In this talk, we first report the roadmap and development of advanced perpendicular magnetic tunnel junctions based on low-damping magnetic materials with crystalline anisotropy that can be scaled down to sub-3-nm diameter with 10 years’ retention time [1]. Then, we will present the perpendicular magnetic tunnel junctions (p-MTJs) switched utilizing bipolar electric fields. Traditional voltage-controlled magnetic anisotropy only linearly lowers the energy barrier of ferromagnetic layer via electric field effect and efficiently switches p-MTJs only with a unipolar behavior. Here we propose and demonstrate a bipolar electric field effect switching of 100-nm p-MTJs through voltage-controlled exchange coupling (VCEC) [2]. The switching current density, ~1.1×105 A/cm2, is one order of magnitude lower than that of the best-reported spin-transfer torque devices. Theoretical results suggest that electric field induces a ferromagnetic-antiferromagnetic exchange coupling transition and generates a field-like interlayer exchange coupling torque, which cause the bidirectional magnetization switching. We will further report our recent results to switch the p-MTJs using the interplay of SOT and VCEC, which lowers the VCEC switching current density to ~5×103 A/cm2[3]. These results could eliminate the major obstacle in the development of spin memory devices beyond their embedded applications. This technology can be used directly for the future spin-orbit-torque (SOT) MRAM [3] and spin logic devices [4]. 
**References** [1] D. Zhang, et al, Phys. Rev. Applied, (2018) vol. 9, 044028 
[2] D. Zhang, et al, Nano Letters (2022), doi.org/10.1021/acs.nanolett.1c03395 
[3] B. Zink, et al, Advanced Electronic Materials, (2022), DOI: 10.1002/aelm.202200382 
[4] M. Mankalale, et al, IEEE Journal on Exploratory Solid-State Computational Devices and Circuits, 3 (2017) 27-36; 10.1109/JXCDC.2017.2690629 

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Perpendicular Magnetic Tunnel Junctions with Crystalline Anisotropy and Energy efficient Switching of Synthetic Antiferromagnet pMTJs by voltage

Magnetic random access memory (MRAM) is a promising candidate for beyond-CMOS memory and logic technologies and has been developed for commercial purposes in recent years (1). The key building block of commercial MRAM is magnetic tunnel junction (MTJ), which is operated through either spin transfer torque (STT) or spin-orbit torque (SOT). STT-based MRAM benefits from higher cell density but suffers from reliability and energy-efficiency due to the current flow through the MTJ whereas SOT-based MRAM has superior reliability, endurance, and faster switching times (2). For realizing the ultralow switching current density and ultrafast switching MRAM cell, two or more driving forces are normally employed to switch MTJs. One acts as the principal switching mechanism and the other serves to assist in switching. For example, ultrafast switching speed of ~ 0.27 ns has been reported in SOT switching of perpendicular MTJs (p-MTJs) assisted with STT and voltage-controlled magnetic anisotropy (VCMA), however, the key challenge is still large switching current densities (Jc) ~ 2.0×108 A/cm2 for SOT and ~ 2.3×106 A/cm2 for STT (3).
Voltage controlled exchange coupling (VCEC) occurs in perpendicular MTJs (p-MTJs) with synthetic antiferromagnetic (SAF) free layers and can realize bidirectional switching at switching current densities as low as 1 x 105 A/cm2 (4). In this work, we designed and fabricated the p-MTJ stacks with SAF free layers on bi-layered spin Hall channels (Ta/Pd). The p-MTJ stacks were patterned into 150-nm pillars, and MTJ devices are switched through combination of SOT and VCEC effects, as illustrated in Fig. 1(a). Bidirectional switching of p-MTJs were obtained with current densities as low as 3×103 A/cm2 for VCEC and 6.8×107 A/cm2 for SOT, as shown in Fig. 1(b), where the VCEC switching current density is two orders of magnitude lower than the reported value for VCEC-only switching [4]. Furthermore, by studying the contribution of SOT and VCEC for MTJ switching, it is found that SOT plays a crucial role for ultralow-energy VCEC bidirectional magnetization switching. 

**References:** 
(1) J.-P. Wang et al, DAC '17: Proceedings of the 54th Annual Design Automation Conference 16, pp. 1-6 (2017). 
(2) Y. C. Liao et al, IEEE J. Explor. Solid-State Computat. 6, pp. 9-17 (2020). 
(3) E. Grimaldi et al, Nat. Nanotechnol. 15, pp. 111-117 (2020). 
(4) D. Zhang et al, Nano Lett., 22, pp. 622-629 (2022). 

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Ultra low current density switching via voltage

Application of electric fields (E-fields) on perpendicular magnetic tunnel junctions (p-MTJs) is a promising solution for improving energy efficiency in spintronics based memory and logic systems [1]. Many studies on E-field effects on p-MTJs have focused on voltage-controlled magnetic anisotropy (VCMA) of the p-MTJs free layer to reduce the energy required for p-MTJs to switch magnetization states [2]. In this approach, VCMA temporarily decreases the energy barrier separating the two magnetization states and bidirectional switching is achieved using either an external magnetic field or a spin-polarized current. Another approach is to utilize voltage controlled exchange coupling (VCEC) between two ferromagnetic layers in synthetic antiferromagnetic (SAF) structures. Recently, bidirectional switching through VCEC has been demonstrated on p-MTJs with bottom dual-SAF free layers [3]. Benchmarking on MRAM technologies has shown VCEC-MRAM to have much lower energy dissipation than spin-orbit torque (SOT) MRAM [4]. However, SOT-MRAM has faster write speeds since SOT allows for ultra-fast (≤ 400ps) switching [5]. In this work, we explore the potential for SOT-assisted VCEC switching on p-MTJs with bottom dual-SAF free layers fabricated on bilayer spin hall channels. Fig. 1 shows that the SOT current can tune the coercivity of the p-MTJ, which illustrates that SOT can assist in VCEC switching. Fig. 2 shows that a voltage applied to the MTJ causes either a positive or negative shift in its stray field (center of the field switching curve), which indicates that voltage influences interlayer exchange coupling in the SAF free layer, thus demonstrating VCEC. Since the coercivity of the p-MTJs can be tuned with both VCEC and SOT, dual-biasing of SOT+VCEC can possibly generate stochastic signals for probabilistic-bits. By utilizing the effects of both VCEC and SOT, we introduce a potential solution for low-energy and ultra-fast MRAM technology as well as more novel computing paradigms such as probabilistic spin logic.

![](https://assets.underline.io/uploads/markdown_image/1/image/c46c24b4bba3f3242ec62765e893ac77.jpg)

Fig. 1. Anomalous hall resistance versus field measurements at SOT currents ranging from 1mA to 2.25mA.

![](https://assets.underline.io/uploads/markdown_image/1/image/98c15c49ed5dd8b81e43a340598f2316.jpg)

Fig. 2. Tunneling magneto-resistance (TMR) vs. field measurements at VCEC voltages of +1.1V and -1.1V.

Spin-orbit torque assisted switching with voltage controlled exchange coupling

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SB: Recent Developments in Advanced Magnetic Tunnel Junctions

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DPA: Spintronic Devices for Memory and Computing

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GPE - Magnetoresistance and Voltage-Controlled Magnetic Properties

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Deyuan Lyu

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Perpendicular Magnetic Tunnel Junctions with Crystalline Anisotropy and Energy efficient Switching of Synthetic Antiferromagnet pMTJs by voltage

Ultra low current density switching via voltage

Spin-orbit torque assisted switching with voltage controlled exchange coupling

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