There are mainly two types of exchange interactions in a multiple-spins system. First being the symmetric exchange, which is directly responsible for the existence of ferromagnetism (FM) and antiferromagnetism (AFM) in which the spins collectively form either parallel or antiparallel configurations. On the other hand, the antisymmetric exchange, or the Dzyaloshinskii-Moriya interaction (DMI) [1-3] has gained significant attention lately. For the majority of cases, DMI is an intralayer effect, i.e. the nonlinear spin configurations are confined within a single magnetic layer. However, pioneering predictions [4] have recently expanded the possibility to an interlayer case, where the spins are from two distinct FM layers. We hereby demonstrate an interlayer DMI in a CoFeB/Pt/Co/Pt multilayer where CoFeB has in-plane anisotropy (IMA), while Co has perpendicular magnetic anisotropy (PMA). The wedge sputtering provides sufficient symmetry breaking for the interlayer DMI to manifest (Fig.1 (a)). A field sweep protocol involving the out of plane external field Hz sweep while applying an in-plane field Hφ with various φ orientation is used to capture the effective field experienced by the PMA layer. The resultant hysteresis loops of the Co layer are substantially shifted (Fig.1 (b)) and follow a sinusoidal form with respect to the φ direction (Fig.1 (d)). The importance of in-plane symmetry breaking is confirmed by comparing results with control samples without symmetry breaking (Fig.1 (c)). The reciprocal effect experienced by the IMA layer is captured by magneto-optical Kerr effect experiments (Fig.1 (d)). Moreover, it is shown that current-induced field-free switching of a PMA layer is achievable utilizing the DMI induced effective field and the switching polarity can be controlled by the interlayer DMI landscape (Fig.2). 
**References:** [1] Dzyaloshinsky, A thermodynamic theory of ‘weak’ ferromagnetism of antiferromagnetics, J. Phys. Chem. Sol. 4, 241 (1957). 
[2] T. Moriya, Anisotropic Superexchange Interaction and Weak Ferromagnetism, Phys. Rev. 120, 91 (1960). 
[3] A. Fert and P. M. Levy, Role of Anisotropic Exchange Interactions in Determining the Properties of Spin-Glasses, Phys. Rev. Lett. 44, 1538 (1980). 
[4] E. Y. Vedmedenko, P. Riego, J. A. Arregi, and A. Berger, Interlayer Dzyaloshinskii-Moriya Interactions, Phys. Rev. Lett. 122, 257202 (2019). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/e96cf0a6ee880782704d00f31d5c561c.png) 
Fig.1 (a) Sketch of the magnetic multilayers, external fields, coordinate system. Interlayer DMI’s manifestation on both IMA and PMA hysteresis loops presented in (b) and (d), respectively. Comparison between samples without symmetry breaking presented in (c). 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/8f09170333a6d60da6ba318c9fd2d8ae.png) 
Fig.2 Interlayer DMI landscape demonstrating field free switching and the influence of the device angle on the switching polarity.

Growth dependent Interlayer Chiral Exchange and Field

Current-induced spin-orbit torque (SOT) is an efficient mechanism to manipulate the magnetization in magnetic materials [1]. In principle, the sizable SOT is believed to originate from the bulk spin Hall effect (SHE) and/or the interfacial spin Rashba- Edelstein effect (SREE) in heavy metals (HMs)-based magnetic heterostructures with strong spin-orbit coupling (SOC). Strikingly, recent reports suggest that the light metals (LMs) can also generate non-negligible SOTs and these sizable charge-to-spin conversions are attributed to the orbital angular momentum (OAM) induced orbital Hall effect (OHE) and/or orbital Rashba-Edelstein effect (OREE) [2]. The OAM dominating SOT generation has been experimentally explored in various LM-based systems such as Ti, Cr, and Cu-based magnetic heterostructures [3].
In this work, we prepare the Pt/Co/CuNx perpendicular magnetic heterostructures by reactive sputtering. By controlling the doping concentration, the overall SOT efficiency can be tuned from 6% to ~ 40%. The enhancement is attributed to combining the spin currents from Pt and the orbital currents generated by the nitrided Cu capping layer. Besides the conventional SOT characterizations, we further demonstrate the neuromorphic switching and show that the orbital currents can effectively manipulate the perpendicular magnetization. These results provide valuable information for building energy-efficient spin-orbitronic devices utilizing both the spin and the orbital effects [4]. 
**References** [1] L. Liu et al., Science 336, 555 (2012).
[2] D. Lee et al., Nature Communications 12, 6710 (2021).
[3] S. Lee et al., Communications Physics 4, 234 (2021).
[4] T.-Y. Chen et al., Physical Review Applied 17, 064005 (2022). 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/8f0fb0ea18b1a4f2b0a17ef693b20045.png)

Spin orbit torque switching mediated by orbital currents from CuNx

Spin-related magnetoresistance (MR) from versatile condensed matter systems can be employed as the main mechanism to detect the orientation of magnetization in next generation magnetoresistive random-access memories (MRAM). Recently, novel MR effects such as spin Hall MR (SMR) [1,2] and spin-dependent unidirectional MR (SD-UMR) [3,4] are extensively studied due to the correlation between MR and spin-orbit interactions in both classical and emergent materials systems. In our work, we disentangle the underlying mechanisms of the UMR in W/CoFeB magnetic heterostructures by systematic current-dependent and field-dependent MR measurements at room temperature. As shown in Fig.1, the current-dependent UMR of W/CoFeB heterostructures can be roughly divided into three regimes. In the first and second regimes, UMR increases linearly and then nonlinearly to the applied current density J as J reaches ~5×1010 A/m2, which originates from the SD-UMR [Fig.2(a)] and spin-flip UMR (SF-UMR) [Fig.2 (b)] [4]. In the third regime, as J reaches ~ 1011 A/m2, the UMR ratio will further saturate at 0.36%, which is larger than the typical UMR magnitudes reported previously in metallic system. With the aid of numerical simulations, this large UMR can be attributed to the extra contribution from SMR, which stems from the tilting of magnetization M caused by the competition between the spin Hall effect induced spin torques (STs) from W layer and the external field Hy. This tilting behavior is illustratively shown in Fig.2 (c). Consequently, we define this UMR as the spin-transfer torque UMR (STT-UMR). The damping like-ST (DL-ST) efficiency of W/CoFeB heterostructures can be also roughly estimated by using both the experimental UMR data and the simulation results. Our finding therefore verifies the correlation between the STs and this large room-temperature UMR, thereby providing an alternative approach to determine DL-ST efficiency.

![](https://assets.underline.io/uploads/markdown_image/1/image/40f566960248469705c79873e22caf42.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/7ec5345f8d8c1cc8ce22f8a88abc6e0c.jpg)

Observation of Large Spin Torques-Related Unidirectional Magnetoresistance in Metallic Heterostructures

Observation of Large Spin Torques-Related Unidirectional Magnetoresistance in Metallic Heterostructures

Spin-orbit torque (SOT) has been demonstrated to manipulate the magnetization in various types of heterostructures. Besides the conventional current-induced SOT switching with the assistance of in-plane bias field, field-free SOT switching and memristive SOT switching have also drawn lots of interests due to the possible applications in commercial magnetic random access memory (MRAM) and neuromorphic computing devices. To further decrease power consumption in such devices, most works commit to enhance the SOT efficiency. However, the thermal stability of device is also essential for reliable applications. Especially when SOT devices are integrated with CMOS that the thermal dissipations from transistors can cause significant increase of ambient temperature. In this work, we systematically investigate the SOT efficiencies, thermal stability factors (Δ), and memristive SOT switching behaviors of a standard W/CoFeB/MgO device and a field-free CoFeB/W/CoFeB/MgO device. In both samples, the SOT efficacies are characterized to be constant from 25 °C (298 K) to 80 °C (353 K), where the thermal stability factors (Δ) are slightly degraded as increasing the temperature. The memristive SOT switching behaviors in both systems with various pulse-widths and temperatures are also examined.1 The memristive switching windows are found to shrink at elevated temperatures, which poses the potential challenge of employing SOT devices in realistic neuromorphic applications. Our results suggest that, although the SOT efficacy is robust against thermal effects, the reduction of Δ at elevated temperatures could be detrimental to standard memory as well as neuromorphic (memristive) device applications.2

**References:**

[1] S. Z. Rahaman, I. Wang, T. Chen, C. Pai, D. Wang, J. Wei, H. Lee, Y. Hsin, Y. Chang, S. Yang, Y. Kuo, Y. Su, Y. Chen, K. Huang, C. Wu, and D. Deng, IEEE Electron Device Lett. 39, 1306 (2018). [2] W.-B. Liao, T.-Y. Chen, Y.-C. Hsiao, and C.-F. Pai, Appl. Phys. Lett. 117, 182402 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/e09c6a1eb4904ce1c7b8e57544c4a31b.jpg)
The memristive current-induced switching of a field-free device with various Jmax.

![](https://assets.underline.io/uploads/markdown_image/1/image/cd07b004a65ffb6019e31583614cec14.jpg)
Switching percentages as functions of the applied Jmax for temperature-dependent measurement.

 Pulse-width and thermal effects on field-free memristive spin-orbit torque switching

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 in contemporary magnetic memory devices. When it comes to materials with sizable efficiency of generating spin current from charge current, 5d transition metals such as Pt [1], Ta [2], and W [3] are the representative candidates due to the sizable spin Hall effect (SHE). More recently, topological insulators (TIs) are reported to possess high SOT efficiency from spin momentum locking of the topologically-protected surface states (TSSs) [4]. 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. Particularly, for exfoliated WTe2, it is proposed to exhibit anomalous spin torques and extremely large damping-like (DL) SOT efficiency [5, 6]. However, most of the previously studied crystalline WTe2 are prepared by mechanical exfoliation, which limits their potentials for industrial application. It is therefore crucial to explore the possibilities of employing WTe2 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 WTe2 and co-sputtered W100-xTex-based magnetic heterostructures prepared by high vacuum sputter depositions. Both series of deposited multilayer stacks are amorphous. We first quantify the damping constant of WTe2/CoFeB devices with in-plane magnetic anisotropy (IMA) through ST-FMR measurements, from which the damping constant is determined to be αWTe2/CoFeB=0.009±0.001, smaller than those obtained from control samples with classical spin Hall transition metals, namely Pt/CoFeB (αPt/CoFeB=0.033±0.003) and W/CoFeB (αW/CoFeB=0.014±0.002). The largest DL-SOT efficiency from the stoichiometric WTe2/CoTb devices with perpendicular magnetic anisotropy (PMA) is further estimated to be ξDLWTe2∼0.20 by current-induced hysteresis loop shift measurement, which are greater than those of the W-based control sample (ξDLW∼-0.04). To give further evidence of the sizable DL-SOT efficiency in WTe2, we perform the current-induced magnetization switching measurement from WTe2/CoTb devices. As shown in Fig. 1, the switching polarities depend on the applied Hx, which is consistent with the SOT-driven switching mechanism. More importantly, the critical switching current density Jc of WTe2/CoTb structure is further demonstrated to be Jc∼7.05×109 (A/m2), which is much lower than those from the 5d transition metal-based heterostructures (Jc∼1011 A/m2). Note that this value is of the same order as the exfoliated 80 nm-thick-WTe2 case reported by Shi. S et al. (Jc∼3×109 A/m2) [6]. All of the advantages mentioned above indicate that the amorphous sputtered WTe2 is a competitive SOT source in the emergent chalcogenide materials category. To explore the possible origins of SOT in the amorphous WTe2 heterostructures, we turn to compare the stoichiometric WTe2 with co-sputtered W82Te18 samples. Fig. 2 shows the SOC layer thickness dependence of │ξDL│ from both type of samples. For the co-sputtered case, the thickness dependence of efficiency can be well fitted by the spin diffusion model, which suggests that the SOC within W82Te18 is of bulk origin (such as the SHE). In contrast, for WTe2, │ξDLWTe2│ 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. Lastly, we turn our focus on the electrical transport properties of amorphous WTe2. Most of the previous works claimed that the textured WTe2 is one kind of Weyl semimetal, while our sample is predominately amorphous and therefore should not fall into this category. To get insight into the electron transport behavior behind, we investigate the temperature dependence of resistivity from the amorphous WTe2. The amorphous WTe2 is inclined to behave as semiconductor rather than metal or semimetal. Within the absence of periodic lattices and band structure, the hopping mechanism such as the small polaron hopping (SPH) model is more suitable to describe the phenomenon [7]. In high temperature regime (323-423K), the resistivity of WTe2 can be well fitted by the SPH model, which indicates that thermally-activated hopping polarons from electron-lattice interaction dominate the transport mechanism behind, and therefore suggests that the sputtered amorphous WTe2 is semiconducting with hopping transport property.

**References:**

[1] I. M. Miron, K. Garello, G. Gaudin, Nature, Vol. 476, p.189-193 (2011) [2] L. Liu, C.-F. Pai, Y. Li, Science, Vol. 336, p.555 (2012) [3] C.-F. Pai, L. Liu, Y. Li, H. W. Tseng, Appl. Phys. Lett., Vol. 101, 122404 (2012) [4] A. R. Mellnik, J. S. Lee, A. Richardella, Nature, Vol. 511, p.449-451 (2014) [5] D. MacNeill, G. M. Stiehl, M. H. D. Guimaraes, Nat. Phys., Vol. 13, p.300-305 (2017) [6] S. Shi, S. Liang, Z. Zhu, K. Cai, Nat. Nanotechnol., Vol. 14, p.945-949 (2019) [7] N. F. Mott, E. A. Davis, Clarendon press: Oxford (1979)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/91639aef8ebe4f7aaa5df27345844586.jpg)
Current-induced magnetization switching from a WTe2(10)/CoTb(6)/Ta(2) device with applied bias fields Hx=±900 Oe.

![](https://assets.underline.io/uploads/markdown_image/1/image/aa865ea0cb95c12b66c607866970e5df.jpg)
Buffer layer thickness dependence of │ξDL│ for W82Te18-based and WTe2-based devices. The blue dashed line represents the fitting of the │ξDL(tW82Te18)│ data to a spin diffusion model with spin diffusion length λsW82Te18∼4.1nm.

 Efficient Spin-Orbit Torque Generation in Semiconducting WTe2 with Hopping Transport

The spin current generated from the spin Hall effect of the heavy metal layer can exert spin orbit torques (SOTs) on the adjacent ferromagnetic layer and further produce current-induced SOT-driven magnetization switching. However, for the ferromagnetic layer with in-plane magnetic anisotropy (IMA), probing such switching phenomenon typically relies on tunneling magnetoresistance of three-terminal devices[1-3], differential planar Hall resistance in Hall-bar devices[4], or Kerr effect imaging approach[5]. In this work, we provide a reliable and all-electrical scheme to characterize both the damping-like SOT (DL-SOT) and field-like SOT (FL-SOT) in micron-sized heterostructures with IMA. Firstly, for magnetic heterostructures containing heavy metals, there exist current-induced in-plane spin Hall effective fields (Hyeff) and unidirectional magnetoresistance (UMR) that will modify their anisotropic magnetoresistance (AMR) behavior (Fig.1). The electrical detection of SOT-driven magnetization switching (Fig.2 (b) and (d)) in W/CoFeB heterostructures is demonstrated by means of AMR measurement under such influences. Through switching measurements with various pulse widths, the effective DL-SOT efficiency (|ξDL| ≈ 0.32) can be extracted for Hall-bar devices with 2.0 nm ≤ tCo-Fe-B ≤ 3.5 nm, which is fairly consistent with the values for thin W layer obtained by other approaches[3, 6-8]. The effective FL-SOT efficiency ( (|ξFL| ≦ 0.05) ) can be obtained by AMR loops shift measurements and is found to be slightly enhanced by inserting a MgO layer on top of CoFeB. Moreover, we demonstrate that for a micro-sized device with a well-defined easy axis (along y-direction), as prepared from an 8-inch CMOS-compatible fabrication facility, the detection of SOT switching can also be achieved by the UMR readouts. Our studies suggest that this pump (DL-SOT) -probe (SHF-modified AMR or UMR) approach allows for characterizing key parameters of SOT switching from IMA heterostructures without lengthy device fabrication processes and complicated measurement protocols.

**References:**

[1] M. Cubukcu, O. Boulle, M. Drouard, et al., Appl. Phys. Lett., Vol. 104, 042406 (2014). [2] L. Liu, C.-F. Pai, Y. Li, et al., Science, Vol. 336, 555 (2012). [3] C.-F. Pai, L. Liu, Y. Li, et al., Appl. Phys. Lett., Vol. 101, 122404 (2012). [4] G. Mihajlović, O. Mosendz, L. Wan, et al., Appl. Phys. Lett., Vol. 109, 192404 (2016). [5] S. Shi, S. Liang, Z. Zhu, et al., Nat. Nanotechnol., Vol. 14, 945-949 (2019). [6] R. Bansal, G. Nirala, A. Kumar, et al., Spin, Vol. 08, 1850018 (2019). [7] Q. Hao, G. Xiao, Phys. Rev. Appl., Vol. 3, 034009 (2015). [8] S. Mondal, S. Choudhury, N. Jha, et al., Phys. Rev. B, Vol. 96, 054414 (2017).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/e2f0127374b5a84b126e71ea2a9a7422.jpg)
Fig. 1. Representative shifted AMR loops measured from a W(3)/CoFeB(1.8) sample with dc currents Idc = ±3mA.

![](https://assets.underline.io/uploads/markdown_image/1/image/5edd50eccb0c9cfaf3b331791e9cd810.jpg)
Fig. 2. (a, c) Measurement sequence for the switching experiment and (b, d) current-induced SOT switching results of a W(3)/Co40Fe40B20(1.8) Hall-bar sample with opposite sense currents Isense = +1 mA and Isense = -1 mA.

 Detection of Spin-Orbit Torques for In-plane Magnetized Heterostructures by Utilizing Spin Hall Effective Field

FOB: Spin Hall and Spin-Orbit Torque I

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Presentations

Growth dependent Interlayer Chiral Exchange and Field

Spin orbit torque switching mediated by orbital currents from CuNx

Observation of Large Spin Torques-Related Unidirectional Magnetoresistance in Metallic Heterostructures

Pulse-width and thermal effects on field-free memristive spin-orbit torque switching

Efficient Spin-Orbit Torque Generation in Semiconducting WTe2 with Hopping Transport

Detection of Spin-Orbit Torques for In-plane Magnetized Heterostructures by Utilizing Spin Hall Effective Field

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