Perpendicular Magnetic Anisotropy and its Voltage Control in MgO/CoFeB/MgO Junctions with Atomically Thin Ta Adhesion Layers

We study the perpendicular magnetic anisotropy (PMA) and the voltage-controlled magnetic anisotropy (VCMA) effect in MgO/Ta/CoFeB/MgO junctions. A few monolayers (3 − 5 A) of Ta acts as an adhesive layer, allowing for the formation of flat and ultrathin (∼ 10 A) CoFeB films with large PMA on top of MgO. In the MgO/Ta/CoFeB/MgO junctions, the diffusion of Ta to the upper CoFeB/MgO interface is effectively eliminated, and superior thermal stability up to 400◦C is demonstrated. Detailed nanostructural analyses reveal the appearance of Co-Fe compositional variation in the CoFeB layer associated with the insertion of Ta layer, which explains the observed changes in PMA and VCMA. Our results will facilitate the development of VCMA-based memory devices and also pave the way toward controlling the magnetic and electric properties of CoFeB thin films through subnanometer-scale compositional modulation.