Enhanced Stabilization and Dynamic De-Pinning of Spin Textures by Exchange Bias

Devices using electron spin are being explored for next-gen data storage and logic applications. A promising approach involves using spin textures like domain walls or skyrmions as bits in racetrack-based devices, which are moved using current, offering advantages such as high speed, low energy consumption and high density. Significant progress has been made in perpendicularly magnetized heterostructures, where nanoscale chiral spin textures are stabilized by interfacial spin–orbit coupling and driven by spin–orbit torques (SOTs) from a heavy metal layer.<br/>In addition to achieving small bits that can be moved at high speeds, a long-standing challenge in racetrack device research has been minimizing pinning, which sets the currents required for displacement. However, when pinning is reduced, thermal energy alone is sufficient to drive spin textures from their nominal resting place, adversely affecting device operation. This is the basis of a general tradeoff between stability and writability and is akin to the similar phenomenon in hard drive storage and MRAM: the anisotropy barrier must be high to ensure thermal stability, but low to allow for low-power writing. Although a lot of work has been done to understand and develop material systems such as ferrimagnets or synthetic antiferromagnets that can host nanoscale spin textures and allow for ultra-fast movement of them, the basic tradeoff between stability and writability remains in all these systems.<br/>In this work, we present a novel approach to address the stability/writability tradeoff by incorporating an exchange biasing antiferromagnet (AFM) layer into the racetrack. The AFM is tuned so that its blocking temperature is above the device's nominal standby temperature but below the temperature reached during current pulse injection for moving spin textures at device relevant speeds. In this design, an injected current pulse deactivates the exchange bias, allowing the spin textures to move freely while after the write current pulse, a field-cooling effect imprints the spin texture onto the AFM, creating a strong pinning component independent of the disorder-induced pinning potential of the FM. We show that current pulses as short as ~2 ns can induce this effect in Pt/Co heterostructures with CoNiO as the AFM. Our experiments confirm this strategy's effectiveness for both domain walls and bubble skyrmions, providing additional stability in the standby state while maintaining dynamic behavior and achieving domain wall motion up to ~90 m/s. Our findings establish exchange biasing as a promising method to overcome the stability/writability tradeoff in racetrack devices, applicable to any spin texture hosting heterostructure.