DFT-NEGF simulation study of Co$_2$FeAl-based magnetic tunnel junctions under biaxial strain

Abstract

Conventional spin-transfer torque-based magnetoresistive random access memory (STT-MRAM) with CoFeB electrodes has great potential as universal memory. However, state-of-the-art STT-MRAM technology has encountered the issues such as high writing current density and low thermal stability for scaling down to 1x nm. Heusler alloy has been suggested as an alternative for resolving these problems by significantly decreased Gilbert damping constant while preserving approximately 100\% spin polarization. In particular, $L2_1$-ordered Co$_2$FeAl (CFA)-based magnetic tunnel junction (MTJ) exhibits outstanding half-metallicity and perpendicular magnetocrystalline anisotropy characteristics arising from Co(Fe)-O orbital hybridization at the interface. In this dissertation, the biaxial strain effects of CFA-based MTJ are investigated by adjusting in-plane lattice constants from -4\% to +4\%. DFT-NEGF calculations present strain-tunable tunneling magnetoresistance ratio and magnetocrystalline anisotropy energy in both Co$_2$-O and FeAl-O interfaced MTJs. The strain-engineered CFA-MTJs with FeAl-O interfaces have shown stable TMR ratios and magnetic anisotropies, whereas Co$_2$-O interfaced structures have shown trade-off relationship.