The central role of tilted anisotropy for field-free spin–orbit torque switching of perpendicular magnetization

Abstract

The discovery of efficient magnetization switching upon device activation by spin Hall effect (SHE)-induced spin–orbit torque (SOT) changed the course of magnetic random-access memory (MRAM) research and development. However, for electronic systems with perpendicular magnetic anisotropy (PMA), the use of SOT is still hampered by the necessity of a longitudinal magnetic field to break magnetic symmetry and achieve deterministic switching. In this work, we demonstrate that robust and tunable field-free current-driven SOT switching of perpendicular magnetization can be controlled by the growth protocol in Pt-based magnetic heterostructures. We further elucidate that such growth-dependent symmetry breaking originates from the laterally tilted magnetic anisotropy of the ferromagnetic layer with PMA, a phenomenon that has been largely neglected in previous studies. We show experimentally and in simulation that in a PMA system with tilted anisotropy, the deterministic field-free switching exhibits a conventional SHE-induced damping-like torque feature, and the resulting current-induced effective field shows a nonlinear dependence on the applied current density. This relationship could be potentially misattributed to an unconventional SOT origin.

Field-free magnetization switching with full scale in Pt/Tm3Fe5O12 bilayer on vicinal substrate

“…In our study, we observed the existence of the loop shift in the device on vicinal substrate when H x = 0 Oe. As interpreted in the previous study, 38) this phenomenon might indicate that the The switching behavior of field-free SOT is explored in Fig. 4, revealing that the tilted anisotropy induced by vicinal substrates is crucial for achieving field-free SOT switching in the TmIG film.…”

“…37) Hu et al investigated field-free SOT switching in Pt/Co/Pt magnetic heterostructures with a Pt underlayer prepared by oblique angle deposition. 38) Both works thus underscore the pivotal role of anisotropy engineering in accomplishing field-free SOT switching of perpendicular magnetization. In contrast to previously proposed approaches for achieving field-free switching, establishing a tilted perpendicular magnetization not only simplifies the fabrication process but also proves to be well-suited for mass production.…”

“…Therefore, we consider that the loop shift is induced by the tilted magnetic anisotropy, and a previous study also observed a similar phenomenon in Pt/Co/Pt with tilted magnetic anisotropy. 38) Generally, for the SOT measurement using the loop-shift method, an external magnetic field H x along the current direction (x-axis) will be applied. 43) The application of H x realigns the magnetic moments of the Néel-type domain walls, and then the current-induced damping-like torque works as an additional effective field along the z-axis, which favors one type of the domain.…”