Influence of HfO2 interlayers on magnetocrystalline anisotropy in Fe|MgO|Fe magnetic tunnel junction: First-principles investigation

Perpendicular magnetic anisotropy (PMA) is important for MgO based magnetic tunnel junction and magnetic random access memory to be integrated on a large scale due to high thermal stability and low critical switching current. Here, we applied the density functional theory to study the effect of HfO2 inserting layers on PMA of Fe|MgO|Fe tunnel junction. It was found that the magnetocrystalline anisotropy (MCA) of the junction for 5 layers Fe electrode was up to 1.95 mJ/m2 with one unit cell HfO2 interlayer, while it was 1.72 mJ/m2 without the interlayer. More importantly, analyzed by the layer and orbital-resolved MCA based on the second-order perturbation theory, MCA characters and the underlying mechanism of PMA become very different after inserting HfO2. The remarkable difference is the MCA contribution of the second interfacial Fe layers, which is about 0.4 mJ/m2 for Fe|MgO|Fe junctions, while it was larger than 0.7 mJ/m2 for Fe|HfO2|MgO|HfO2|Fe junctions. Furthermore, Fe-dz2 and O-pz hybridization plays a crucial role in MCA contribution from the first interfacial Fe layers since the interfacial Fe–O bond length reduces from 2.20 Å to 1.77 Å with inserting HfO2 layers. Besides, the reduction in Fe–O bond length can redistribute the orbital-resolved electrons of the second and third closest Fe layer to the interface to enhance their absolute values of MCA contributions, which results in the strong dependence of MCA on Fe thickness.