Spin-Orbit Torque Switching in Asymmetric Structures with Double Non-Magnetic Metal Layers*

Spin-orbit torque (SOT) switching of magnetization is a promising emerging technology for non-volatile memory and logic applications. However, deterministic switching with SOTs requires breaking of inversion symmetry, usually provided by an external magnetic field, which is not suitable for applications. It has been shown that structures with lateral asymmetry can eliminate the need for external field and realize field-free SOT switching [1]. In this work we study field-free SOT switching in asymmetric structures with double non-magnetic metal layers, where current-induced out-of-plane effective magnetic fields are created. We break the structural symmetry by inserting a wedge-shaped second metallic layer between the ferromagnet and the first heavy metal. We investigate the creation and origin of current-induced out-of-plane effective fields, H_Z^eff, in various material systems (W/IrMn, W/Ta, Pt/IrMn, W/Ti), and we show the realization of field-free SOT switching in these structures. Our work provides a route for practical application of SOT devices and challenges the current understanding of the origins of SOTs in structures with lateral asymmetry.
[1] G. Yu et al., Nature Nanotechnology, 2014. 9: p. 548.