Lawrence Berkeley National Laboratory

Ultrafast magnetic writing using laser, current, and field pulses has caught the interest of scientists because of the fascinating physics underlying the exchange of energy and angular momentum in a magnetic material and because of possible applications in data storage. However, experiments in ultrafast magnetism put stringent requirements onto the experimental technique, both in terms of temporal and of spatial resolution. Mature x-ray microscopy techniques and the availability of picoseond and in the future femtosecond x-ray pulses have now opened the door to such studies.First, I will discuss time-resolved imaging of magnetic vortices with 100 nm spatial resolution using the Photoemission Electron Microscope PEEM-2 at beamline 7.3.1.1 of the Advanced Light Source [1]. The bunch length of the storage ring sets the time resolution of 80 ps. Magnetic vortices appear in soft-magnetic micron- and sub-micron-size structures and are characterized by a curling magnetization. We observed that the chirality or handedness of the vortex, which is determined by the out-of-plane magnetization of the vortex core, governed the sub-ns dynamics of the structure in response to field pulses, generated by an Auston switch. The switch was triggered by a synchronized titanium-sapphire laser. The measured vortex speed and the internal magnetic field at the core agreed well with results of micromagnetic simulations. On a faster time scale, most magnetization dynamics experiments rely on direct laser excitation of the material. Earlier experiments using the time-resolved magneto-optical Kerr effect (TR-MOKE) demonstrated the possibility of manipulating magnetism using a femtosecond laser pulse. The validity and consequences of these measurements have been widely debated. X-ray magnetic dichroism is an ideal tool to probe such dynamics and resolve the debate because x-ray sum rules quantify spin moment, orbital moment and magnetic anisotropy, element-by-element. Furthermore, x-rays are sen