  We demonstrate the first time-resolved X-ray resonant magnetic scattering
(tr-XRMS) experiment at the N edge of Tb at 155 eV performed using a tabletop
high-brightness high-harmonic generation (HHG) source. In contrast to static
X-ray imaging applications, such optical-pump X-ray-probe studies pose a
different set of challenges for the ultrafast driver laser because a high
photon flux of X-rays resonant with the N edge must be attained at a low
repetition rate to avoid thermal damage of the sample. This laboratory-scale
X-ray magnetic diffractometer is enabled by directly driving HHG in helium with
terawatt-level 1 um laser fields, which are obtained through pulse compression
after a high-energy kHz-repetition-rate Yb:CaF2 amplifier. The high peak power
of the driving fields allows us to reach the fully phase-matching conditions in
helium, which yields the highest photon flux (>2x10^9 photons/s/1% bandwidth)
in the 100-220 eV spectral range, to the best of our knowledge. Our
proof-of-concept tr-XRMS measurements clearly resolve the spatio-temporal
evolution of magnetic domains in Co/Tb ferrimagnetic alloys with femtosecond
and nanometer resolution. In addition to the ultrafast demagnetization, we
observe magnetic domain expansion with a domain wall velocity similar to that
induced by spin transfer torque. The demonstrated method opens up new
opportunities for time-space-resolved magnetic scattering with elemental
specificity on various magnetic, orbital and electronic orderings in condensed
matter systems.
