Perpendicular magnetic anisotropy and its voltage control in MgO/CoFeB/MgO junctions with atomically thin Ta adhesion layers

Abstract

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 Å) of Ta acts as an adhesive layer, allowing for the formation of flat and ultrathin (∼10 Å) 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.