Among the existing random access memory technologies, magnetoresistive random access memory (MRAM) is an attractive alternative due to its potential for fast operation, energy-efficiency, non-volatility, and high endurance. Although some of these favourable attributes have been realized, a single material/device system that combines all of them simultaneously remains a challenge. 

In conventional computing systems known as the von Neumann architecture, the computing core and memory are usually separated from each other with a gap in processing speeds of different units. Non-volatile in-memory computing devices could potentially overcome this obstacle. However, the memory speed is limited and has posed a significant challenge. Specifically, due to a relatively long latency and the high critical switching current of spin-transfer torque (STT)-MRAM, it is unsuitable for achieving ultrafast operation in the sub-nanosecond regime. Spin-orbit torque (SOT) switching could overcome these limitations due to the fact that the incubation time for switching can be negligible, and the read and write current are applied with two separated channels, which results in possible sub-nanosecond operation and lower energy-consumption with improved reliability.

We have been working on the current-induced magnetization switching in various magnetic materials. Novel physics and interesting phenomenon in ferrimagnets have been reported in our previous works, in which we found an enhancement of SOT efficiency in ferrimagnets due to the negative exchange interaction [1]. Recently, we found a long spin coherence length and associated bulk-like torque characteristics in a ferrimagnetic multilayer, necessary for the development of efficient ferrimagnet based spintronic devices [2]. The above works highlight strong current induced torques and high switching efficiency in ferrimagnet SOT devices.

Here, our current work directly demonstrates the ultrafast operation in magnetization switching of a CoGd ferrimagnetic device, combined with low-energy consumption [3]. We also clarify that the antiferromagnetically coupled Co–Gd links accelerate the spin angular momentum transfer resulting in high speed switching of ferrimagnet SOT devices. In particular, we used a stroboscopic pump–probe technique to perform time-resolved measurements. Thus, we can directly observe the dynamics of SOT switching in the time domain. From our experiments, the pulse current duration and switching time can be directly obtained, which was assumed to be similar in previous works. The ferrimagnetic devices can be switched by a sub-nanosecond current pulse within sub-nanosecond switching time. In addition, we extracted the domain wall velocity during SOT switching. The domain wall speed as high as 5.7 km/s was obtained. To the best of our knowledge, this value is the highest current-induced domain wall velocity at room temperature so far.

It is of utmost importance to reduce the switching time and power at the same time in modern memory devices. We have demonstrated a switching time of sub-nanosecond and energy consumption that is one to two orders of magnitude lower than that of ferromagnetic SOT systems. Considering that similar ferrimagnetic materials was commercialized for magneto-optical disks in 1998 and scaled up to gigabyte in early 2000s, it may find a viable commercial route for memory technologies in the near future.

For more information, please refer to our recent publication in Nature Electronics, “Ultrafast and energy-efficient spin–orbit torque switching in compensated ferrimagnets” (https://doi.org/10.1038/s41928-019-0345-8).

[1] R. Mishra et al. “Anomalous current-induced spin torques in ferrimagnets near compensation” Phys. Rev. Lett. 118, 167201 (2017)

[2] J. Yu et al. “Long spin coherence length and bulk-like spin–orbit torque in ferrimagnetic multilayers” Nature Mater. 18, 29 (2019)

[3] K. Cai et al. “Ultrafast and energy-efficient spin–orbit torque switching in compensated ferrimagnets” Nature Electron (2020) doi:10.1038/s41928-019-0345-8

Fig. Schematic of antiferromagnetic exchange coupling in a ferrimagnet CoGd alloy and the accelerated spin angular momentum transfer via Co–Gd links.