Field-free magnetization switching in a wedge-shaped CoFeB system with suppressed Dzyaloshinskii–Moriya interaction

Field-free deterministic switching of perpendicular magnetization is essential for spin–orbit torque (SOT) devices. Lateral structural asymmetry via a wedge-shaped structure has been widely explored to enable field-free switching in heavy-metal/ferromagnet heterostructures. However, due to the coexistence of the Dzyaloshinskii–Moriya interaction (DMI), its mechanism remains obscure, leading to contradictory switching behaviors. Here, we fabricate β-W/Ti/CoFeB(wedge)/MgO devices with a thin Ti insertion layer that strongly suppresses DMI, thereby decoupling lateral asymmetry from DMI-induced chiral spin texture asymmetry. This allows us to present the switching physics governed solely by the wedged ferromagnetic layer. We achieve robust field-free SOT-driven magnetization switching. The switching polarity remains identical across all devices and independent of the sign change of the magnetic anisotropy gradient, revealing a unidirectional in-plane built-in electric field originating from the wedged CoFeB layer. Furthermore, a slight tilt of the magnetic easy axis facilitates switching, particularly in the thinner CoFeB region. Our results demonstrate the universal contribution of the in-plane built-in electric field and provide a comprehensive understanding of field-free switching in wedge-shaped lateral asymmetric structures.