  Current-induced spin-orbit torques (SOTs) are of interest for fast and
energy-efficient manipulation of magnetic order in spintronic devices. To be
deterministic, however, switching of perpendicularly magnetized materials by
SOT requires a mechanism for in-plane symmetry breaking. Existing methods to do
so involve the application of an in-plane bias magnetic field, or incorporation
of in-plane structural asymmetry in the device, both of which can be difficult
to implement in practical applications. Here, we reported bias-field-free SOT
switching in a single perpendicular CoTb layer with an engineered vertical
composition gradient. The vertical structural inversion asymmetry induces
strong intrinsic SOTs and a gradient-driven Dzyaloshinskii-Moriya interaction
(g-DMI), which breaks the in-plane symmetry during the switching process.
Micromagnetic simulations are in agreement with experimental results, and
elucidate the role of g-DMI in the deterministic switching. This
bias-field-free switching scheme for perpendicular ferrimagnets with g-DMI
provides a strategy for efficient and compact SOT device design.
