  Non-collinear spin textures in ferromagnetic ultrathin films are attracting a
renewed interest fueled by possible fine engineering of several magnetic
interactions, notably the interfacial Dzyaloshinskii-Moriya interaction. This
allows the stabilization of complex chiral spin textures such as chiral
magnetic domain walls (DWs), spin spirals, and magnetic skyrmions. We report
here on the ultrafast behavior of chiral DWs after optical pumping in
perpendicularly magnetized asymmetric multilayers, probed using time-resolved
circular dichroism in x-ray resonant magnetic scattering (CD-XRMS). We observe
a picosecond transient reduction of the CD-XRMS, which is attributed to the
spin current-induced coherent and incoherent torques within the continuously
dependent spin texture of the DWs. We argue that a specific demagnetization of
the inner structure of the DW induces a flow of hot spins from the interior of
the neighboring magnetic domains. We identify this time-varying change of the
DW textures shortly after the laser pulse as a distortion of the homochiral
N'eel shape toward a transient mixed Bloch-N\'eel-Bloch textures along a
direction transverse to the DW. Our study highlights how time-resolved CD-XRMS
can be a unique tool for studying the time evolution in other systems showing a
non-collinear electric/magnetic ordering such as skyrmion lattices,
conical/helical phases, as well as the recently observed antiskyrmion lattices,
in metallic or insulating materials.
