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We use time-resolved x-ray resonant magnetic scattering (TR-XRMS) at the Co ${M}_{2,3}$ and Tb ${O}_{1}$ edges to study ultrafast demagnetization in an amorphous ${\mathrm{Co}}_{88}{\mathrm{Tb}}_{12}$ alloy with stripe domains. Combining the femtosecond temporal with nanometer spatial resolution of our experiment, we demonstrate that the equilibrium spin texture of the thin film remains unaltered by the optical pump pulse on ultrashort timescales ($l1\phantom{\rule{0.28em}{0ex}}\mathrm{ps}$). However, after $\ensuremath{\simeq}4$ ps, we observe the onset of a significant domain wall broadening, which we attribute to a reduction of the uniaxial magnetic anisotropy of the system, due to energy transfer to the lattice. Static temperature-dependent magnetometry measurements combined with analytical modeling of the magnetic structure of the thin film corroborate this interpretation.

We use femtosecond time-resolved hard x-ray scattering to detect coherent acoustic phonons generated during ultrafast laser excitation of ferromagnetic bcc Fe films grown on MgO(001). We observe the coherent longitudinal-acoustic phonons as a function of wave vector through analysis of the temporal oscillations in the x-ray scattering signal. The width of the extracted strain wave front associated with this coherent motion is $\ensuremath{\sim}100$ fs. An effective electronic Gr\"uneisen parameter is extracted within a two-temperature model. However, ab initio calculations show that the phonons are nonthermal on the time scale of the experiment, which calls into question the validity of extracting physical constants by fitting such a two-temperature model.