Ultrafast perturbation of magnetic domains by optical pumping in a ferromagnetic multilayer

I will present pump-probe experimental results that clearly demonstrate the ultrafast modification of magnetic domains and domain walls after optical pumping in a sample supporting a labyrinth domain pattern (1). The sample is a 40 nm Co/Ni multilayer. The experiments were conducted at the LCLS free electron laser. Domain alteration was measured by ultrafast magnetic X-ray scattering. 3 concentric diffraction rings were seen; the first 3 Fourier intensities of the domain pattern. The initial domain wall width is 39 nm from Fourier analysis of the ring intensities. The ring radii correspond to a domain width of 80 nm. After pumping, the three rings partly quenched, with almost 100% quenching for the highest order ring, while the first order ring quench by less than 50%. Our analysis determined that the domain walls broaden by 12 nm in <1.2 ps. We also found that the 1^stand 3^rd order rings broaden and shift to lower scattering vector. The quantitative measurement of the wall broadening proves the ring shift is not from wall broadening, contrary to the assertion of Pfau, et al. (2). Ring broadening implies a reduction in the domain correlation length from 845 nm to 712 nm, requiring wall speeds >2km/s. The ring radii contraction is consistent with an ultrafast increase in the average domain size. New measurements at Eu-XFEL confirm ring broadening and radius shift, but only for labyrinths, not stripes (3).

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

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

(3) Hagstrom, et al., PRB under review (2022)