Skyrmion Based Synaptic Device Having Perpendicular Magnetic-Tunnel-Junction Spin-Valve Sensor
Authors: Choi Yohan , Jun Han-Sol , Kwon Ho-Jung , Lee So-Hyun , Shin Yeon-Soo , Shim Tae-Hun , Park Jea-gun 
Published: February 2024

Perpendicular spin-transfer-torque magnetic random access memory (p-STT MRAM) has been reported as an application of binary neural networks (BNN) [1]. In addition, magnetic skyrmions have been reported as information carriers for neuromorphic devices due to their small nanoscale diameter, topological stability, and complete linearity [2-3]. A previous study has demonstrated the synaptic characteristics of skyrmion based synapses using a Hall bar measurement [4]. However, skyrmion based synapses using the Hall bar measurement were not able to detect electrically a synaptic behavior. Thus, for the first time, the skyrmion based synaptic device detected resistance change via sensing the perpendicular magnetic-tunnel-junction (p-MTJ) spin-valve, as shown in Fig.1(a). The detailed synaptic behavior and mechanism will be presented.
In this study, we introduced the skyrmion based synaptic device having p-MTJ spin-valve sensor designed by CoFeB free layer, MgO tunneling barrier, CoFeB pinned layer, SyAF layer, and interface reducing W layer between the CoFeB free layer and MgO tunneling barrier, as shown in Fig.1(a). The spin direction of the CoFeB free layer was changed by the generated skyrmions, and the resistance of the synaptic device changed depending on the parallel or anti-parallel state between CoFeB free and pinned layer. In particular, the skyrmion generation was determined critically by the combination of DMI and PMA (i.e., saturation magnetization (Ms), squareness), and the interface reducing W layer was designed to reduce PMA. The M-H loop of the synaptic device with the interface reducing W layer thickness of 0.110 nm showed reduced saturation magnetization (Ms ~ 60 μemu) and squareness (Hc ~ 8 Oe) via a vibrating-sample magnetometer (VSM), as shown in [Figs.1(b), 1(c), and 1(d)]. In addition, uniform state, generation of skyrmions, and stripe domains in the synaptic device were achieved at the external magnetic field of 0, 3.5, and 7 Oe, respectively, as shown in [Figs.1(e), 1(f), and 1(g)]. Moreover, we observed that the number of skyrmions in the synaptic device with a cell diameter of 5μm changed depending on the external magnetic field via the magneto-optical Kerr effect (MOKE) image, as shown in [Figs.1(h), 1(i), and 1(j)]. Finally, we demonstrated a multi-level operation in which the electrical resistance of the synaptic device changed as the number of skyrmions. In our presentation, we will review in detail the synaptic behavior and mechanism for the synaptic device.

Acknowledgements

This research was supported by National R&D Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (2021M3F3A2A01037733)

References

[1] Thi-Nhan Pham et al., IEEE JOURNAL ON EMERGING AND SELECTED TOPICS IN CIRCUITS AND SYSTEMS, 12, 569-579 (2022)
[2] Nagaosa, N. & Tokura, Y. Nat. Nanotechnol. 8, 899–911 (2013).
[3] C Back, V Cros et al., J. phys. D. Appl. phys. 53, 363001 (2020).
[4] Kyung Mee Song et al., Nature Electronics. 3, 148–155 (2020)

Figure 1