Engineering-Free Deterministic Spin-Orbit Torque Switching in Ultrathin Perpendicular Synthetic Antiferromagnets

Spin-orbit torque (SOT) offers a promising route for energy-efficient magnetization manipulation in low-power spintronic devices. Synthetic antiferromagnet (SAF) structure, in particular, enhances device performance through ultrafast magnetization switching and magnetic stray-field resilience. However, achieving deterministic SOT switching in perpendicularly magnetized SAFs typically requires an additional in-plane magnetic field or structural engineering to break inversion symmetry, which complicates their practical implementation. Here, we demonstrate field-free deterministic SOT switching in ultra-thin perpendicular SAFs operating at room temperature, designed with a symmetric CoFeB (0.6 nm)/W/CoFeB (0.6 nm) structure. This switching occurs only under antiferromagnetic coupling conditions between the two CoFeB layers, and the switching chirality can be tuned via the spacer layer thickness and temperature. Micromagnetic modeling reveals that symmetry breaking arises intrinsically from the interplay between Ruderman-Kittel-Kasuya-Yosida and Dzyaloshinskii-Moriya interactions, which stabilize the asymmetric Néel-Bloch mixed domain walls of both CoFeB layers. These domain walls manifest as dynamic spin textures with a net chiral driving force, enabling deterministic magnetization switching without the need for external fields or engineered structural asymmetry. Our findings provide a streamlined path toward practical applications of ultrathin SAFs in structurally simplified, fabrication-friendly SOT memory and logic devices.