Field‐Free Spin‐Orbit Torque Switching in Synthetic Ferro and Antiferromagents with Exchange Field Gradient

Abstract

Deterministic switching of perpendicularly magnetized synthetic antiferromagnets using spin-orbit torque (SOT) usually requires an in-plane auxiliary magnetic field, which limits its practical applications. Here, an exchange field gradient is introduced into perpendicularly magnetized synthetic ferro-and antiferromagnets (SFs and SAFs) through the insertion of a slightly wedged Ru between two thin ferromagnetic layers, which induces field-free switching of perpendicular SFs and SAFs with a switching ratio up to 81% regardless of the nature of the coupling. Temperature-dependent measurement shows a robust field-free switching even at low temperature. The experimental results show that the field-free switching ratio and the effective SOT field are directly related to the exchange field gradient. The theoretical model and numerical simulation indicate that the dynamic noncollinear spin textures induced by the exchange field gradient lead to the field-free switching, while the sign of the exchange field gradient determines the field-free switching polarity. It is further revealed that the SOT efficiency is positively correlated with the antiferromagnetic exchange field for both Ru wedged and non-wedged samples. These results provide a new avenue for simultaneously achieving field-free switching and high SOT efficiency of perpendicularly magnetized SAFs for highly stable, high-density, low-stray-field, and low-power magnetic memory devices.

Switching of Perpendicular Magnetization by Spin–Orbit Torque

“…Unveiling the in‐depth mechanism of this unexpected hider effect of the so‐called assisting field may significantly advance the understanding of switching phenomena of perpendicular magnetization by spin–orbit torque. Perpendicularly magnetized synthetic antiferromagnets (SAFs) (e.g., ferromagnet/Ru or Ir/ferromagnet or ferromagnet/Ru or Ir/ferrimagnet) can integrate the merits of ferromagnets and antiferromagnets. Specifically, SAFs can have enhanced thermal stability, low stray field, compensated magnetization, tunable interlayer exchange coupling via the Ruderman–Kittel–Kasuya–Yosida interaction and DMI, [ 195,196 ] external‐field‐free magnetization switching by SOTs, [ 197,198 ] etc. SAFs can thus be potentially compelling information storage layers of magnetic memories, logic gates, or neurons.…”