Multi-Level Synaptic Device Having Perpendicular Magnetic-Tunnel-Junction Spin-Valve Sensor Via Skyrmion Generated By Nano-Scale Current Emitter
Authors: Choi Yohan , Jun Han-Sol , Lee So-Hyun , Shin Yeon-Soo , Hong Jin-Pyo , Shim Tae-Hun , Park Jea-gun 
Published: August 2024

Recently, with the expansion of deep learning-based artificial intelligence into various fields, high integration density and recognition accuracy are essentially necessary for neuromorphic devices. In particular, skyrmion-based artificial synaptic devices have been actively researched due to complete linearity, topological stability, and small nanoscale size. This device can emulate the synaptic plasticity (i.e., long-term potentiation (LTP) and long-term depression (LTD)) depending on the number of generated skyrmions. Magnetic skyrmions have been characterized by various methods such as perpendicular magnetic-tunnel-junction (p-MTJ) spin-valve structure, Hall bar measurements [3], and non-collinear magnetoresistance [4]: However, in the previous study using a p-MTJ spin-valve, they only measured the resistance of a single skyrmion and did not demonstrate synaptic behavior (i.e., multi-level characteristics) [5]. In addition, they essentially require an additional external magnetic field to generate skyrmions. Moreover, the reported device has low resistance margins, indicating it is extremely difficult to sense in bit-lines.
In this study, for the first time, we introduced a novel multi-level synaptic device having a p-MTJ spin-valve sensor via skyrmion generated by a nano-scale current emitter. To generate skyrmion, we employed nano-scale oxygen vacancy filaments as a current emitter on the p-MTJ spin-valve sensor, and interface reducing W layer between the free layer and MgO layer, as shown in Fig.1(a). In addition, we performed a vibrating sample magnetometer (VSM) analysis to measure the PMA characteristics (i.e., squareness, saturation magnetization) of the SyAF layer and the free layer, as shown in Fig.1(b). Moreover, the generation of skyrmions depending on the thickness of the interface reducing W layer was investigated using VSM and magneto-optical Kerr effect (MOKE) analyses, as shown in Fig.1(c-f). Furthermore, MOKE analysis was conducted to analyze the mechanism of skyrmion generation in the synaptic device having the p-MTJ spin-valve sensor patterned by 5μm diameter, as shown in Fig. 1(g). Finally, we have directly observed the generation of skyrmions using a nano-scale current emitter and demonstrated synaptic plasticity via the skyrmion-based synaptic device having the p-MTJ spin-valve sensor into a 300 nm diameter cell. In our presentation, we will review in detail the synaptic behavior and generation mechanism of the skyrmions.

Acknowledgements

This work was supported by the Technology Innovation Program (or Industrial Strategic Technology Development Program-Public-private joint investment semiconductor R&D program(K-CHIPS) to foster high-quality human resources) ("RS-2023-00235634", Development of high speed/low power/high reliability 2-terminal field-free SOT-MRAM) funded By the Ministry of Trade, Industry & Energy(MOTIE, Korea)(1415187787) and Institute of Information & communications Technology Planning & Evaluation (IITP) under the artificial intelligence semiconductor support program to nurture the best talents (IITP-(2023)-RS-2023-00253914) grant funded by the Korea government(MSIT).

References

[1] Nagaosa, N. & Tokura, Y. Nat. Nanotechnol. 8, 899–911 (2013).
[2] C Back, V Cros et al., J. phys. D. Appl. phys. 53, 363001 (2020).
[3] Kyung Mee Song et al., Nature Electronics. 3, 148–155 (2020)
[4] Christian Hanneken, Nature Electronics. 10 (2015)
[5] Yao Guang, Adv. Electron. Mater. 9, 2200570 (2023).

Figure 1