Ultra Low Power Composite Free Layer Spin-Orbit Torque MRAM

Spin-transfer torque MRAM (STT-MRAM) serves as a promising candidate for next generation memory. However, STT-MRAM suffers from tunnel barrier degradation under rapid operations and a poor spin polarization. In contrast, spin-orbit torque MRAM (SOT-MRAM) utilizes spin-orbit interaction at the interface of heavy metal (HM) and ferromagnetic (FM) layers via mechanisms such as the Rashba effect and spin Hall effect (SHE^{1 2}. Since current only flows through the heavy metal in SOT-MRAM, device performance is no longer limited by insulator breakdown. Because no charge current through the FM layer is needed in SOT devices, the other unrecognized benefit of SOT is its compatibility with low-damping magnetic insulators (MI). SOT experiments with YIG/Pt, YIG/Ta, and YIG/W have demonstrated the potential for ultra-low damped switching of YIG in SOT systems^{3 4}. SOT switching has also been demonstrated in TmIG based system⁵. Here, we propose a composite free layer SOT-MRAM (Fig. 1a). The MTJ is a cylinder structure with a diameter D. The free layer consists of an easily switched YIG layer and a thermally stable high anisotropy (K_u ~ 10⁷ ergs/cc) L1₀- FePt or L1₀-FePd layer. These two layers are ferromagnetically coupled through the RKKY exchange provided by the Pd layer. It has been shown that exchange coupling between magnetic soft and hard layers can reduce the critical current⁶. Current CoFeB based devices necessitate larger diameters due to the low anisotropy. By adopting FePt or FePd, we can reduce the device area, which is favorable for high density applications. We optimized our device by running micromagnetic simulations in the macrospin approximation. A device of small diameter (D <; 30 nm) with low write energy ( E_w <; 1 fJ) can be achieved for 1 ns switching while maintaining thermal stability Δ =60_B T at room temperature.