Field-Free Type-x Spin-Orbit-Torque Switching by Easy-Axis Engineering

The current-induced type -x spin-orbit-torque (SOT) switching configuration describes the orthogonal relationship between the magnetic easy axis (EA) of a ferromagnetic (FM) layer and the injected spin polarization, sigma, from a heavy-metal (HM) layer, which has the potential to eclipse the conventional type -y scenario (EA parallel to sigma) at the sub-nanosecond pulse regime. Here, we show that, in HM/FM bilayer heterostructures, current-driven differential planar Hall signals can serve as efficient means to probe type -x SOT switching in a simple measurement fashion. Through this approach, we demonstrate field-free type -x SOT switching in all devices with a canted EA, which are engineered via field-annealing processes. By analyzing the switching-phase diagrams, we further verify that such field-free switching stems from a z-direction effective field contributed to by both the dampinglike torque and the canted EA. Our work indicates that the canted EA, as engineered by a field-annealing process, can give rise to a robust field-free SOT switching and further reduce the critical switching current density.