Tunable PMA and Interfacial Microstructure Induced by a Hf(HfO2) Interfacial Spacer in MTJs with Two MgO Layers

The insertion of a metal (metal oxide) layer in a ferromagnetic layer is highly effective for tailoring the interfacial microstructures of magnetic multilayers and consequently their physical, chemical, and magnetic properties. In this study, the authors add Hf (HfO2) spacer layers to magnetic tunnel junctions (MTJs) with two MgO layers to ensure the perpendicular magnetic anisotropy (PMA) of the films after annealing at 375 °C. High‐resolution transmission electron microscopy shows that the MgO and Ta layers are partially crystallized, while the CoFeB layers are predominantly amorphous. The different Ta, MgO, and CoFeB layers can be easily distinguished by energy‐dispersive X‐ray spectroscopy. X‐ray photoelectron spectroscopy indicates that some HfO2 is formed when the film is deposited, and more HfO2 is formed during annealing. More Fe and Co oxides are also formed as oxygen reduced from BOx and MgO combines with Fe and Co in the CoFeB layer, which strengthens the PMA. The results indicate that the insertion of a metal (metal oxide) layer is an effective way to improve the magnetic properties of MTJs.