Time-resolved imaging of magnetic nanostructures in the visible and soft X-ray spectral range

To unravel the nature of magnetism, an investigation at its fundamental length- and time-scales has to be tackled. Besides pure research interest of the processes occurring in nanoscopic magnetic systems in the femto- and picosecond range, advancing knowledge experiences promising applications toward faster non-volatile memory devices with increased data storage density. However, this exploration demands state-of-the-art experimental setups sensitive to detect the magnetization with superior spatial and temporal resolution. In this work two new instruments for time-resolved imaging of magnetic nanostructures have been set up:A tabletop time-resolved scanning Kerr microscope (TR-SKM) capable of analyzing the dynamics of patterned magnetic media using the magneto-optic Kerr effect (MOKE) in combination with femtosecond laser pulses to carry out stroboscopic pump-probe experiments. With a novel pump approach utilizing a magnesium photocathode as electrooptical switch, the generation of intense, broadband electronic current pulses becomes possible for excitation of magnetic systems with the transported transient magnetic field. This enables jitter-free measurements on isolated spin-wave packets in permalloy (Ni80Fe20) with a temporal resolution < 30 ps and a spatial resolution < 560 nm. The spatially and temporally resolved data set permits a global analysis of the dynamic parameters defining the wave-packet. For the first time, a direct observation of backward volume modes with counterpropagating group and phase velocities in permalloy became possible.For examinations requiring an increased spatial resolution, a mobile synchrotron radiation based full-field time-resolved magnetic transmission X-ray microscope (TR-MTXM) has been set up at the soft X-ray beamline P04 of the high-brilliance storage ring PETRA III at DESY with a temporal resolution < 250 ps and a spatial resolution < 65 nm. The magnetization of a sample is accessible via the X-ray magnetic circular dichroism (XMCD). Experiments to stimulate magnetic permalloy nanostructures with synchronized femtosecond laser pulses as well as picosecond electronic current pulses of large amplitudes have been carried out. Vortex magnetization patterns were forced into a non-equilibrium state by aligning the majority of the magnetic moments into one direction and the evolution of the domain pattern destruction and recovery could be monitored in time. A reproducible recovery of the vortex pattern with stable chirality and polarity and unusually high vortex core velocities could be observed. Feasibility studies on nonreversible switching processes in need of an active reset stimulus in magnetic chains and arrays have been initiated