Sub-wavelength extreme ultraviolet microscopy reveals domain-wall stability during ultrafast demagnetization.

Ultrafast control of magnetic textures relies on understanding how domain walls respond to femtosecond laser excitation. Previous X-ray scattering studies suggested transient domain-wall broadening and motion during reversible demagnetization. Yet such dynamics have never been observed directly, owing to insufficient spatiotemporal resolution in existing techniques. Here we introduce femtosecond extreme ultraviolet microscopy with sub-wavelength 13.5-nm spatial resolution, enabling direct real-space imaging of domains in ferri- and ferromagnetic thin films during laser-induced demagnetization. Within an experimental precision better than 2 nm, domain-wall positions and widths remain unchanged up to the onset of irreversible magnetic switching at 50%-80% demagnetization. These findings establish quantitative limits for reversible domain-wall motion and spin transport upon ultrafast demagnetization, while implying that irreversible reconfiguration underlies characteristic changes in diffraction signatures at high fluence. The approach expands time-resolved microscopy into the nanometre and femtosecond regime, enabling element-specific studies of coupled spin, charge and lattice dynamics in quantum and functional materials.