Non-linear evolution of magnetic domain structure following ultrafast laser excitation

We utilized time-resolved small-angle X-ray magnetic scattering at FERMI free electron laser to measure the ultrafast response of mixed domain state in CoFe/Ni multilayers. The mixed domain state consists of coexisting perpendicularly magnetized labyrinth and stripe domains. 2D fitting routine was employed to disentangle the individual ultrafast change in period (1/q) and inhomogeneity (1/ Γ) of labyrinth and stripe domains from magnetic scattering [1]. We observed a linear dependence of scattered intensity as a function of pump fluence. A nonlinear ultrafast contraction and broadening in the scattering was observed for labyrinth domains where below a certain fluence threshold (≈ 7 mJ/cm²), small changes were observed. Above this threshold, q and Γ increased drastically as a function of fluence and at the highest measured fluence of 13.4 mJ/cm², a change of 5.5 % and 27 % for q and Γ respectively was observed for labyrinth domains as reported previously [2, 3]. In contrast, stripe domains show significantly weaker change when pumped [1, 4]. We report clear evidence of difference in quench and recovery timescales for magnetization and domain structure evolution. Our studies hint to domain wall velocities of up to 10⁴ m/s and a clear dependence of the magnetization dynamics on the domain pattern highlighting the role of nanoscale morphology on the ultrafast magnetization behavior.

(1) Hagstrom et al., arXiv (2021)

(2) Zusin et al., PRB accepted (2022)

(3) Pfau et al., Nat. Comm. 3 (2012)

(4) Hennes et al., PRB (2020)