Spin Torques Estimation and Magnetization Dynamics in Dual Barrier Resonant Tunneling Penta-Layer Magnetic Tunnel Junctions

摘要

We investigate electronic transport and magnetization dynamics associated with current induced spin-torque effects in dual barrier magnetic tunnel junctions using Non-Equilibrium Green's Function formalism and Landau-Lifshitz-Gilbert (LLG) equation self-consistently. In a dual barrier penta-layer MTJ, a set of geometry and band-structure parameters including the free-layer thickness, oxide barrier height, width of the tunneling barrier and applied voltage jointly determines the position of resonant peaks and valleys within the energy range of interest. The combined effect of these design parameters to enhance the in-plane and out-of-plane spin-torque efficiencies in both aligned and antialigned penta-layer MTJs [Fig. 1] has been studied comprehensively. We quantify the impact of non-monotonic quantum well states for majority and minority spin electrons inside the thin free layer on the spin-torque effects in penta-layer MTJs. We essentially explore the design space for both the aligned and anti-aligned penta-layer MTJs optimized for read/write stabilities, improved TMR and low power. The crucial role of anti-aligned penta-layer MTJs in reducing the Energy-Delay-Product (EDP) during write over tri-layer MTJs has also been reported quantitatively. Fig. 3 and 4 demonstrate the angular dependence of spin-torque components in aligned (P) and anti-aligned (AP) penta-layer MTJs under normal condition and at resonance respectively and compare those with an identical tri-layer. A narrow energy spacing (～25.9meV at 300K) between the adjacent spin states at resonance promotes spin-flip scattering inside the free layer and thereby enhances net spin-torque efficiencies [defined in Eq. 1] almost by an order in magnitude. In presence of dual barrier resonant tunneling, the average switching delay of an anti-aligned penta-layer MTJ is significantly lower than that of a state-of-the-art tri-layer structure under iso-voltage condition as exhibited in Fig. 2. Symmetric switching characteristics for AP-P and P-AP state transitions have also been observed due to the presence of two anti-aligned pinned layer on either side of the thin free layer. We estimate the average Energy-Delay-Product (EDP) density for anti-aligned penta-layer MTJs significantly smaller (almost by 40%) than that of an identical tri-Layer during write under dual barrier resonance. In Fig. 5, the in-plane spin-torque efficiencies [defined in Eq. 1] in both aligned and anti-aligned penta-layer structures have been compared over the free layer thickness-applied bias design space. For in-plane torque, more light-shaded areas than darker (blackish-red) areas means significantly higher spin-torque efficiencies over the design space in anti-aligned penta-layer configurations than in aligned structures. Fig. 4 also shows the fluctuating nature of spin-torque efficiencies of penta-layer MTJs unlike the monotonic behavior observed in tri-layer MTJs. This can be attributed to the non-monotonic density of quantum well states for both majority and minority spin carriers.