Large voltage-induced coercivity change in Pt/Co/CoO/amorphous TiO_x structure and heavy metal insertion effect

There is urgent need for spintronics materials exhibiting a large voltage modulation effect to fulfill the great demand for high-speed, low-power-consumption information processing systems. Fcc-Co (111)-based systems are a promising option for research on the voltage effect, on account of their large perpendicular magnetic anisotropy (PMA) and high degree of freedom in structure. Aiming to observe a large voltage effect in a fcc-Co (111)-based system at room temperature, we investigated the voltage-induced coercivity (H_c) change of perpendicularly magnetized Pt/heavy metal/Co/CoO/amorphous TiO_x structures. The thin CoO layer in the structure was the result of the surface oxidation of Co. We observed a large voltage-induced H_c change of 20.2 mT by applying 2 V (0.32 V/nm) to a sample without heavy metal insertion, and an H_c change of 15.4 mT by applying 1.8 V (0.29 V/nm) to an Ir-inserted sample. The relative thick Co thickness, Co surface oxidation, and large dielectric constant of TiO_x layer could be related to the large voltage-induced H_c change. Furthermore, we demonstrated the separate adjustment of H_c and a voltage-induced H_c change by utilizing both upper and lower interfaces of Co.