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ISBN 978-3-8439-1815-2

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978-3-8439-1815-2, Reihe Physik

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

197 Seiten, Dissertation Universität Hamburg (2014), Softcover, A5

Zusammenfassung / Abstract

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 capable of analyzing the dynamics of patterned magnetic media using the magneto-optic Kerr effect in combination with femtosecond laser pulses to carry out stroboscopic pump-probe experiments. With a novel pump approach utilizing a magnesium photocathode as electro-optical switch, the generation of intense electronic current pulses becomes possible for excitation of magnetic systems. 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 full-field time-resolved magnetic transmission X-ray microscope has been set up at the soft X-ray beamline P04 at the storage ring PETRA III 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. Experiments to stimulate magnetic permalloy nanostructures with 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 non-reversible switching processes in magnetic chains and arrays have been initiated.