Ultrafast internal dynamics of chiral domain walls probed by time-resolved XRMS

We measure picosecond dynamics of labyrinthine stripe domains with chiral N\'eel-type domain walls using time-resolved x-ray resonant magnetic scattering (XRMS). At the stripe-domain wavevector, the helicity-summed signal shows ultrafast demagnetization, recovery, and a weak oscillatory contribution that we identify as a signature of laser-launched coherent surface phonons. In contrast, the dichroic signal, which is sensitive to the in-plane magnetization inside the N\'eel walls, shows a strong oscillatory response whose frequency decreases with increasing pump fluence. We attribute this softening to pump-induced changes in the effective anisotropy and saturation magnetization, which modify the restoring field of an internal domain-wall mode. Time-resolved XRMS thus isolates wall-specific dynamics and provides access to internal domain-wall motion in disordered stripe textures on picosecond time scales.