Picosecond-scale coherent toggle switching of topological spin helicity.

The switching of conventional magnetization states is a cornerstone of modern spintronics, enabling control over binary ('0' and '1') information bits. Although the coherent control of helicity switching in topological spin configurations is promising for applications such as high-speed multistate memory and neuromorphic and probabilistic computing, realizing it has been challenging. This difficulty stems from the requirement for coherent spin precession while maintaining the intrinsic topology of the spin configurations, which is usually disrupted by conventional excitations. Here we report an experimental realization of coherent helicity toggle switching in nanoscale magnetic vortices occurring on timescales of several hundred picoseconds. This switching behaviour is driven by femtosecond laser pulse excitation under an out-of-plane magnetic field. The mechanism is governed by ultrafast photothermal demagnetization and coherent spin precession in the subsequent remagnetization process, during which the intrinsic topology and symmetry of the vortex are preserved. Crucially, the helicity switching dynamics can be tuned precisely using the laser fluence and magnetic field strength, enabling deterministic to stochastic control over the two energy-degenerate helicity states. This control was reproduced in micromagnetic simulations when the parameters were optimized within a physically reasonable range.