Study of ultrathin Pt/Co/Pt trilayers modified by nanosecond XUV pulses from laser-driven plasma source

We have studied the structural mechanisms responsible for the magnetic reorientation between in-plane and out-of-plane magnetization in the (25 nm Pt)/(3 and 10 nm Co)/(3 nm Pt) trilayer systems irradiated with nanosecond XUV pulses generated with laser-driven gas-puff target plasma source of a narrow continuous spectrum peaked at wavelength of 11 nm. The thickness of individual layers, their density, chemical composition and irradiation-induced lateral strain were deduced from symmetric and asymmetric X-ray diffraction (XRD) patterns, grazing-incidence X-ray reflectometry (GIXR), grazing incidence X-ray fluorescence (GIXRF), extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) measurements. In the as grown samples we found, that the Pt buffer layers are relaxed and that the layer interfaces are sharp. As a result of a quasi-uniform irradiation of the samples, the XRD, EXAFS, GIXR and GIXRF data reveal the formation of two distinct layers composed of Pt1-xCox alloys with different Co concentrations, dependent on the thickness of the as grown magnetic Co film but with similar ∼1% lateral tensile residual strain. For smaller exposure dose (lower number of accumulated pulses) only partial interdiffusion at the interfaces takes place with the formation of a tri-layer composed of Co-Pt alloy sandwiched between thinned Pt layers, as revealed by TEM. The structural modifications are accompanied by magnetization changes, evidenced by means of magneto-optical microscopy. The difference in magnetic properties of the irradiated samples can be related to their modification in Pt1-xCox alloy composition, as the other parameters (lateral strain and alloy thickness) remain almost unchanged. The out-of-plane magnetization observed for the sample with initially 3 nm Co layer can be due to a significant reduction of demagnetization factor resulting from a lower Co concentration

Damage accumulation in thin ruthenium films induced by repetitive exposure to femtosecond XUV pulses below the single shot ablation threshold

The process of damage accumulation in thin ruthenium films exposed to multiple femtosecond XUV free electron laser FEL pulses below the critical angle of reflectance at the Free electron LASer facility in Hamburg FLASH was experimentally analyzed. The multi shot damage threshold is found to be lower than single shot damage threshold. Detailed analysis of the damage morphology and its dependence on irradiation conditions justifies the assumption that cavitation induced by the FEL pulse is the prime mechanism responsible for multi shot damage in optical coating

Structural pathways for ultrafast melting of optically excited thin polycrystalline Palladium films

Due to its extremely short timescale, the non-equilibrium melting of metals is exceptionally difficult to probe experimentally. The knowledge of melting mechanisms is thus based mainly on the results of theoretical predictions. This work reports on the investigation of ultrafast melting of thin polycrystalline Pd films studied by optical laser pump - X-ray free-electron laser probe experiments and molecular-dynamics simulations. By acquiring X-ray diffraction snapshots with sub-picosecond resolution, we capture the sample's atomic structure during its transition from the crystalline to the liquid state. Bridging the timescales of experiments and simulations allows us to formulate a realistic microscopic picture of melting. We demonstrate that the existing models of strongly non-equilibrium melting, developed for systems with relatively weak electron-phonon coupling, remain valid even for ultrafast heating rates achieved in femtosecond laser-excited Pd. Furthermore, we highlight the role of pre-existing and transiently generated crystal defects in the transition to the liquid state.Comment: main manuscript 33 pages, 9 figures; supplemental material 19 pages, 13 figures - all in one fil

Characterization of megahertz X ray laser beams by multishot desorption imprints in PMMA

Proper diagnostics of intense free electron laser FEL X ray pulses is indisputably important for experimental data analysis as well as for the protection of beamline optical elements. New challenges for beam diagnostic methods are introduced by modern FEL facilities capable of delivering powerful pulses at megahertz MHz repetition rates. In this paper, we report the first characterization of a defocused MHz 13.5 nm beam generated by the free electron laser in Hamburg FLASH using the method of multi pulse desorption imprints in poly methyl methacrylate PMMA . The beam fluence profile is reconstructed in a novel and highly accurate way that takes into account the nonlinear response of material removal to total dose delivered by multiple pulses. The algorithm is applied to experimental data of single shot ablation imprints and multi shot desorption imprints at both low 10 Hz and high 1 MHz repetition rates. Reconstructed response functions show a great agreement with the theoretical desorption response function mode

Mechanism of single shot damage of Ru thin films irradiated by femtosecond extreme UV free electron laser

Ruthenium is a perspective material to be used for XUV mirrors at free electron laser facilities. Yet, it is still poorly studied in the context of ultrafast laser matter interaction. In this work, we present single shot damage studies of thin Ru films irradiated by femtosecond XUV free electron laser pulses at FLASH. Ex situ analysis of the damaged spots, performed by different types of microscopy, shows that the weakest detected damage is surface roughening. For higher fluences we observe ablation of Ru. Combined simulations using Monte Carlo code XCASCADE 3D and the two temperature model reveal that the damage mechanism is photomechanical spallation, similar to the case of irradiating the target with optical lasers. The analogy with the optical damage studies enables us to explain the observed damage morphologie

Experimental study of EUV mirror radiation damage resistance under long term free electron laser exposures below the single shot damage threshold

The durability of grazing and normal incidence optical coatings has been experimentally assessed under free electron laser irradiation at various numbers of pulses up to 16 million shots and various fluence levels below 10 of the single shot damage threshold. The experiment was performed at FLASH, the Free electron LASer in Hamburg, using 13.5 nm extreme UV EUV radiation with 100 fs pulse duration. Polycrystalline ruthenium and amorphous carbon 50 nm thin films on silicon substrates were tested at total external reflection angles of 20 and 10 grazing incidence, respectively. Mo Si periodical multilayer structures were tested in the Bragg reflection condition at 16 off normal angle of incidence. The exposed areas were analysed post mortem using differential contrast visible light microscopy, EUV reflectivity mapping and scanning X ray photoelectron spectroscopy. The analysis revealed that Ru and Mo Si coatings exposed to the highest dose and fluence level show a few per cent drop in their EUV reflectivity, which is explained by EUV induced oxidation of the surfac

Role of heat accumulation in the multi-shot damage of silicon irradiated with femtosecond XUV pulses at a 1 MHz repetition rate

The role played by heat accumulation in multi-shot damage of silicon was studied. Bulk silicon samples were exposed to intense XUV monochromatic radiation of a 13.5 nm wavelength in a series of 400 femtosecond pulses, repeated with a 1 MHz rate (pulse trains) at the FLASH facility in Hamburg. The observed surface morphological and structural modifications are formed as a result of sample surface melting. Modifications are threshold dependent on the mean fluence of the incident pulse train, with all threshold values in the range of approximately 36-40 mJ/cm². Experimental data is supported by a theoretical model described by the heat diffusion equation. The threshold for reaching the melting temperature (45 mJ/cm²) and liquid state (54 mJ/cm²), estimated from this model, is in accordance with experimental values within measurement error. The model indicates a significant role of heat accumulation in surface modification processes