Precise interface engineering using a post-oxidized ultrathin MgAl layer for the voltage-controlled magnetic anisotropy effect

The voltage-controlled magnetic anisotropy (VCMA) effect has been proposed as an energy efficient approach for controlling the direction of magnetization. To demonstrate the scalability of a voltage-controlled magnetoresistive random access memory, we need to optimize the perpendicular magnetic anisotropy (PMA), tunnel magnetoresistance (TMR), and VCMA properties. Here, we performed a systematic investigation of the effects of inserting a post-oxidized MgAl layer on PMA, TMR, and VCMA in epitaxial magnetic tunnel junctions (MTJs). PMA and TMR have substantial dependences on the thickness of the MgAl layer, and their maximum values occurred when the MgAl layer was 0.20 nm thick, resulting in threefold and twofold increases in the PMA energy and TMR ratio, respectively, compared with the case without a MgAl layer. On the other hand, the VCMA coefficient increased as the MgAl layer thickness decreased and had a maximum value of -350 fJ/Vm when the MgAl layer was 0.16 nm thick, suggesting that the weakly oxidized interface provides a larger VCMA effect. Interface engineering using a post-oxidized ultrathin MgAl layer provides us with a valuable technique for precisely controlling the PMA, TMR, and VCMA properties of voltage-controlled MTJs.