Domain dynamics of a ferromagnetic CoFe/Ni multilayer in response to ultrafast optical pumping*

We studied a response of nanoscale magnetic domains in a 50 nm thick [Co90Fe10/Ni] multilayer to a femtosecond laser pulse by use of time-resolved soft X-ray resonant magnetic scattering (RMS) at the Linear Coherent Light Source. By collecting X-ray diffraction at high scattering angles, we observe the 1st, 3rd and 5th order RMS intensity peaks and thus capture the dynamics of fine magnetic features. We observe a rapid shift in the position of all diffraction orders towards smaller scattering angles and a simultaneous reduction of their magnitude. Surprisingly, higher orders start to decline relative to the 1st order only after a few picoseconds. With our data, we develop a method to extract real-space domain dynamics. We find that the dynamics across the domains are non-uniform, with smaller domains demagnetizing stronger than their larger neighbors. The domain walls remain relatively unchanged and start to broaden only after a few picoseconds. Using our models we attribute this behavior to a combined effect of non-uniform spin transport and a gradient in the absorption of the laser pump throughout the sample.