Using Dopants and Alloying to Optimize Spin Hall Materials for Mram Devices

While commercial spin-transfer-torque MRAM chips are now available, there are still key power and endurance challenges that must be overcome to bring this technology to a broader market. Optimizing materials and interfaces can help in solving these issues. In this talk, we will discuss our recent theoretical work on spin Hall materials for MRAM applications. Using a spin Hall material to flip the spin orientation of a neighboring magnetic layer offers one potential route to low power, high endurance MRAM devices. The spin Hall angle (SHA) is defined as the ratio between the induced transverse spin Hall conductivity due to spin-orbit interactions and the longitudinal electrical conductivity. In this work, we consider two leading spin Hall materials (β-W and Pt alloys) where dopants play a key role. While the metastable β-W (A15) phase of W has one of the largest spin Hall angles measured (~0.3-0.4) [1] , the origin of its high spin Hall conductivity (SHC) is still controversial. Since large concentrations (~11%) of dopants (O or N) are required to stabilize β-W, it is unclear if the high SHC is due to intrinsic or extrinsic (skew-scattering) contributions. Platinum possesses a respectable SHA (~0.1) and low resistivity, making it a leading competitor for integration into MRAM applications. Recent studies have shown that alloying can boost the SHA with minimal impact on other device properties. Work is still needed to determine the best binary Pt alloy composition. Using a multi-code approach, we have developed a comprehensive framework to address and optimize all aspects of SHC in complex alloys.