[en] In the theories of fs demagnetization of a ferromagnetic sample after exposure to a fs laser pulse it is assumed that the angular momentum of the sample is conserved. It is shown that this is not strictly valid. However, it is argued that the effect of the deviations from strict angular momentum conservation is very small. - Highlights: • Theory of fs-demagnetization after laser pulse excitation. • Angular momentum transfer out of the electronic spin system. • Non-isolated system, non-central forces, angular momentum conservation

[en] Solid coatings, when heated above their melting points, often break up by forming small round holes, which then grow, coalesce and finally turn the initially contiguous film into a pattern of isolated droplets. Such dewetting has been intensively studied using thin polymer films on Si. Three different hole nucleation mechanisms were discovered: homogeneous (spontaneous) nucleation, heterogeneous nucleation at defects, and spinodal dewetting by self-amplifying capillary waves. We have recently found that swift heavy ion (SHI) irradiation of thin oxide films on Si results in similar dewetting patterns, even though the films were kept far below their melting points. Using our new in-situ SEM at the UNILAC accelerator of GSI, we were now able to identify the mechanisms behind this SHI induced dewetting phenomenon. By varying the film thickness and introducing defects at the interface, we can directly address the hole nucleation processes. Besides homogeneous and heterogeneous nucleation, we also found a process, which very much resembles the spinodal mechanism found for liquid polymers, although in the present case the instable wavy surface is not generated by capillary waves, but by ion beam induced stresses.

[en] Recently we have found that swift heavy ion (SHI) irradiation of thin oxide films on Si results in restructuring similar to the dewetting patterns formed when melting polymer films on Si. However, in these ex-situ experiments we could only investigate the average development of the surface structure, since a new sample had to be taken for each irradiation fluence. With our new in-situ SEM/EDX at the UNILAC accelerator of GSI we are now able to follow the history of individual objects from the first ion impact until their final state. The nucleation and growth mechanisms of SHI induced dewetting can now be easily studied, since the irradiation process can be controlled in very small fluence steps and dewetting stops as soon as the ion beam is switched off. This allows to immediately freeze the actual state of dewetting for careful analysis, which is hardly possible in case of liquid films. After the next irradiation step, one and the same spot can be analyzed again and such the nucleation and growth kinetics of individual dewetting holes can be determined. We report our first results on SHI induced dewetting of iron-oxide layers with varying film thicknesses and artificial defect concentrations.

[en] We have installed a High Resolution Scanning Electron Microscope in the beam line of the UNILAC ion accelerator at the GSI Helmholtz Centre for Heavy Ion Research, which has recently been extended also with an EDX-system. This instrument allows us to in-situ investigate the structural and compositional development of individual objects in the μm- and nm-range under swift heavy ion bombardment, from the very first ion impact up to fluences of the order of several 10"1"5 cm"-"2. The sample is irradiated in small fluence steps and in between SEM-images (and EDX-scans) of one-and-the-same surface area are taken. The irradiation can be carried out at any incidence angle between 0° and 90° and also under stepwise or continuous azimuthal rotation of the sample. At present the set-up will be equipped with a micro-manipulator system, which will allows us to perform additional in-situ analysis like electrical and mechanical characterization as well as substrate-free EDX at sub-μ m objects. We also will be able to irradiate almost free-standing sub-μm structures (pasted on a nanoscale tip). In addition, a Peltier-cooled sample holder is under construction. In this report an overview over this unique instrument and its capabilities and advantages will be given, illustrated by the results of our first in-situ studies on ion induced modification of thin films (e.g. decomposition, dewetting, self-organisation) and on shaping of sub-μm objects by swift heavy ion effects (e.g. ion sputtering, ion hammering, ion induced visco-elastic flow). (author)