Spin-orbit Torque Driven Magnetization Switching for Non-volatile Memory and Beyond

2020-03-12T18:02:39Z (GMT) by Yang Liu
Research efforts in discovering and gaining better understanding of various spin-based physical phenomena over the past decades have propelled the innovation and developments of new generations of memory and logic devices. With utilization of non-volatility inherent in magnetism and low-power consumption characteristics, these novel device concepts present new opportunities for future electronics and computers. In recent years, magnetization switching via spin-orbit torques (SOTs) has come out as a promising candidate for advanced memory and computing applications, as it gives the advantages of low power consumption as well as ultrafast writing speed. The underlying mechanisms by which the SOTs induce the magnetization switching, however, turns out to be quite complex. Furthermore, for perpendicularly magnetized systems, i.e., perpendicular MRAM, the SOT driven magnetization switching of the free layer often requires an external in-plane field that significantly hinders the technological viability of commercial implementations.
In this research work, we aim to gain a deeper understanding of the SOTs and their roles in inducing the magnetization switching; in turn, by means of material or device engineering, we can control the SOTs to achieve the desired switching outcomes. We particularly focus our study on the perpendicularly magnetized systems because the high perpendicular magnetic anisotropy (PMA) in these systems makes them appealing for practical applications.
A major part of this research work emphasizes on the elimination of the need for an external magnetic field in the SOT switching of a perpendicular magnet. One strategy to achieve the field-free perpendicular SOT switching is through creating a magnetic field that’s localized within the device. The origin of such internal field can come from the interlayer exchange coupling. Based on this idea, we demonstrate robust field-free perpendicular magnetization switching by utilizing the spin Hall effect and interlayer exchange coupling of iridium (Ir). This is the first reported clear experimental demonstration that a heavy metal layer, Ir in particular, is capable of serving as both a spin current source and an interlayer exchange coupling layer. An additional important characteristic of Ir is that its interface with either Co or FeCoB facilitates strong perpendicular magnetic anisotropy. These combined properties allow us to achieve the SOT driven magnetization switching of a perpendicular Co layer in absence of an external field. Besides the field-free switching of a single layer, we also show that the switching scheme can be well integrated with the MgO-based magnetic tunnel junction (MTJ). We show that the three-terminal MTJ device with the Ir-enabled switching exhibits reliable writing and reading operations at zero external field, moving a step closer to the practical applications of the SOT-related magnetoresistive devices.
In addition to engineering the SOT materials, we provide another solution by altering the device design. The idea is based on the well-known phenomenon that a current carrying wire produces an effective magnetic field around it. Compared to the conventional three-terminal device, our device contains an additional current line orthogonal to the write path, which can generate an in-plane Oersted field during the SOT writing. Facilitated by this Oersted field, reliable SOT switching of the perpendicular MTJs is obtained without applying an external field. The switching characteristic also renders our device unique advantages in terms of preventing the half selecting issue.
In the study of the switching dynamics, we find the switching process in our devices often starts with domain nucleation followed by the domain wall motion (DWM) to expand the reversed domains. This inspires us to dig deeper into the SOT driven DWM and explore the ways in manipulating the DWM so as to control the magnetization state of a perpendicular magnet. In this work, we investigate the DWM in a system with two heavy metal underlayers that have the opposite spin Hall angles. By simply varying the relative thicknesses of these two underlayers, we can manipulate the polarity of the SOTs exerting on the DWs, which further allows us to control the direction of DWM. Based on our findings, we propose a wedge DW device where the SOT driven DWM can effectively give rise to the expansion of reversed domains and thereby realize the magnetization switching.
Lastly, we show the initial experimental works for developing a novel DW device known as mCell, which can be used as the computing unit in non-volatile logic circuit without the integration with CMOS. We develop a magnetic oxide (FeOx) layer that can serve as the electric-insulating magnetic layer inserted in between the write path and read path of mCell. The FeOx insertion layer not only provides sufficient magnetic coupling between the adjacent magnetic layers, but also significantly enhances the DWM in terms of the DW velocity and power efficiency.