Integrate Nature of Perpendicular Spin-Orbit-Torque Neuron Depending on Crystal Structure of Tungsten

Abstract

With the expanding use of artificial intelligence (AI), a higher integration density and lower power consumption is required of the neuromorphic devices. In particular, 2-terminal perpendicular spin-transfer-torque (p-STT) neuron has been reported as a candidate of spin neuron. [1-2] However, p-STT devices cannot reliably operate at ns and sub-ns scale because of large incubation delays. [3] In addition, the shared read/write path compromise the read reliability where the write operation can impose severe stress to the MgO tunneling barrier, leading to a possible time dependent degradation of the p-MTJ spin-valve. As a solution to these issues, Spin-Orbit-Torque (SOT) based MRAM was proposed where the read/write path is separated.The crystallinity of the W film used for SOT channel ex-situ annealed at 350oC was investigated depending on the W thickness from 4 to 10 nm, as shown in Figs. 1(a) and (b), where the transition from β (A15 crystal structure) to α-phase (b.c.c.) occurs between thickness of 5-nm and 6-nm. Also, the resistivity of the W film was greatly reduced from 150 to 40 Ω cm at this thickness which further shows the transition from β to α-phase, as shown in Fig. 1(c). The the b.c.c. (110) crystallinity can be confirmed in the 6-nm thick W, as shown in Fig. 1(d). In our sample, β-phase tungsten was confirmed below 6-nm at ex-situ annealing temperature of 350oC which is known to have the highest SOT efficiency (>0.3) [4]. The p-SOT neuron was patterned into a hall-bar to confirm the SOT efficiency depending on the crystal structure of the W film, as shown in Fig. 1(e). The hysteresis loop showed perpendicular magnetic anisotropy for both 4 and 6-nm W, but the change of the Hall resistance was negligible in the case of 6-nm W thick p-SOT neuron, as shown in Fig. 1(f). The normalized Hall resistance was measured as a function of nearly in-plane magnetic field (β=4o: angle between external field and injected current) under a positive or negative current (±1 mA) to calulate the spin-torque per magnetic moment (τST), as shown in Figs. 1(i)-(k), where the slope of [B+(θ)-B-(θ)] is τST. The τST increases with applied current for both α and β-phase W, but the spin torque of α-phase is about 1/3 of the β-phase W. In addition, the spin hall angle of the β-phase W (-0.288) was about 6 times larger than that of the α-phase W (-0.045), as shown in Fig. 1(l). In our presentation, we will review in detail the integrate nature of the p-SOT neuron depending on the spike amplitude and crystal structure of the W. Acknowledgement *This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (No. 2016M3A7B4910249) and the Brain Korea 21 PLUS Program in 2014. Reference [1] Kondo, K., Choi, J., Baek, J., and Jun, H. A two-terminal perpendicular spin-transfer torque based artificial neuron. J. Phys. D Appl. Phys. 51:504002. (2018)[2] Dong Won Kim, et al. Double MgO-Based Perpendicular Magnetic Tunnel Junction for Artificial Neuron. Front. Neurosci. 14:309 (2020)[3] Ya-Jui Tsou, Jih-Chao Chiu, Huan-Chi Shih, Chee Wee Liu, Write Margin Analysis of Spin–Orbit Torque Switching Using Field-Assisted Method, Exploratory Solid-State Computational Devices and Circuits IEEE Journal on, vol. 5, no. 2, pp. 173-181, 2019.[4] Qiang Hao and Gang Xiao, Giant Spin Hall Effect and Switching Induced by Spin-Transfer Torque in a W/Co40Fe40B20/MgO Structure with Perpendicular Magnetic Anisotropy, PHYS. REV. APPLIED 3, 034009 (2015) Figure 1