Engineering of Co/MgO interface with combination of ultrathin heavy metal insertion and post-oxidation for voltage-controlled magnetic anisotropy effect

The voltage-controlled magnetic anisotropy (VCMA) effect in ferromagnet/insulator junctions provides an effective way to manipulate electron spins, which can form the basis of future magnetic memory technologies. Recent studies have revealed that the VCMA effect can be strongly tuned by a process of "interface engineering" exploiting ultrathin heavy metal layers and an electron depletion effect. To further decrease the numbers of electrons, chemical reactions, such as surface oxidation of ferromagnets, may also be an effective way to achieve this depletion. However, the knowledge of combined effect of heavy metal layers and oxidation is still lacking. Here, we demonstrate that dual interfacial engineering using an insertion of heavy metals (Pt or Re) and a post-oxidation process can have a remarkable effect on the perpendicular magnetic anisotropy and the VCMA effect. Interestingly, a strong enhancement of the perpendicular magnetic anisotropy is observed by dual interfacial engineering with Pt insertion, although it does not occur with Pt insertion or surface oxidation alone. Furthermore, even a sign reversal of the additional VCMA effect due to the ultrathin heavy metal layers is observed by utilizing dual interfacial engineering. These findings provide another degree of freedom for designing voltage-controlled spintronic devices and pave the way to interfacial spin-orbit engineering for the VCMA effect.