Field-Free Spin–Orbit Torque Magnetization Switching in a Perpendicularly Magnetized Semiconductor (Ga,Mn)As Single Layer

Current-induced
spin–orbit torque (SOT) in a perpendicularly
magnetized single layer has a strong potential to switch the magnetization
using an extremely low current density, which is generally 2–3
orders of magnitude smaller than that required for conventional metal
bilayer systems. However, an in-plane external magnetic field has
to be applied to break the symmetry and achieve deterministic switching.
To further enhance the high-density integration and accelerate the
practical application of highly efficient SOT magnetic random-access
memory (SOT-MRAM) devices, field-free SOT magnetization switching
in a ferromagnetic single layer is strongly needed. In a spin–orbit
ferromagnet (a ferromagnet with strong spin–orbit interaction)
with crystal inversion asymmetry and a multi-domain structure, the
internal Dzyaloshinskii–Moriya effective fields are considered
to induce field-free switching. Here, combined with strong spin–orbit
coupling and a tilted anisotropy axis induced by a nonuniform Mn distribution
and a possible magnetocrystalline anisotropy resulting from a slight
substrate tilting, we successfully achieve magnetization switching
in a spin–orbit ferromagnet (Ga,Mn)As single layer by utilizing
SOT without applying any external magnetic field. Our findings help
to deeply elucidate the SOT switching mechanism and can advance the
development of a highly efficient MRAM with better scalability.