Extrinsic and Intrinsic Symmetry Breaking for Field-Free Spin-Orbit Torque Switching

Spin-orbit torque (SOT) enables efficient manipulation of magnetization. Owing to its advantages of high speed, superior endurance, and low power consumption, SOT is attractive for non-volatile memory and logic devices, and emerges as a core writing mechanism for next-generation magnetic random-access memory. In conventional heavy-metal/ferromagnet heterostructures, the spin Hall effect generates only in-plane spin polarization. Consequently, deterministic SOT switching of perpendicular magnetization relies on an external magnetic field to break symmetry, which imposes a critical limitation on high-density device integration. Over the past several decades, field-free switching has been realized through various strategies, such as wedge-shaped films, composition grading, exchange bias, and canted magnetic moments. However, these approaches typically suffer from slow switching speed, high power consumption, and complex fabrication. Low-symmetry van der Waals (vdW) topological semimetals—including WTe₂, TaIrTe₄, and NbIrTe₄—offer a fundamentally new route for field-free switching. Originating from intrinsic crystal symmetry breaking, their unconventional spin polarization drives robust field-free SOT switching without relying on an external magnetic field. This review systematically summarizes the evolution of field-free SOT switching. We analyze the intrinsic bottlenecks of conventional SOT architectures, then elaborate various categories of symmetry-breaking strategies toward field-free switching, and highlight the unique merits of low-symmetry vdW spin-source materials. In addition, we discuss functional implementations of all-vdW SOT spintronic devices for in-memory logic, nonvolatile magnetic memory, and neuromorphic computing, and conclude by outlining unresolved challenges and prospective research pathways.