Real-Time Measurements of Field-Free Spin-Orbit Torque Switching in 3-Terminal Magnetic Tunnel Junctions INVITED

Abstract

Current-induced spin-orbit torques (SOT) have attracted great attention for their unique ability to manipulate the magnetization of ferromagnets and antiferromagnets [1]. SOT-induced switching offers speed and reliability comparable to or better than spin transfer torque (STT) switching, as was shown in nanodots [2,3] and magnetic tunnel junctions (MTJ) [4,5]. However, whereas the STT-driven switching dynamics of MTJs has been extensively studied [6-8], less is known about the transient dynamics and actual speed of the magnetization reversal induced by SOT. In this study we investigate the reversal dynamics of SOT switching in 3-terminal MTJs and show how the interplay of voltage control of magnetic anisotropy (VCMA) and STT can be used to optimize the switching efficiency [9,10]. We report real-time measurements of in-field and field-free SOT switching and disentangle the combined impact of SOT, STT, and VCMA on the switching speed and efficiency. We show that the combination of these effects leads to reproducible sub-ns switching with an extremely narrow distribution of the switching times without the aid of an external field. Our 3-terminal devices are fabricated using a fully compatible CMOS process [11] and are based on CoFeB/MgO/CoFeB MTJs grown on a b-W SOT-injection line and patterned into 80 nm pillars (Fig. 1a). The field-free switching functionality is enabled by an in-plane magnet embedded into a hard-mask [12], which provides a symmetry-breaking magnetic field to the magnetization of the free layer (Fig. 1b). Our electrical setup combines after-pulse and real-time detection [13] allowing for the exploration of individual SOT switching events in the time domain as well as of their statistical distributions. The voltage time traces of individual switching events exhibit a finite incubation delay followed by a single-jump reversal, after which the magnetization remains quiescent in the final state (Fig. 1c,d) [13]. Whereas complete magnetization reversal occurs within ≈1 ns, the incubation delay can take a substantial part of the total switching time in close-to-critical conditions. This incubation time, which is not expected for the “instant-on” SOT in perpendicular MTJs, can be minimized by a moderate increase of the SOT current (compare Figs. 1c and 1d), resulting in a very narrow distribution of the switching latency (Fig. 1e) [14]. A similar effect is achieved by increasing the external magnetic field strength or by biasing the MTJ. We attribute the two-steps switching process to the nucleation of a reversed domain and propagation of a domain-wall across the free layer, as supported by micromagnetic simulations (Fig. 1f). The model reveals that a key element to the observed dynamics is the reduction of magnetic anisotropy of the free layer related to the current-induced temperature rise [13]. The results further evidence a significant dependence of the switching speed and reversal onset on the MTJ bias. In Fig. 1g, averaged time traces obtained for different bias conditions clearly show that the bias accelerates the switching process. Finally, from symmetry considerations, we show that the contributions of STT and VCMA can be separated, unraveling their distinct impact on the magnetization dynamics.