High perpendicular magnetic anisotropy and voltage-controlled magnetic anisotropy coefficients in crystalline CoFe/Fe-based top-free multilayers fabricated using cryogenic temperature deposition

Cryogenic-temperature sputtering deposition at 100 K was used to fabricate crystalline CoFe/Fe stacked ultrathin films grown on a MgO tunneling barrier. The CoFe/Fe stacked layers exhibited a high perpendicular magnetic anisotropy (PMA) energy density of 0.78 mJ/m2 and a voltage-controlled magnetic anisotropy coefficient of 161 fJ/Vm even after annealing at 673 K. The high PMA can be attributed to the addition of the Fe overlayer, which enhances the interfacial PMA originating from orbital hybridization between Fe and O atoms. Although the CoFe/Fe stacked free layers exhibited a clear PMA, Fe films directly grown on MgO exhibited rounded magnetization curves, indicating that the CoFe layer helped grow ultrathin Fe films. Stacked crystalline ultrathin films are expected to be used for the development of free layers, and the CoFe/Fe stacked layers developed in this study can be applied to various applications based on perpendicular magnetic tunnel junctions, e.g., voltage-controlled magnetoresistive random-access memory, achieving extremely low power consumption.