A Spin-Orbit Torque Knob to Tailor Stochasticity for Physically Secured Applications

The deterministic switching of spin-orbit torque (SOT) memory has long been the primary challenge for implementation. Conversely, stochastic switching has recently sparked significant interest for physically secured applications. This study investigates the effects of an external field (Hx) applied to break the SOT symmetry for deterministic switching in a Pt/Co bilayer with perpendicular anisotropy. This external field serves as a tuning mechanism to adjust the stochasticity at the intermediate state during SOT switching. It is observed that moderate Hx could lead to a substantial variation in electrical output during the repeating write/read operations. In contrast, a large Hx enables precise control over the intermediate states of the Pt/Co bilayer, effectively functioning as a memristor. The stochasticity derives from small Hx resulting from the dynamics of dendritic domain wall propagation during SOT switching. Herein, the oblique domain spiking from the initially reversed domain is notably influenced by the transverse component of SOT. Increasing Hx suppresses this oblique dendritic domain growth, allowing for controllable domain propagation and reducing stochasticity. By utilizing Hx as an external knob to tailor the stochasticity at the intermediate state, the high stochastic state can be employed as an ideal entropy source for an SOT-based random number generator. This work reports a spin-orbit torque knob to tailor the magnetic stochasticity at the intermediate state of a Pt/Co bilayer device, achieved under a field of 500 Oe. It is found that the magnetic stochasticity on the electrical output enables functioning as a natural entropy source to generate a random number sequence with high unpredictability for the physically secured functionality. image