Efficient Spin-Orbit Torque Generation in Semiconducting WTe2 with Hopping Transport
11 Efficient Spin-Orbit Torque Generation in Semiconducting WTe with Hopping Transport Cheng-Wei Peng,
Wei-Bang Liao,
Tian-Yue Chen, and Chi-Feng Pai Department of materials science and engineering, National Taiwan University, Taipei 10617, Taiwan Center of Atomic initiative for new materials, National Taiwan University, Taipei 10617, Taiwan a) These authors contributed equally to this work. b) Author to whom correspondence should be addressed:
Email: [email protected]
Abstract
Spin-orbit torques (SOTs) from transition metal dichalcogenides systems (TMDs) in conjunction with ferromagnetic materials are recently attractive in spintronics for their versatile features. However, most of the previously studied crystalline TMDs are prepared by mechanical exfoliation, which limits their potentials for industrial applications. Here we show that amorphous WTe heterostructures deposited by magnetron sputtering possess a sizable damping-like SOT efficiency WTeDL and low damping constant . Only an extremely low critical switching current density J is required to achieve SOT-driven magnetization switching. The SOT efficiency is further proved to depend on the W and Te relative compositions in the co-sputtered W Te x samples, from which a sign change of WTeDL is observed. Besides, the electronic transport in amorphous WTe is found to be semiconducting and is governed by a hopping mechanism. With the above advantages and rich tunability, amorphous and semiconducting WTe serves as a unique SOT source for future spintronics applications. Keywords: Transition metal dichalcogenide, spin-orbit torque, spintronics, damping constant Ⅰ. Introduction
With the advancement of spintronics and next generation magnetic random access memory (MRAM) technologies, spin-orbit torques (SOTs) originated from the spin-orbit interactions in various types of materials systems and magnetic heterostructures have been shown to be an effective mechanism to manipulate and switch the magnetization similar to or more efficient than the traditional spin-transfer torque (STT) approach.
When it comes to materials with sizable efficiency of generating spin current from charge current, the 5d transition metals like Pt,
Ta, and W are the typically chosen due to the strong spin-orbit coupling (SOC) therein, which can induce spin currents and SOTs through the spin Hall effect (SHE). More recently, topological insulators (TIs) such as BiSe,
BiSb, and (Bi Sb ) Te are reported to possess high SOT efficiency from spin momentum locking of topologically-protected surface states (TSSs). Interestingly, various non-epitaxial Bi-based chalcogenides without TSS have also been reported to show giant SOT efficiencies. Another family of emergent materials, transition metal dichalcogenides (TMDs), have also gained lots of attention due to not only the strong SOC but also their unconventional SOT features from the lack of inversion symmetry.
These unique properties lead to the possibility of electric field control over SOT and generation of out-of-plane damping-like (DL) SOTs. Particularly, for exfoliated WTe , MacNeill et al . confirmed the existence of anomalous DL-SOT in WTe /Py through spin-torque ferromagnetic resonance (ST-FMR) measurements. Li et al . observed an enhanced spin conductivity in WTe /Py originated from spin momentum locking. Shi et al . reported an extremely large DL-SOT efficiency up to 0.5 for WTe /Py, which was attributed to a bulk origin. Although all these studies suggest that WTe is a promising SOT material with rich tunability, most of them require mechanical exfoliation of single-crystalline WTe , which is of great challenge to realize in mass production. It is therefore crucial to explore the possibilities of employing WTe layers prepared by conventional materials growth approaches, such as magnetron sputter deposition, to see their spin transport properties as compared to the exfoliated cases. In this work, we report the DL-SOT efficiencies from both stoichiometric WTe and co-sputtered W Te x -based magnetic heterostructures prepared by high vacuum sputter depositions. Both series of deposited multilayer stacks are amorphous. We first quantify the damping constant of WTe /CoFeB devices with in-plane magnetic anisotropy (IMA) through ST-FMR measurements, from which the damping constant is determined to be WTe /CoFeB , smaller than those from the transition metal-based control samples, namely
W/CoFeB and
Pt/CoFeB . The largest DL-SOT efficiency from the stoichiometric WTe /CoTb devices with perpendicular magnetic anisotropy (PMA) is further estimated to be WTeDL by current-induced hysteresis loop shift measurement. Current-induced magnetization switching at a low critical switching current density J further confirms the sizable DL-SOT efficiency. By studying the co-sputtered W Te x samples, we find that
100 x x
DLW Te is strongly composition dependent in terms of both magnitude and sign, which can explain the huge discrepancies of observed DL in several recent reports. Additionally, the distinct difference between the layer thickness dependence of WTeDL and
100 x x
DLW Te suggests that both bulk and interfacial origins of SOTs are possible to be generated by sputter-deposited chaclogenides. These unconventional features together with its low and sizable DL , make amorphous WTe -based heterostructure an intriguing system for novel SOT applications. Ⅱ. Materials and Methods
The samples in this work are all prepared by magnetron sputtering and deposited onto thermally oxidized silicon substrates with base pressure ~ -8 Torr. We use dc sputtering for metallic targets under 3 mTorr of Ar working pressure, while MgO target is rf sputtered under 10 mTorr of Ar working pressure. Three series of samples are prepared, namely WTe (10)/CoFeB(5)/MgO(1)/Ta(1) for ST-FMR measurements, WTe ( t )/CoTb(6)/Ta(2) for hysteresis loop shift and current-induced magnetization switching measurements, and co-sputtered W Te x (10)/CoTb(4.5)/Ta(2) (units in nanometers) for further comparisons. The WTe layer is deposited directly from a stoichiometric WTe target, while the CoFeB layer is sputtered from a Co Fe B target. The Co Tb (CoTb) layer and W Te x layer are deposited by co-sputtering Co and Tb targets as well as W and Te targets, respectively. Note that WTe layer is grown at a substrate temperature of
300 C to ensure the uniformity. The thicknesses of each layer are checked by atomic force microscopy (AFM). Electron probe X-ray microanalyzer (EPMA) is employed to confirm the atomic ratios of the sputtered WTe and co-sputtered W Te x thin films. For further electrical measurements, the films are patterned into microstrip with lateral dimensions of
40 m 100 m for ST-FMR measurement and Hall bar devices with lateral dimensions of
10 m 60 m for hysteresis loop shift and current-induced magnetization switching measurements through standard photolithography and followed by Ar ion-milling.
Ⅲ. Results A.
Materials characterizations
The structural properties of the sputtered WTe films are first examined by cross-sectional field-emission transmission electron microscopy (FE-TEM, JEOL 2010F). The TEM sample is prepared by a lift-out technique with SEIKO SMI-3050SE focused ion beam (FIB). As shown in Fig. 1(a), the sputtered WTe in a representative WTe (10)/CoFeB(5)/MgO(1)/Ta(1) sample is amorphous. Figure 1(b) shows the normalized secondary ion mass spectrometer (SIMS) signals from the same thin film during Ar ion-milling process. The peaks of Mg, Co, W, and Te correspond to the signals from the MgO layer, the CoFeB layer, and the WTe layer being etched, respectively. According to the EPMA analysis, the atomic ratio between W and Te are 33% and
67% in our sputtered WTe films, therefore we keep the stoichiometric notation of WTe to represent samples deposited from the WTe target (see the supplementary material S1). The resistivities of each material are measured by four-point probe measurement, among which WTe (see the supplementary material S2). We further examine the electrical transport properties by measuring the temperature dependence of resistivity from 323 K to 423 K, which suggests our sputtered WTe and W Te x films are semiconducting rather than metallic (see the supplementary material S3). We further examine the electrical transport properties by measuring the temperature dependence of resistivity from 323 K to 423 K. Most of the previous works claimed that the textured WTe is one kind of Weyl semimetal, while our sample is predominately amorphous and therefore should not fall into this category. According to the electrical transport measurement shown in supplementary material S3, the sputtered amorphous WTe is semiconducting and the temperature dependent resistivity can be well described by small polaron hopping (SPH) model, where the thermally-activated hopping polarons from electron-phonon interaction dominate the electrical transport. B. ST-FMR measurement
We then investigate the spin transport properties of WTe (10)/CoFeB(5)/MgO(1)/Ta(1) with ST-FMR. CoFeB is chosen to be the ferromagnetic (FM) layer because it is widely used in contemporary MTJs. MgO(1)/Ta(1) layers are employed to prevent the layers beneath from oxidation. In a typical ST-FMR measurement, a rf current is applied, generating a rf spin current from the SOC buffer layer (WTe ) to induce the precessional magnetization of the FM layer, which leads to an oscillation of the resistance owing to the anisotropic magnetoresistance. With the mixing of oscillating resistance and rf current, we can measure a rectified dc voltage mix V of the device through a bias tee, as illustrated in Fig. 2(a). A rf current is generated from the signal generator with the amplitude modulated at 500 Hz, which allows the use of lock-in amplifier for mix V detection. A sweeping in-plane field ext H is applied at a fixed angle of 135 o between ext H and the current channel. Typical field-swept ST-FMR spectra of a WTe (10)/CoFeB(5) microstrip under frequencies of f
10 GHz f is further scrutinized in Fig. 2(c), in which the data points of mix V can be fitted with ( ) ( ) ( ) H HV S AH H H H , (1) where S , A , , and H denote symmetric Lorentzian coefficient, anti-symmetric Lorentzian coefficient, linewidth, and resonant field, respectively. The symmetric part of mix V originates from the in-plane DL torque ( ), while the anti-symmetric part comes from the out-of-plane field-like (FL) torque ( ) and the Oersted field. The torque ratio is further evaluated by ff0 e MSA H , (2) where eff M is the demagnetization field extracted from the fitting of Kittel formula (see the supplementary material S4). Note that the contribution from the Oersted field is negligible in our system. For the WTe (10)/CoFeB(5) device, the torque ratio is 2.75 and spin-pumping effect might plague the estimation of WTeDL from torque ratio. Therefore, we choose to quantify WTeDL via other approaches, which will be addressed in the following section. Nevertheless, ST-FMR provides us other valuable information, such as damping constant , which is one of the key figures of merit to realize efficient current-induced switching. can be estimated from f , where is the linewidth from inhomogeneous broadening, and γ is the gyromagnetic ratio. Figure 2(d) shows the linewidths as functions of frequencies of WTe , Pt and W-based magnetic heterostructures. Surprisingly, the WTe (10)/CoFeB(5) device shows WTe /CoFeB , which is smaller than those obtained from control samples with classical spin Hall transition metals, namely Pt(6)/CoFeB(5) (
Pt/CoFeB ) and W(4)/CoFeB(5) (
W/CoFeB ). Note that WTe /CoFeB from our sputtered WTe /CoFeB is comparable to those observed from exfoliated WTe /Py heterostructures. C. Hysteresis loop shift measurement
Next, we employ hysteresis loop shift measurement to quantify the DL-SOT efficiency WTeDL of WTe /FM heterostructures, with FM being ferrimagnetic CoTb alloy with PMA. Ferrimagnets are widely used as SOT detector due to their robust bulk PMA on diverse materials with SOC. The setup for loop shift measurement on a WTe (10)/CoTb(6)/Ta(2) Hall bar device with suitable PMA ( c
60 Oe H ) is depicted in Fig. 3(a). A dc current ( dc I ) is applied along the current channel with a parallel in-plane bias field x H . With x H overcoming the effective field from interfacial Dzyaloshinskii-Moriya interaction (DMI) DMI H and realigning the domain wall moments, the current-induced effective field from DL-SOT will assist domain wall propagation and domain expansion. When x DMI H H , the current-induced DL-SOT from WTe can fully act on the domain wall moments of CoTb and it can be seen as an out-of-plane effective field effz H causing the hysteresis loop to shift. This can be monitored via anomalous Hall voltage measurements, as shown in Fig. 3(b). By obtaining the slope of effz H vs dc I , as shown in Fig. 3(c), the DL-SOT efficiency ( DL ) can be estimated through effCoTb WTe WTe CoTb zDL 0 s CoTb WTe CoTb WTe dc t t He M t wtћ t I , (3) where s M
48 emu/cm is the saturation magnetization of the CoTb layer (characterized by vibrating sample magnetometer (VSM)), = 10 m w is the width of the Hall bar device, and represents the layer resistivity. The estimated DL-SOT efficiency of the sputtered WTe (10)/CoTb(6) heterostructure is WTeDL , which is comparable with the exfoliated crystalline WTe /FM. It is worth noting that this sizable WTeDL is in sharp contrast to a recent work by Fan et al. , in which a relatively small DL but a large FL are reported for the sputtered W x Te . Figure 3(d) shows effz dc / H I ( DL ) as a function of x H for WTe /CoTb and W/CoTb (control sample) devices. It can be observed that the signs of effz dc / H I for the two devices are opposite ( WTeDL and WDL ), which is consistent with the exfoliated WTe case. We emphasize that both the DL effective field ( effz dc / H I ) and the estimated DL-SOT efficiency of WTe -based device ( WTeDL ) are greater than those of the W-based control sample (
WDL ) (see the supplementary material S5). It is also interesting to note that WTe WDMI DMI
H H , which suggests that the WTe -based device might have a stronger interfacial SOC than the W-based case. D. Current-induced magnetization switching
We further demonstrate current-induced SOT-driven magnetization switching in WTe (10)/CoTb(6)/Ta(2) magnetic heterostructures. Pulsed currents are injected into the current channel of Hall bar devices, and again, an x H is applied to overcome DMI H such that the deterministic magnetization switching can be achieved. As shown in Fig. 4(a), the switching polarities depend on the applied x H , which is consistent with the SOT-driven switching mechanism. More importantly, the critical switching current density c J of WTe /CoTb structure is further demonstrated to be J , which is much lower than those from the 5d transition metal-based heterostructures (
11 2c
10 A/m J ) and comparable to the TI cases. Note that this value is of the same order as the exfoliated 80 nm-thick-WTe case reported by Shi. S et al. ( J ), and smaller than the sputtered WTe x case reported by Li et al. (
10 2c J ). In addition to c J , the SOC layer thickness also needs to be taken into account to estimate the overall switching performance. We evaluate the ideal critical switching current through idealc c I J t w ,where t is the required SOC buffer layer thickness and w is the ideal current channel width assumed to be 100 nm. Figure 4(b) compares the reported c J and estimated idealc I from several TIs and WTe x -based magnetic heterostructures (see the supplementary material S6 for parameter details). Both benchmark results indicate that amorphous sputtered WTe is a competitive SOT source in the emergent chalcogenide materials category. E. Co-sputtered W Te x devices Lastly, we turn our focus on the possible origins of SOT in the amorphous WTe heterostructures. Due to the lack of crystalline phase (and therefore no TSS) in our sample, we speculate that the SOC should have a bulk origin and the concentration of constituent elements will strongly affect DL . Figure 5(a) shows DL obtained from a series of co-sputtered samples,
100 x x
W Te (10) / CoTb(4.5) / Ta(2) . It is found that DL gradually increases and reaches a maximum at 18% of Te doping, where
82 18
W TeDL . Further increasing the Te concentration will lead to a decrease of DL . Note that DL changes sign from co-sputtered W Te x /CoTb (negative) to WTe /CoTb (positive). This sign-change suggests that the governing SOC behind the W-rich and the Te-rich regimes could be different. Although there is few work discussing the SOC or SOT from pure Te, previous studies have pointed out that elements with p -orbitals are able to possess strong SOCs. A recent first principles calculation also indicates the spin Hall conductivity (SHC) is sensitive to the Fermi energy of WTe , where the sign of SHC could reverse in a narrow energy window. To understand the possible difference in SOT generation from WTe and W Te x , we further examine the SOC layer thickness dependence of DL from both type of samples. For the co-sputtered case, we use W Te /CoTb as the representative sample due to its maximized efficiency. As summarized in Fig. 5(b), the co-sputtered W Te and the stoichiometric WTe cases have a drastically difference in SOC layer dependence. For the co-sputtered case, the thickness dependence of efficiency can be well fitted by the spin diffusion model
82 18 82 1882 18 82 18
W Te W TeDL W Te DL W Te s t t , with
82 18
W TeDL and spin diffusion length
82 18
W Tes , which suggests that the SOC within W Te is of bulk origin (such as the SHE). This is perhaps due to a dominating contribution of the SHE from W in the W-rich regime. In contrast, for WTe , WTeDL decays with increasing the stoichiometric layer thickness, which indicates the possible existence of interfacial contributions of SOC besides the bulk effect. We speculate that interfacial effects could play crucial roles on the SOT generation in the Te-rich (stoichiometric) regime, even if the TMD layer is amorphous. Further studies are required to elucidate the SOC and SOT generation mechanisms in these amorphous TMD systems.
Ⅳ. Conclusions In summary, we use conventional magnetron sputtering to deposit WTe instead of mechanical exfoliation, where the structure is confirmed to be amorphous by TEM imaging. From the ST-FMR measurements, we obtain a sizable DL-SOT response with a low damping constant in WTe /CoFeB heterostructure ( WTe /CoFeB ). Through hysteresis loop shift measurements, the largest DL-SOT efficiency of WTe /CoTb heterostructure is characterized to be WTeDL . This large charge-to-spin conversion efficiency is further verified by current-induced magnetization switching with a low critical switching current density of J . By comparing the stoichiometric WTe to the co-sputtered W Te x -based devices, the sign and magnitude of DL is found to be strongly composition dependent. Moreover, not only bulk SHE but also other interfacial SOC may contribute to the SOTs from WTe . Our observation therefore suggests that the relative composition between W and Te is the key factor, rather than the crystallinity, to affect the resulting spin transport properties, and the transport mechanism in amorphous sputtered WTe can be described by the SPH model. With the features of wide-range tunable DL , low , and low c J , amorphous sputtered WTe can serve as a potential SOT source material in the semiconducting regime. Supplementary Material
See the supplementary material associated with this article.
Acknowledgements
We thank Tsao-Chi Chuang and Hsia-Ling Liang for the support on AFM and VSM measurements. We also thank Mr. Chung-Yuan Kao of Ministry of Science and Technology (National Taiwan University) for the assistance in EPMA, and thank Ms. Chia-Ying Chien of Ministry of Science and Technology (National Taiwan University) for the assistance on focused ion beam (FIB). This work is supported by the Ministry of Science and Technology of Taiwan (MOST) under grant No. MOST-109-2636-M-002-006 and by the Center of Atomic Initiative for New Materials (AI-Mat) and the Advanced Research Center of Green Materials Science and Technology, National Taiwan University from the Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan under grant No. NTU-109L9008. The authors declare no competing financial interests. Author Contributions
Cheng-Wei Peng prepared the IMA samples and performed ST-FMR measurement. Wei-Bang Liao prepared the PMA samples, performed hysteresis loop shift measurement and current-induced magnetization switching. Tian-Yue Chen conceived the experimental protocols and performed the analysis on the experimental data with Cheng-Wei Peng and Wei-Bang Liao. Chi-Feng Pai proposed and supervised the study.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Structural characterization of WTe -based heterostructure. (a) Cross-sectional TEM image and (b) SIMS signals with respect to the etching (Ar ion-milling) time of a representative WTe (10)/CoFeB(5)/MgO(1)/Ta(1) heterostructure. FIG. 2.
ST-FMR measurement of WTe (10)/CoFeB(5)/MgO(1)/Ta(1) device. (a) Illustration of ST-FMR measurements setup. (b) The ST-FMR spectra of a WTe (10)/CoFeB(5)/MgO(1)/Ta(1) microstrip for
7 to 12 GHz f . (c) Representative ST-FMR result of WTe (10)/CoFeB(5)/MgO(1)/Ta(1) at
10 GHz f . The raw data is fitted by the sum of a symmetric and an antisymmetric Lorentzian. (d) f dependence of linewidth for Pt(6)/CoFeB(5)/MgO(1)/Ta(1), W(4)/CoFeB(5)/MgO(1)/Ta(1) and WTe (10)/CoFeB(5)/ MgO(1)/Ta(1) structures. FIG. 3.
Hysteresis loop shift measurement of WTe (10)/CoTb(6)/Ta(2) device. (a) Illustration of a Hall bar device for hysteresis loop shift measurement. (b) Representative shifted hysteresis loops of a WTe (10)/CoTb(6)/Ta(2) device with dc I and x H . (c) The effective fields as functions of applied currents under x H . (d) effz dc / H I as functions of x H for WTe (10)/CoTb(6)/Ta(2) and W(4)/CoTb(6)/Ta(2) (control) heterostructures. FIG. 4.
Current-induced magnetization switching in WTe (10)/CoTb(6)/Ta(2) device. (a) Current-induced magnetization switching from a WTe (10)/CoTb(6)/Ta(2) device with applied bias fields x
900 Oe H . (b) Comparison of critical switching current density c J (left y axis) and ideal critical switching current idealc I (right y axis) among TIs and WTe based heterostructures. FIG. 5.
Comparison of SOT from WTe and co-sputtered W Te x . (a) The DL-SOT efficiency DL of W Te x /CoTb heterostructures as a function of Te concentration. The blue open squares represent the devices with co-sputtered W Te x , and the red open circle represents the stoichiometric WTe case. The dashed line serves as guide to the eye. (b) Buffer layer thickness dependence of DL for W Te -based and WTe -based devices. The blue dashed line represents the fitting of the
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DL W Te ( ) t data to a spin diffusion model with spin diffusion length
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W Tes