switching. Our results establish the potential of the β-phase W-V alloy layer as a spin current source layer in spintronic devices.

Spin–orbit torque driven perpendicular magnetization switching in Re/CoFeB/MgO with high thermal stability

Writing using spin–orbit torque (SOT) has been widely investigated in the field of magnetic random-access memory (MRAM). Heavy metal (HM)/CoFeB/MgO is the core of this SOT-MRAM structure. The heterostructure consisting of Ta as the spin current source and CoFeB/MgO as the perpendicular magnetic anisotropy (PMA) material is the most researched structure, owing to its high tunneling magnetoresistance ratio. However, Ta is difficult to integrate into the CMOS process due to its poor thermal stability against annealing at temperatures greater than 350 °C. Currently, β-tungsten (W) is the only HM with the CoFeB/MgO system, which can provide both thermal stability and SOT switching. Nevertheless, to achieve the high resistive β phase of W is a challenging task. Here, we report another material rhenium (Re) capable of providing thermally stable PMA up to temperature 425 °C with a perpendicular anisotropic field greater than 5000 Oe; Re possesses a spin hall angle (ϴSH) of 0.065 ± 0.003, and SOT switching can be achieved with a current density around 1.36 × 1011 A/m2. Our findings pave a new avenue for the material design of perpendicular SOT-based MRAM.

Thermal stability study of Weyl semimetal WTe&lt;sub&gt;2&lt;/sub&gt;/Ti heterostructures by Raman scattering

Weyl semimetal Td-phase WTe&lt;sub&gt;2&lt;/sub&gt;, a novel topological matter, possesses a strong spin-orbit coupling and non-trivial topological band structure, and thus becomes a very promising superior spin current source material. By constructing the WTe&lt;sub&gt;2&lt;/sub&gt;/Ti heterostructures, the issue that the ferromagnetic layer with perpendicular magnetic anisotropy cannot be directly prepared on WTe&lt;sub&gt;2&lt;/sub&gt; layer can be well addressed, and meet the requirements for high-performance spin-orbit torque devices. To be compatible with the semiconductor technology, the device integration usually involves a high temperature process. Therefore, the thermal stability of WTe&lt;sub&gt;2&lt;/sub&gt;/Ti is critical for practical device fabrication and performance. However, the thermal stability of WTe&lt;sub&gt;2&lt;/sub&gt;/Ti interface has not been very clear yet. In this work, the micro-Raman scattering technique is used to systematically study the WTe&lt;sub&gt;2&lt;/sub&gt;/Ti interface annealed at different temperatures. It is found that the thermal stability of the interface between WTe&lt;sub&gt;2&lt;/sub&gt; and Ti is related to the thickness of WTe&lt;sub&gt;2&lt;/sub&gt; flake; appropriate increase of the WTe&lt;sub&gt;2&lt;/sub&gt; thickness can lead to the improvement of thermal stability in WTe&lt;sub&gt;2&lt;/sub&gt;/Ti heterostructures. In addition, high temperature annealing can cause a significant interfacial reaction. After annealed at 473 K for 30 min, the interface between WTe&lt;sub&gt;2&lt;/sub&gt; (12 nm) and Ti changes dramatically, leading to the formation of Ti-Te interface layer. This observation is highly consistent with the observations by high-resolution transmission electron microscopy and the elemental analysis results as well. This study will provide useful information for further exploring the influence of the WTe&lt;sub&gt;2&lt;/sub&gt;/Ti interface on the spin-orbit torque effect, and greatly invigorate the research area of energy efficient spintronic devices based on WTe&lt;sub&gt;2&lt;/sub&gt; and other novel topological materials.

Thermal stability study of Weyl semimetal WTe2 by polarized micro-Raman scattering

Weyl semimetal Td-phase WTe2, a novel topological matter, possesses non-trivial band structures with strong spin-orbit coupling and thus exhibits a pronounced charge-to-spin conversion efficiency, holding promises as a superior spin current source for spin memory and logic devices. Good thermal stability of WTe2 is crucial for device fabrication and performance, which should be compatible with the semiconductor technology usually involving high temperature process of ∼350–400 °C. However, the thermal stability of this novel material has not been very clear yet. Here, we study the exfoliated Td-WTe2 flakes by in-situ polarized micro-Raman scattering with a much higher heating temperature up to 550 °C. We find that the structure and composition of WTe2 flakes can remain unchanged up to 425 °C. However, around 450 °C, the composition of WTe2 flakes starts to change due to the decomposition and sublimation of Te atoms in WTe2. Our study demonstrates that Weyl semimetal WTe2 shows good thermal stability and has great potentials for the energy-efficient spintronic device application.

Spin–orbit torque magnetization switching in a perpendicularly magnetized full Heusler alloy Co2FeSi

To optimize the writing and reading performance of magnetic random-access memory (MRAM) devices, achieving current-induced spin–orbit torque (SOT) magnetization switching in perpendicularly magnetized full Heusler alloys is vitally important. For conventional SOT-metal bilayer systems, heavy metals (HMs) with a large spin Hall angle (θSH) are generally used for generating a spin current, which is injected into the adjacent ferromagnet (FM) layer and exerts a torque on the magnetization to switch it. However, the large resistivity of generally used HMs such as β-Ta and β-W can increase the Ohmic loss. In this article, we achieve full SOT switching in Heusler alloy Co2FeSi using low-resistivity Pd as a spin current generation source. The critical switching current density is found to be 3.7 × 107 A cm−2, which is in the same order of magnitude as that required for conventional HM/FM systems even though Pd has a smaller θSH than that of generally used HMs. Using harmonic Hall measurements, the damping-like and field-like effective fields per unit current density are estimated to be 56.9 (10−7 Oe A−1 cm2) and 39.8 (10−7 Oe A−1 cm2), respectively. This high efficiency can be attributed to the excellent lattice matching between Co2FeSi and Pd (only 2% mismatch), to a slight Pd diffusion, and possibly to the additional SOTs induced by the in-plane spin component generated in the Co2FeSi layer. Our finding will advance the development of SOT-MRAM devices with both better reading and writing performance.

Low power spin–orbit torque switching in sputtered BiSb topological insulator/perpendicularly magnetized CoPt/MgO multilayers on oxidized Si substrate

Topological insulators (TIs) are promising for efficient spin current sources in spin–orbit torque (SOT) magnetoresistive random access memory (MRAM). However, TIs are usually deposited by molecular beam epitaxy on single crystalline III–V semiconductor or sapphire substrates, which are not suitable for realistic applications. Here, we studied SOT characteristics in sputtered BiSb topological insulator—Pt/Co/Pt—MgO heterostructures deposited on oxidized Si substrates, where Pt/Co/Pt trilayers have a large perpendicular magnetic anisotropy field of 4.5 kOe. We show that the BiSb layer has a large effective spin Hall angle of θSHeff = 2.4 and a high electrical conductivity of σ = 1.0 × 105 Ω−1 m−1. The magnetization can be switched by a small current density of 2.3 × 106 A cm−2 at a pulse width of 100 µs, which is 1 or 2 orders of magnitudes smaller than those in heavy metals. Our work demonstrates the high efficiency and robustness of BiSb as a spin current source in realistic SOT-MRAM.

Spin-orbit torques induced by spin Hall and spin swapping currents of a separate ferromagnet in a magnetic trilayer

Spin-orbit torques (SOTs) have been investigated most widely in normal metal/ferromagnet bilayers where the spin Hall effect of normal metal is a main source of spin currents. Recently, ferromagnets are found to also serve as spin-current sources through spin-orbit coupling. In this work, we theoretically investigate SOT acting on ferromagnet2 in ferromagnet1/normal metal/ferromagnet2 trilayers, which is caused by the spin Hall and spin swapping effects of ferromagnet1. Our result provides an analytical expression of SOT in the trilayers, which may be useful for quantifying the spin Hall and spin swapping effects of ferromagnets and also for designing and interpreting SOT experiments where a ferromagnet is used as a spin-current source instead of a normal metal.

Ultralow-current magnetization switching in nearly compensated synthetic antiferromagnetic frames using sandwiched spin sources

Spin–orbit torque (SOT)-induced magnetization switching via electric currents in large spin–orbit-coupled heavy metal (HM)/ferromagnetic (FM) frames is one of the most reliable approaches to achieve increased performances in various spintronic applications. However, its widespread utilization is still challenging owing to the inherent requirement of a high electric current density for efficient magnetization switching. In this paper, we introduce a sandwiched synthetic antiferromagnetic (S-SyAF) frame, comprising CoFeB (FM)/W (HM)/CoFeB (FM) stacks, whose operation is mediated by the Ruderman–Kittel–Kasuya–Yosida interaction. The middle HM layer serves as a spin current source for the top and bottom FM layers simultaneously. In addition, the S-SyAF frame contributes to a substantial reduction in the net magnetization, which ensures a considerably increased SOT switching efficiency; further, the switching current was almost 10 times smaller than that in a conventional HM/FM frame. Thus, we believe that these experimental findings will provide a new avenue for designing future spintronic devices.