Effect of insertion layer on reducing switching current density in spin-orbit torque magnetic tunnel junction

Abstract

As the crucial device element for next-generation cache and edge computing technologies, the magnetic tunnel junction (MTJ) has been widely studied with recent focus towards using spin-orbit torque (SOT) to switch the free layer magnetization. Writing the MTJ using SOT approach is attractive, because it can increase the junction endurance as switching current path does not go through the tunneling barrier. In this work, we aim to enhance the SOT switching efficiency in the MTJ through various interlayer insertion designs in the free layer. Our study is carried out using in-plane MTJ (IP-MTJ), which has been demonstrated that no external magnetic field is required for SOT switching [1]. We fabricated W underlayer-based IP-MTJs, and investigate how the insertion layer between the W and CoFeB free layer affects the switching current density of the MTJ. Interestingly, with the insertion of ultra-thin layers such as MgO, Ru and TiW, the switching current drops significantly with the MgO insertion offering ~70% of reduction.Here, standalone 3-terminal IP-MTJ devices of elliptical sizes 300 nm x 600 nm are fabricated using Canon X5 stepper lithography on 8 inch Si wafers. We measure the pulse current switching loops with our home-built probe station [2]. Without insertion layer, the switching current is ~11.9 mA, equivalent to ~1.9x1011 A/cm2. The large value of switching current density is because of the dramatic reduction in the spin Hall angle (SHA) of W underlayer after annealing at 330°C, which also leads to a structural phase change to α-W. While the introduction of an insertion layer universally reduces the current density. The MgO interlayer reduces it by more than 3 times making it superrior for insertion layer selection.To understand it better, we performed spin-torque ferromagnetic resonance (ST-FMR) measurement [3] to determine the damping constant, spin Hall angle, and effective saturation magnetization (Ms_eff). A representative example of ST-FMR spectra for MgO insertion layer is shown in Fig. 1. We found the thin insertion layer has little influence on the damping constant, but some modifications in Ms_eff and SHA are found, which can be correlated with the switching current reduction results. Further investigations on the role of insertion layer thickness are ongoing and are expected to reveal the optimal insertion layer for SOT current reduction. **