Role of Rashba spin–orbit coupling for voltage-controlled magnetic anisotropy at magnetic tunnel junction interface Fe/MgO

Abstract

At the Fe/MgO interface in magnetic tunnel junctions (MTJs), a sharp electrostatic potential gradient results in the breaking of inversion symmetry of the system and gives rise to the Rashba-type spin–orbit coupling. Based on the first-principles method, we find that this coupling governs voltage-controlled magnetic anisotropy (VCMA) of MgO-based MTJs. The Rashba splitting, arising from the asymmetric character of wavefunction along the out-of-plane direction, is driven by orbital admixture between the intra-atomic d±1 and p±1 at the interfacial Fe near the Fermi energy, together with hybridization with O p±1 orbitals. For the pristine Fe/MgO model, the calculated VCMA coefficient is 29.1 ε fJ/Vm, where ε is a dielectric constant of the insulator barrier. The study further predicts that interfacial modification by AlN/MgO and LiF/MgO, which have lower and higher electronegativity difference than that of MgO, offers a promising MTJ design candidate in controlling VCMA. The former goes with VCMA coefficient of 46.9 ε fJ/Vm, enhanced by 60%, while the latter is 24.7 ε fJ/Vm with a 15% reduction than that of the pristine Fe/MgO model. The origin is attributed to the depth of the anion p-orbital energy level at the barrier interface.