The continuous growth in the demand to store, process, and access data is one of the main drivers of spintronic research. Emerging big data and AI computing technologies require writing data in less time and storing them at a smaller scale, and consuming less energy [1]. Magnetic memories (MRAM) using spin-transfer torques have proven their place among leading non-volatile memory technologies and are currently in production for eFlash replacement applications [2]. In parallel, spin-orbit torques (SOT) have emerged and proven to be an efficient way of writing reliably magnetization at nanosecond time scale, broadening the scope of MRAM to applications that run close to the clock speed of the central processing unit, leading to novel spintronic memory and computing approaches [3], [4].
I will review across this presentation the fundamental characteristics of SOT and their use to switch perpendicularly magnetized magnetic tunnel junction (MTJ) devices. I will cover different challenges to take up SOT-MRAM from material and stack optimization to technology large-scale integration and circuit design [5]. After describing typical full-scale integration process of SOT-MRAM devices on 300mm wafers using CMOS compatible processes, I will discuss manufacturable field free methods and figures of merit for SOT-based memories. Finally, I will introduce bit-cell design considerations on scaling and performance in the perspective of circuit macro-design architectures [6], and discuss the interest to combine SOT, STT and voltage-gate (VCMA) effects to improve density and leverage performances [7]. I will conclude with opportunities to diverse SOT-MRAM application spectra, notably in the field of in-memory computing [8]. 

**References:** 

[1] G. Molas and E. Nowak, “Advances in emerging memory technologies: From data storage to artificial intelligence,” Appl. Sci., vol. 11, no. 23, (2021), [2] B. Dieny et al., “Opportunities and challenges for spintronics in the microelectronics industry,” Nat. Electron., vol. 3, no. 8, pp. 446–459 (2020), [3] Q. Shao et al., “Roadmap of Spin-Orbit Torques,” IEEE Trans. Magn., vol. 57, no. 7 (2021), [4] A. Manchon et al., “Current-induced spin-orbit torques in ferromagnetic and antiferromagnetic systems,” Rev. Mod. Phys., vol. 91, no. 3 (2019), [5] K. Garello, F. Yasin, and G. S. Kar, “Spin-orbit torque MRAM for ultrafast embedded memories: From fundamentals to large scale technology integration,” IEEE IMW 2019, pp. 2019–2022 (2019), [6] M. Gupta et al., “High-density SOT-MRAM technology and design specifications for the embedded domain at 5nm node,” Tech. Dig. - IEDM, vol. 2020-Decem, pp. 24.5.1-24.5.4 (2020), [7] Y. C. Wu et al., “Voltage-Gate-Assisted Spin-Orbit-Torque Magnetic Random-Access Memory for High-Density and Low-Power Embedded Applications,” Phys. Rev. Appl., vol. 15, no. 6, pp. 1–10 (2021), [8] J. Doevenspeck et al., “SOT-MRAM based Analog in-Memory Computing for DNN inference,” Dig. Tech. Pap. - Symp. VLSI Technol., vol. 2020-June, no. 1, pp. 2020–2021 (2020) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/9bc65a5e3946864df38958e52573eb17.png)

Spin orbit torque switching of magnetic tunnel junctions for memory applications

Magnetic random access memory (MRAM) with its inherent non-volatility is believed to address the large stand-by energy issues in the present memory hierarchy [1]. In recent years, the spin-transfer torque (STT)-MRAM has gradually matured and started to appear in the market. Typically, STT writing of perpendicular magnetic tunnel junction (pMTJ) is limited to a few nanoseconds. This is because STT is collinear and competing with the intrinsic damping of the free-layer (FL), resulting in a long incubation delay [2] and large energy consumption. To mitigate STT limitations, spin-orbit torque (SOT) and voltage control of magnetic anisotropy (VCMA) were proposed as alternative MRAM writing mechanisms. SOT is able to switch the FL of a pMTJ by injecting an in-plane current in an adjacent SOT layer, which enables a three-terminal cell structure with energy-efficient and reliable sub-ns writing capabilities [3]. On the other hand, VCMA promises significant advances toward low-power MRAM. The electronic-based VCMA effect can instantly modify the perpendicular magnetic anisotropy (PMA) of the FL to induce precessional switching at GHz rates and ultra-low energy consumption (fJ) [4]. However, both mechanisms also present technological challenges. The challenges for SOT are currently related to the density and write efficiency: the 2-transistor 1-MTJ cell structure limits the array density, and the SOT write current remains larger than for STT-MRAM, which imposes a large selector transistor to accommodate its write current. For VCMA, the write margin is small, and is subject to the variations in VCMA coefficient and field amplitudes, making write control difficult in dense arrays. Further, a large VCMA effect (&gt;1000fJ/Vm) is mandatory to avoid compromising retention in sub-30nm pMTJ, which remains as a major challenge as typical values in pMTJs are 30-60 fJ/Vm at device level [5]. To overcome the above-cited limitations and to combine the best of these two approaches, voltage-gate assisted spin-orbit torque (VGSOT)-MRAM concept [Fig. 1(a)] has been proposed [6]. In this work, we report on VGSOT switching properties in pMTJ, integrated with our 300mm SOT-MRAM platform [3]. The stack is composed of W(3.5nm)/CoFeB(1nm)/MgO(1.7nm, RA ~ 5kΩ.μm2)/RL/SAF, where the thick MgO ensures pure VCMA gate control without STT contribution. We use an electrical scheme that allows for individual control over SOT/gate pulse amplitudes (VSOT/Vg), and variable pulse duration (tp). Fig. 1(b) shows exemplary SOT switching probability (Psw) curves under different Vg, at tp = 0.4ns. We observe a clear decrease (increase) in VSOT under Vg of 1V (-1V) for both AP-P and P-AP transitions. The critical switching voltage, defined at Psw = 50%, is converted into the critical switching current (Ic) and plotted as a function of 1/tp in Fig. 1(c). It shows a typical linear scaling, i.e. Ic = Ic0 + q/tp in the sub-ns regime for all gate values [7], where Ic0 is the intrinsic critical current and q indicates the nucleation energy for magnetization reversal. The linear dependences of Ic0 and q on Vg [Fig. 1(d)] reflect the direct modification of PMA upon Vg applications and the change of nucleation energy, respectively. We report here a 25% reduction in Ic under Vg = 1V, which corresponds to a 45% reduction in total switching energy to 30fJ at 0.4ns. We further demonstrate that VGSOT offers reliable sub-ns switching with a low write error rate (&lt;10-5) and that VGOST-pMTJ is highly durable against large read/write stresses (&gt;1012) Finally, we benchmark the VGSOT performance at the 5nm technology node against other embedded memory technologies to highlight the VGSOT benefits. It reveals that VGSOT-4MTJ design can reduce the effective cell area to &lt;0.5x of the high-density SRAM at minimal performance degradation. Meanwhile, the constrain of achieving challenging material parameters for VCMA-MTJ (ξ &gt; 1000fJ/Vm) and SOT-MTJ (θSH &gt; 1.5) can be relaxed to reasonable values: ξ = 300fJ/Vm and θSH = 0.45. Our study shows the great potential of this VGSOT approach for fast, dense and low power embedded memory applications.

**References:**

[1] B. Dieny et al., Nat. Electron. 3, pp. 446–459 (2020) [2] T. Devolder et al., Phys. Rev. Lett. 100, 057206 (2008) [3] K. Garello et al., IEEE VLSI Circ., pp. 81-82 (2018) [4] Y. C. Wu et al., IEEE VLSI Tech., TMFS.4 (2020) [5] C. Grezes et al., IEEE Magn. Lett. 8, 3102705 (2017) [6] H. Yoda et al., IEEE IEDM, pp. 27.6.1-27.6.4 (2016) [7] K. Garello et al., Appl. Phys. Lett. 105, 212402 (2014)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/704e6e8a749f9a7c8eea400c0ddf393c.jpg)
Fig. 1. (a) Illustration of the VGSOT writing scheme. (b) SOT switching probability (Psw) as a function of VSOT under different Vg at tp = 0.4ns. (b) Critical SOT switching current and current density at Psw = 50% as a function of 1/ tp, showing typical linear scaling for all Vg. (c) Ic0 and q as a function of Vg.

 Voltage-Gate Assisted Spin-Orbit Torque MRAM for High-Density and Low Power Embedded Applications INVITED

Fast domain wall (DW) motion in magnetic nanostructures is crucial for future spintronic devices, such as racetrack memory [1], spin logic [2] and neuromorphic computing [3]. The discovery of fast DW transport governed by spin-orbit torque (SOT) and the Dzyaloshinskii-Moriya interaction (DMI) in ultrathin magnetic layers on heavy metals was a breakthrough in the development of low energy devices based on DW motion. However, a lack of all-electrical control of DWs in nanoscale devices impedes to bring these advanced materials to practical applications. Therefore, finding energy-efficient ways to electrically write, read and transport DWs is a necessity. As demonstrated in STT-MRAM technology, the magnetic tunnel junction (MTJ) based on a CoFeB/MgO free layer (FL) [4], which offers efficient spin-transfer torque (STT) write and a high tunneling magnetoresistance (TMR) readout signal, could be a potential solution to electrically operate a full functional DW device. However, such an MTJ stack poses significant challenges to realize a DW device, which particularly relate to low DW speed and poor manufacturability of a CoFeB/MgO based DW conduit using industrial integration platforms [5]. Here, we propose and develop a new type of MTJ stack that incorporates typical high DW velocity materials (i.e., Pt/Co) as the second FL into a conventional MTJ with a CoFeB/MgO based FL. We firstly demonstrate that the fundamental properties of a standard MTJ device are not compromised by the integration of DW conduit materials. Complete functional DW devices, consisting of multiple MTJ pillars connected by a common FL, have been fully integrated using such novel MTJ stack, Figure 1 (a). We then demonstrate that all-electrical control of a DW in a nanoscale device, i.e., STT write, TMR read and SOT driven DW motion, can be achieved. As an example, figure 1 (b) presents field-driven domain expansion from P2 through the whole track, after the domain was nucleated by STT in P2. Figure 1 (c) demonstrates the current-driven DW transport from P2 to P1, with small field assistance. No change in resistance was observed in P3, as expected from directional current-driven DW motion. Finally, we show that these DW devices can be used to investigate the SOT driven DW dynamics in nanoscale devices where conventional magnetic imaging techniques become ineffective. Our DW devices show good TMR read-out and efficient STT writing, comparable to current STT-MRAM devices. The devices are fabricated in imec’s 300 mm CMOS fab on full wafers which clears the path for large scale integration. This proof-of-concept thus offers potential solutions for high performance and low-power DW-based devices for logic and neuromorphic applications [5,6].

**References:**

1. Parkin et al., Science 320, 190 (2008) 2. Zhaochu., et al., Nature 579, 214 (2020) 3. Siddiqui., et al., Nano Lett. 20, 1033 (2019) 4. Ikeda, et al., Nat. Mater. 9, 721 (2010) 5. Raymenants, et al., 2018 IEDM, 36.4 6. Raymenants, et al., 2020 IEDM, 21.5

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/84febf49913847a9fbe7f5b8a24e7a0e.jpg)
Figure 1 | (a) TEM image of a DW device. The figure highlights how current can flow through the FL track, transporting DWs from pillar to pillar. (b) Domain was nucleated in P2 by STT, after which external field expanded it to the outer pillars, P1 and P3. (c) A domain was nucleated in P2 by STT, after which directional DW transport was achieved from P2 to P1 when driven by SOT and a small assist field. No DW propagation was observed to P3 as expected [6].

 All-electrical control of nanoscale domain wall devices using magnetic tunnel junction read and write INVITED

The realization of magnetization switching induced by in-plane current injection in heavy metal/ferromagnetic heterostructures has drawn increasing attention to spin-orbitronics, leading to the advent of spin-orbit torques magnetoresistive random access memories (SOT-MRAM). Compared to heavy metals, oxide 2D electron gases have emerged as alternative spin-orbitronics material systems, which benefit from an efficient spin-charge interconversion through the direct and inverse Edelstein effects. Recently, we have demonstrated an enhancement of the spin-to-charge conversion efficiency by two orders of magnitude in SrTiO3-based 2D gas compared to conventional heavy metals [1], along with a non-volatile electric-control of the spin-to-charge conversion [2]. While the sign and efficiency of the SOTs are fixed by the stack of materials in conventional SOT devices, achieving an electric-control of the mirror charge-to-spin conversion would be of great interest for developing reconfigurable SOT-MRAM and logic gates. Here we report electric-field control of spin orbit torques with electrical remanence in a perpendicular ferromagnet - SrTiO3 system. Non-volatile electric-field control of the sheet resistance is achieved with 1150% contrast, with two switchable and remanent high and low resistivity states of the 2D electron gas. Spin-orbit torques effective fields are further measured using second harmonic Hall methods. A remanent electric-field control of the SOT efficiency is demonstrated, with inversion of the sign of the SOT anti-damping-like effective field. These results are consistent with a combination of both intrinsic modulation of the SOT efficiency together with extrinsic modulation due to the non-volatile electric-control of the current injection in the 2D gas. The non-volatile control of the SOT effective field is further evidenced by reproducible inversion of the SOT torques after initializing with negative or positive voltage pulses of ±130 V, opening the way to reconfigurable spin-orbit torque memory and logic gate architectures.

![](https://assets.underline.io/uploads/markdown_image/1/image/c659e26f3f654d16f4c29d73b4605f6d.jpg)

Fig.1: (a) Structure of the sample and measurement geometry. (b) Electric-field control of spin orbit torques with electrical remanence.

Non-Volatile Electric-Field Control of Spin-Orbit Torques and Anomalous Hall Effect in Perpendicular Ferromagnet - SrTiO3 System


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SD: Metrics and Measurements of Spin-Orbit Torque MRAM

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GPC - Fundamental Spintronics II

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Kevin Garello

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Spin orbit torque switching of magnetic tunnel junctions for memory applications

Voltage-Gate Assisted Spin-Orbit Torque MRAM for High-Density and Low Power Embedded Applications INVITED

All-electrical control of nanoscale domain wall devices using magnetic tunnel junction read and write INVITED

Non-Volatile Electric-Field Control of Spin-Orbit Torques and Anomalous Hall Effect in Perpendicular Ferromagnet - SrTiO3 System

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