Interface Effect on the Out-of-Plane Spin-Orbit Torque in the Ferromagnetic CoPt Single Layers

Field-free switching of a perpendicularly magnetized heavy metal/ferromagnetic metal bilayer or single alloy layer through spin-orbit torque (SOT) provides a potential way for next-generation spintronic devices with fast speed and high efficiency. Here, a sizable symmetry dependence of field-free magnetization switching via an out-of-plane torque in CoxPt100-x single layers is reported. It is found that the sign of in-plane SOT in CoxPt100-x when x <= 25 is positive. However, it changes to negative when x > 25, while the out-of-plane SOT changes its sign when x > 30. The polarized neutron reflectometry measurement further suggests an interface layer with rich Pt content near the substrate, which could have a strong interface effect on the out-of-plane SOT. The sign of the out-of-plane SOT, and then the polarity of the field-free magnetization switching, is determined by the competition between the out-of-plane torques arising from the bulk and interface parts in the CoxPt100-x single layers. It is expected that such interface effect of the out-of-plane torque is desirable to the applications in spin logic devices using symmetry dependence of field-free magnetization switching in the ferromagnetic CoPt single layers.