Canted magnetic field assisted spin orbit torque switching in a perpendicularly magnetized nano-magnet

November 29, 2023

Spin orbit torque(SOT) driven magnetization switching [2][3] has attracted attention towards next generation magnetic memory[1]. The SOT driven magnetization switching is suitable for high-speed operation. The high endurance is also expected because the read and write current paths in the SOT device are separated(Figure 1). Furthermore, the SOT device with perpendicular magnetization anisotropy(PMA) has a potential of high-density applications. In this report, we investigated the bias magnetic field dependence of the SOT magnetization switching in a nano-dot with PMA by using a micromagnetic simulation with the SOT term.

Fig.1 SOT-MRAM cell with PMA MTJ

Generally, the SOT driven perpendicular magnetization switching requires the bias magnetic fields in the x direction which is parallel to the SOT current direction(Figure 2(a)). Firstly, the effect of bias field application time was investigated with the magnetic field in x direction(Hx) (Figure 2(b)(c)). The magnetic field and SOT current are applied at the same time. After 1 ns only the SOT current is stopped and the relaxation is studied as a function of magnetic field applying time. The time for relaxation of magnetization oscillates as the magnetic field application time increases, and then settles to a shorter level. It indicates that compared to field applying time same as current applying, a sufficiently-long magnetic field application time makes the SOT driven perpendicular magnetization switching faster.

Fig.2(a) Configuration of simulation conditions
Fig.2(b) Normalized magnetization as a function of time t
Fig.2(c) Relaxation time as a function of magnetic field application time
Copyright (2023) The Japan Society of Applied Physics

However, despite that with lower magnetic fields the magnetization can be switched successfully, it was found that the high bias magnetic fields over Hx=160 Oe caused a switching error where the magnetization is switched back to the original direction(Figure 3 (a)). In order to avoid the switching error, canted bias magnetic fields combining the perpendicular magnetic field Hz and parallel magnetic field Hx are newly introduced. The angle dependence of the magnetization behavior with the constant magnetic fields of H = 160 Oe shows that the addition of perpendicular bias magnetic fields Hz suppresses the switching error(Figure 3 (b)). Unlike the SOT by Hx, the SOT by Hz always assists the precession around z axis, which is considered to suppress the error. It is clarified that the canted bias magnetic fields contribute to a faster and more stable SOT magnetization switching.

(a) Hx
(b) magnetic field applying angle
Fig.3 SOT switching as a function of magnetic field applying time depending on (a)Hx (b) magnetic field applying angle
Copyright (2023) The Japan Society of Applied Physics

In this report, we proposed a new method of SOT perpendicular magnetization switching assisted by the canted magnetic fields, which is applicable to faster and more stable SOT-MRAM with PMA.

This was presented at International Conference on Solid State Devices and Materials in 2023(SSDM2023).

[1] S. Fukami et al., Nat. Nanotech. 11, 621 (2016).
[2] I. M. Miron et al., Nature 476, 189 (2011).
[3] L. Liu et al., Science 336, 555 (2012).