Demonstration of on-chip all-optical switching of magnetization in integrated photonics.

Ultrafast all-optical switching of magnetization holds great promise for next-generation spintronic memory and hybrid spintronic-photonic systems. However, most implementations so far have relied on bulky free-space optical setups, limiting scalability and practical integration. Here, as a critical step towards integrated applications, we demonstrate the single-pulse all-optical switching of magnetization within a silicon nitride photonic integrated circuit. Using trains of femtosecond laser pulses guided through on-chip waveguides, we achieve deterministic toggle switching in a submicrometre out-of-plane Co/Gd Hall cross patterned directly atop the photonic waveguide. Electrical read-out via the anomalous Hall effect reveals a switching contrast of up to 90% for 500-nm-wide devices. In larger Hall crosses, the contrast decreases and switching becomes stochastic, consistent with spatially non-uniform optical absorption as confirmed by finite-element simulations. This behaviour is hypothetically attributed to domain-wall relaxation and thermally assisted (de)pinning processes within partially switched regions. Our results highlight the critical role of device scaling in achieving robust on-chip all-optical switching of magnetization and establish a foundation for ultrafast, energy-efficient and fully integrated spintronic-photonic platforms.