Imaging ultrafast and ultrasmall: Capturing ultrafast magnetization using fs x-rays

Ultrafast control of magnetization with light holds the promise of being a new paradigm for the next generation memory and data storage devices. There is an explosive increase in studies that aim to understand the mechanisms of photon-driven spin dynamics at such short time-scales. However, it has been recently recognized that spatial domain pattern and nanoscale heterogeneities can play a critical role in dictating the ultrafast behavior [1-5].  These and many other experimental findings in ultrafast spin dynamics are now possible due to emerging coherent x-ray/EUV sources that combine the power of x-rays with fs time resolution [6]. I will highlight some of the intriguing data obtained in the recent years with x-ray/EUV free electron lasers (FELs) that hint at the rich variety of magnetic behavior at the frontier of the ultrafast and ultra-small. I will focus on some of our recent results obtained at the European XFEL and FERMI that demonstrate how spin textures are spatially perturbed under ultrafast optical pumping conditions. Surprisingly, we found that the spatial symmetry of spin textures affects ultrafast spatial dynamics at the nm length scale [7,8]. Specifically, we observe nontrivial distortions in the shape of magnetic diffraction patterns depending on the symmetry of the magnetic domain pattern. In addition, these distortions are distinctly nonlinear in pump fluence with a threshold, reminiscent of the threshold behavior for driven domain wall motion. Our results settle the debate as to whether ultrafast optical pumping instigates domain dynamics, and show that the dynamics depend strongly on both the details of the domain pattern and the pump energy. While the evidence supporting ultrafast domain dynamics is now substantial, the mechanism (or mechanisms) underlying these effects is not yet clear. Superdiffusive spin currents [9] have been proposed as one possible explanation [4,5]. Regardless, these intriguing observations suggest that texture-dependent ultrafast spin transport might provide us with novel ways to manipulate spin degrees of freedom.