High spin-orbit torque efficiency in Pt/Co/Tb/W repetition structure with broken spatial inversion symmetry

Abstract Spin-orbit torque (SOT) is an attractive phenomenon as a method of current-induced control of magnetization obtained at the interface between the heavy metal and magnetic layers. We investigated SOT dependence on the stacking structure, namely, [Pt/Co]n, [Pt/Co/Tb]n, or [Pt/Co/Tb/W]n where n is the repetition number because the repetition structure guarantees the thermal stability of magnetization. Whereas the SOT is negligibly small in the repetition structure [Pt/Co]10 because the SOT from the top and bottom Pt is canceled out, we observed finite SOT in [Pt/Co/Tb]10 induced by breaking a spatial inversion symmetry. Furthermore, the SOT of [Pt/Co/Tb/W]10 was more than three times higher than that of [Pt/Co/Tb]10 because the net spin Hall torque was enhanced by sandwiching Co with Pt and W with the opposite polarity of spin Hall angles. In addition, we clarified that the magnetization is reduced due to the intermixing at the Co/Tb interface by X-ray diffractions and composition depth profiles, that is another reason for the SOT improvement. These results are expected to provide a new foothold for the control of magnetization in various spintronic devices.