Superparamagnetic Tunnel Junctions with Nanosecond Relaxation Time for Probabilistic Computing*

Recently, superparamagnetic tunnel junctions (s-MTJs) with a low barrier energy have been shown to compose probabilistic bits (p-bits) for probabilistic computing using 300 times fewer transistors and consuming one-tenth of the operating energy than purely CMOS-based p-bits [1]. Here we investigate the time-domain response of s-MTJs, which characterizes an average switching event time (the relaxation time) between bit states, and affects the computation time. Based on an analysis of the time evolution of entropy for the magnetization direction probability distribution, we propose that for shorter relaxation times, it is necessary not only to reduce the barrier energy but also to increase the precession frequency [2]. According to this design guideline, s-MTJs with in-plane magnetic easy axis are fabricated and achieve relaxation times down to 8 ns [3], which is more than 5 orders of magnitude shorter than that of typical s-MTJs with perpendicular easy axis [1].

[1] W. A. Borders, et al., Nature 573, 390 (2019).

[2] S. Kanai, et al., Phys. Rev. B 103, 094423 (2021).

[3] K. Hayakawa, et al., Phys. Rev. Lett. 126, 117202 (2021).