Unconventional Enhancement of Spin-Orbit Torque with Laterally Extended Heavy Metal Layer

Conventionally, spin-orbit torque (SOT) devices utilize heavy metal (HM) and ferromagnetic (FM) bilayers of identical width. Here, we challenge this design paradigm, demonstrating a substantial and unexpected enhancement of SOT efficiency in a geometry where the HM layer is wider than the FM layer. By systematically fabricating and measuring Pt/Co structures with varying width ratios ( w _FM / w _HM ), we found the SOT efficiency to be strongly dependent on this geometric parameter. Using harmonic Hall measurements, we identified a unconventional improvement of SOT efficiency at the smallest w _FM / w _HM (= 0.1), which represents a 2.5-fold enhancement compared to conventional devices where w _FM / w _HM = 1. Furthermore, SOT-induced domain wall mobility also shows noticeable improvement for the smallest w _FM / w _HM cases. These counterintuitive results suggest an unconventional SOT mechanism arising from the macroscopic device geometry variations, providing further insight into the underlying physics for next-generation spintronic applications.