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The generation of ultrashort hot-electron pulses in metallic heterostructures offers attractive perspectives for the ultrafast spin manipulation on the picosecond time scale. In such approach, the hot-electron pulses are produced by exciting a non-magnetic capping layer with femtosecond infrared laser pulses. These hot-electron pulses propagate towards a buried magnetic layer to trigger ultrafast demagnetization. Lately, it was shown that the demagnetization onset and characteristic demagnetization times are both affected by the transport regime (ballistic or diffusive) of the photo-excited hot-electrons. In this work, we show that the hot-electron pulses produced by photo-exciting a Al(3)/Ta(3)/Cu(60) capping layer undergo a temporal stretching and delays when they go across a [Co(0.1)/Ni(0.6)]x5 multilayer. These information were extracted from a study of hot-electron induced demagnetization in CoDy alloys by means of Time-Resolved X-Ray Magnetic Circular Dichroism.

One of the intriguing problems of modern magnetism is unravelling the non-equilibrium spin dynamics following laser excitation on the nanometer length scale. In this work the ultrafast magnetic behavior of thin Co/Pt multilayers is studied by resonant magnetic SAXS in transmission geometry. We have for the first time observed a very bright transient scattering from nanometer scale magnetic fluctuations.

© 2017 American Chemical Society. Light-matter interaction at the nanoscale in magnetic materials is a topic of intense research in view of potential applications in next-generation high-density magnetic recording. Laser-assisted switching provides a pathway for overcoming the material constraints of high-anisotropy and high-packing density media, though much about the dynamics of the switching process remains unexplored. We use ultrafast small-angle X-ray scattering at an X-ray free-electron laser to probe the magnetic switching dynamics of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. We observe that the combination of laser excitation and applied static magnetic field, 1 order of magnitude smaller than the coercive field, can overcome the magnetic anisotropy barrier between "up" and "down" magnetization, enabling magnetization switching. This magnetic switching is found to be inhomogeneous throughout the material with some individual FePt nanoparticles neither switching nor demagnetizing. The origin of this behavior is identified as the near-field modification of the incident laser radiation around FePt nanoparticles. The fraction of not-switching nanoparticles is influenced by the heat flow between FePt and a heat-sink layer.

Resonant X-ray absorption, where an X-ray photon excites a core electron into an unoccupied valence state, is an essential process in many standard X-ray spectroscopies. With increasing X-ray intensity, the X-ray absorption strength is expected to become nonlinear. Here, we report the onset of such a nonlinearity in the resonant X-ray absorption of magnetic Co/Pd multilayers near the Co L\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{3}$$\end{document}3 edge. The nonlinearity is directly observed through the change of the absorption spectrum, which is modified in less than 40 fs within 2 eV of its threshold. This is interpreted as a redistribution of valence electrons near the Fermi level. For our magnetic sample this also involves mixing of majority and minority spins, due to sample demagnetization. Our findings reveal that nonlinear X-ray responses of materials may already occur at relatively low intensities, where the macroscopic sample is not destroyed, providing insight into ultrafast charge and spin dynamics., X-ray free electron lasers allow for studying the interaction of magnetic materials with intense X-rays beyond a linear response regime. Here, the authors demonstrate the onset of X-ray induced ultrafast demagnetization in Co/Pd multilayers via a redistribution of valence electrons on timescales shorter than 40 fs.

International audience; We report an element-and time-resolved investigation of femtosecond laser induced ultrafast dynamics of the Co 3d and Dy 4f spins in a ferrimagnetic Co80Dy20 alloy as a function of the temperature. We observe that the Co characteristic demagnetization time (τCo) remains nearly constant (~0.2 ps) on increasing the temperature. Conversely, the Dy characteristic demagnetization time (τDy) decreases from ~1 ps to ~0.4 ps with the rise of temperature. Comparing our experimental data with literature shows that τCo and τDy are independent of the alloy composition or the demagnetization amplitude and that τDy scales with the relative temperature T* = TCurie-T.

The slicing facility FemtoSpeX at BESSY II offers unique opportunities to study photo-induced dynamics on femtosecond time scales by means of X-ray magnetic circular dichroism, resonant and non-resonant X-ray diffraction, and X-ray absorption spectroscopy experiments in the soft X-ray regime. Besides femtosecond X-ray pulses, slicing sources inherently also produce a so-called `halo' background with a different time structure, polarization and pointing. Here a detailed experimental characterization of the halo radiation is presented, and a method is demonstrated for its correct and unambiguous removal from femtosecond time-resolved data using a special laser triggering scheme as well as analytical models. Examples are given for time-resolved measurements with corresponding halo correction, and errors of the relevant physical quantities caused by either neglecting or by applying a simplified model to describe this background are estimated.

Polarization dependent X-ray absorption spectroscopy and microscopy enables the element selective investigation of magnetic systems at the nanoscale. At the Swiss Light Source a photoemission electron microscope is used for the study of a broad variety of systems. Here, a review of recent activities is presented with a focus on instrumental and analytical developments. A new procedure for the 3 dimensional determination of the magnetization vector has been developed, and is demonstrated for GdFeCo microstructures displaying in-plane and out-of-plane domains, and sub-20 nm Fe nanoparticles. The recent progress for measurements in applied magnetic fields is presented and a new set-up for time-resolved measurements employing femtosecond laser pulses is described.

Ultrafast magnetization reversal driven by femtosecond laser pulses has been shown to be a promising way to write information. Seeking to improve the recording density has raised intriguing fundamental questions about the feasibility of combining ultrafast temporal resolution with sub-wavelength spatial resolution for magnetic recording. Here we report on the experimental demonstration of nanoscale sub-100 ps all-optical magnetization switching, providing a path to sub-wavelength magnetic recording. Using computational methods, we reveal the feasibility of nanoscale magnetic switching even for an unfocused laser pulse. This effect is achieved by structuring the sample such that the laser pulse, via both refraction and interference, focuses onto a localized region of the structure, the position of which can be controlled by the structural design. Time-resolved photo-emission electron microscopy studies reveal that nanoscale magnetic switching employing such focusing can be pushed to the sub-100 ps regime.

Time-dependent X-ray absorption spectroscopy at the Ge L3 edge is used to study the optical switching dynamics of epitaxial phase change materials grown on Si(111) membranes. Small differences between the static absorption spectra of the crystalline and laser-amorphized phase of GeTe are observed. Furthermore, a new approach for pump-probe studies on phase change materials is presented. However, no time-dependent change in the X-ray absorption > 0.1% has been detected so far. This is mainly attributed to sample deterioration at 3 kHz pump-probe rates.

The effect of structuring GdFeCo thin films has been investigated in view of laser induced magnetic recording applications. It is found that patterning the substrate via electron beam lithography combined with reactive ion etching before the actual magnetic layer deposition can produce a significant magnetic de-coupling between the obtained structures and their immediate magnetic surrounding. However, upon laser heating, dipolar coupling with this surrounding is found to determine completely the magnetization orientation of the structures, a situation which could be problematic for laser induced recording applications. An alternative structuring approach, based on a lift-off technique of the magnetic layer, is immune to this problem since no magnetic surrounding remains.

Transitions between different charge density wave (CDW) states in quasi-two-dimensional materials may be accompanied also by changes in the inter-layer stacking of the CDW. Using MeV ultrafast electron diffraction, the out-of-plane stacking order dynamics in the quasi-two-dimensional dichalcogenide 1T-TaS2 is investigated for the first time. From the intensity of the CDW satellites aligned around the commensurate l = 1/6 characteristic stacking order, it is found out that this phase disappears with a 0.3 ps time constant. Simultaneously, in the same experiment, the emergence of the incommensurate phase, with a slightly slower 2.0 ps time constant, is determined from the intensity of the CDW satellites aligned around the incommensurate l = 1/3 characteristic stacking order. These results might be of relevance in understanding the metallic character of the laser-induced metastable “hidden” state recently discovered in this compound.

Combining femtosecond transmission measurements with picosecond time-resolved photo-emission electron microscopy, both using x-ray magnetic circular dichroism, new insights into the all-optical magnetization switching mechanism in GdFe based rare-earth transition metal ferrimagnetic alloys is provided, with emphasis on the role played by the magnetization compensation temperature TM of the alloy.

The effects of magnetic anisotropy on domain structures in half-metallic Heusler alloy Fe2CrSi thin film elements were investigated using high resolution x-ray magnetic circular dichroism photoemission electron microscopy. The transition of the dominating contribution from the magnetocrystalline anisotropy to the shape anisotropy is observed in square-shaped elements when reducing the size below 2.0–2.5 μm. In particular, we identify in disk-shaped Heusler elements the vortex state as the ground state. The shape-anisotropy dominated, well-defined magnetization configuration shows the potential of the Fe2CrSi Heusler alloy for applications in vortex-core- or domain-wall-devices, where the high spin polarization is desirable.

Ultrafast control of a ferromagnet (FM) via exchange coupling with an antiferromagnet (AFM) is demonstrated in a Co/SmFeO${}_{3}$ heterostructure. Employing time-resolved photoemission electron microscopy combined with x-ray magnetic circular dichroism, a sub-100-ps change of the Co spins orientation by up to 10${}^{\ensuremath{\circ}}$ driven by the ultrafast heating of the SmFeO${}_{3}$ orthoferrite substrate through its spin reorientation phase transition is revealed. Numerical modeling of the ultrafast-laser-induced heat profile in the heterostructure, and the subsequent coupled spins dynamics and equilibration of the spin systems suggest that the localized laser-induced spin reorientation is hindered compared with the static case. Moreover, numerical simulations show that a relatively small Co/SmFeO${}_{3}$ exchange interaction could be sufficient to induce a complete and fast spin reorientation transition (SRT).

X-ray magnetic circular dichroism spectroscopy using an X-ray free electron laser is demonstrated with spectra over the Fe L$_{3,2}$-edges. This new ultrafast time-resolved capability is then applied to a fluence-dependent study of all-optical magnetic switching dynamics of Fe and Gd magnetic sublattices in a GdFeCo thin film above its magnetization compensation temperature. At the magnetic switching fuence, we corroborate the existence of a transient ferromagnetic-like state. The timescales of the dynamics, however, are longer than previously observed below the magnetization compensation temperature. Above and below the switching fluence range, we observe secondary demagnetization with about 5 ps timescales. This indicates that the spin thermalization takes longer than 5 ps., 5 pages, 4 figures

Artificial multiferroic systems, in which novel properties can emerge from elastic coupling between piezoelectric and magnetostrictive phases, are a promising route to obtain significant room-temperature magnetoelectric coupling at the nanoscale. In this work, we have used element-specific soft x-ray photoemission electron microscopy to spatially resolve the effects of ferroelectric BaTiO${}_{3}$ on the magnetic properties of ferromagnetic layers of CoFe${}_{2}$O${}_{4}$ and NiFe${}_{2}$O${}_{4}$. Resulting from the large magnetoelastic anisotropy present in these artificial multiferroic systems, a modification of the orientation and symmetry of the magnetic easy axes of the ferromagnetic film upon changing the underlying domain structure of the ferroelectric has been observed. This opens the possibility to strain-imprint magnetic states in these systems.

The question of how, and how fast, magnetization can be reversed is a topic of great practical interest for the manipulation and storage of magnetic information. It is generally accepted that magnetization reversal should be driven by a stimulus represented by time-non-invariant vectors such as a magnetic field, spin-polarized electric current, or cross-product of two oscillating electric fields. However, until now it has been generally assumed that heating alone, not represented as a vector at all, cannot result in a deterministic reversal of magnetization, although it may assist this process. Here we show numerically and demonstrate experimentally a novel mechanism of deterministic magnetization reversal in a ferrimagnet driven by an ultrafast heating of the medium resulting from the absorption of a sub-picosecond laser pulse without the presence of a magnetic field.

Employing magnetization-sensitive microscopy techniques, we address the light-induced magnetization dynamics of patterned samples of rare-earth transition metal alloys. We experimentally find that smaller structures require a lower energy density to undergo all-optical switching of the magnetization. With the aid of simulations we explain this reduction in terms of enhanced light absorption by interference of the light within the structure. This results in a decrease of about 60% in the energy densities required for all-optical switching compared with those in continuous thin films of the same alloy. Moreover, we envisage that an energy lower than 10 fJ should be sufficient to switch a 20 × 20 nm 2 structure.

Magnetization switching by a single femtosecond laser heat pulse is demonstrated for out-of-plane domains with sizes down to 200 nm in GdFeCo nanostructures. A complex magnetic domain configuration was revealed with a photoemission electron microscope employing x-ray magnetic circular dichroism at the Fe L3 edge and consisted of in-plane magnetized rims and out-of-plane domains, which results from the structuring process. No influence of this complex domain pattern on the switching efficiency of the structures was detected, constituting an important step towards the application of laser induced magnetization switching in storage devices.

International audience; The evolution of the magnetization configurations in highly spin polarized La0:7Sr0:3MnO3 (LSMO) thin film elements (20-60 nm in thickness) as a function of external magnetic field and temperature is studied by direct magnetic imaging using x-ray magnetic circular dichroism photoemission electron microscopy. The sample structuring is done via a pre-patterning process using a Cr mask layer. The LSMO grows amorphous on the Cr layer for the 20 nm thick film but polycrystalline at larger thicknesses. Temperature dependent studies allow for a direct comparison of the properties of the strained and unstrained LSMO regions on a single sample and show that the polycrystalline areas exhibit a higher TC compared to the epitaxial areas. The single crystalline areas are largely magnetically decoupled from the matrix. The magnetic switching between domain states and domain wall spin structures is determined for LSMO ring elements of varying size and thickness. We find that the magnetic field values required to depin domain walls or to nucleate domains increase with decreasing ring width due to the increasing role of shape anisotropy and edge defects. Both transverse and vortex domain walls are stable spin configurations at room temperature and at zero field. In particular, we demonstrate that the desired domain wall type can be selected by applying an appropriate field sequence.

We determine experimentally the spin structure of half-metallic Co2FeAl0.4Si0.6 Heusler alloy elements using magnetic microscopy. Following magnetic saturation, the dominant magnetic states consist of quasi-uniform configurations, where a strong influence from the magnetocrystalline anisotropy is visible. Heating experiments show the stability of the spin configuration of domain walls in confined geometries up to 800 K. The switching temperature for the transition from transverse to vortex walls in ring elements is found to increase with ring width, an effect attributed to structural changes and consequent changes in magnetic anisotropy, which start to occur in the narrower elements at lower temperatures., Comment: 4 pages, 4 figures

We study fast vortex wall propagation in Permalloy wires induced by 3 ns short current pulses with sub 100 ps rise time using high resolution magnetic imaging at zero field. We find a constant domain wall displacement after each current pulse as well as current induced domain wall structure changes, even at these very short timescales. The domain wall velocities are found to be above 100 m/s and independent of the domain wall spin structure. Comparison to experiments with longer pulses points to the pulse shape as the origin of the high velocities.

Studying the interaction of spin-polarized currents with the magnetization configuration is of high interest due to the possible applications and the novel physics involved. High resolution magnetic imaging is one of the key techniques necessary for a better understanding of these effects. Here we present an extension to a magnetic microscope that allows for in-situ current injection into the structure investigated and furthermore for the study of current induced magnetization changes during pulsed current injection. The developed setup is highly flexible and can be used for a wide range of investigations. Examples of current-induced domain wall motion and vortex core displacements measured using this setup are presented., Comment: 11 pages, 5 figures

We propose a novel variant of the algorithm by Simson et al. [R. Simson, E.D. Sheets, K. Jacobson, Biophys. J. 69, 989 (1995)]. Their algorithm was developed to detect transient confinement zones in experimental single particle tracking trajectories of diffusing membrane proteins or lipids. We show that our algorithm is able to detect confinement in a wider class of confining potential shapes than Simson et al.'s one. Furthermore it enables to detect not only temporary confinement but also jumps between confinement zones. Jumps are predicted by membrane skeleton fence and picket models. In the case of experimental trajectories of $\mu$-opioid receptors, which belong to the family of G-protein-coupled receptors involved in a signal transduction pathway, this algorithm confirms that confinement cannot be explained solely by rigid fences., Comment: 4 pages, 3 figures

We report a full analysis of the magnetization dynamics in thin ellipsoidal Permalloy $({\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20})$ elements, made both experimentally, using an all optical pump-probe scheme based on the magneto-optical Kerr effect, and numerically, using micromagnetic simulations. We have found experimentally as well as numerically that the precession frequencies shift with the aspect ratio of the element, with stronger effects for smaller sizes. Moreover, the magnetization dynamics is found to be inhomogeneous, and can be decomposed into three different kinds of localized precession modes: a low frequency edge mode, a main mode which is composed of several modes with frequencies close to each other, and some higher-order modes.

We investigate the effect of electric current pulse injection on domain walls in La(0.7)Sr(0.3)MnO(3) (LSMO) half-ring nanostructures by high resolution x-ray magnetic microscopy at room temperature. Due to the easily accessible Curie temperature of LSMO, we can employ reasonable current densities to induce the Joule heating necessary to observe effects such as hopping of the domain walls between different pinning sites and nucleation/annihilation events. Such effects are the dominant features close to the Curie temperature, while spin torque is found to play a small role close to room temperature. We are also able to observe thermally activated domain wall transformations and we find that, for the analyzed geometries, the vortex domain wall configuration is energetically favored, in agreement with micromagnetic simulations.

We investigated the energy states in compact clusters of ferromagnetic islands with perpendicular anisotropy arranged on a triangular lattice. Due to their finite nature, we were able to determine the energies of all possible cluster states using dipolar energy calculations. We employed photoemission electron microscopy to observe the magnetic states in arrays of clusters of monodomain Co/Pt multilayer islands and following demagnetization, we observed a shift in the energy distribution to lower energies as the dipolar coupling increased. These multistate island clusters not only provide model arrangements of frustrated Ising-type nanomagnets but are also interesting for data storage applications.

Femtosecond laser induced electromagnetic surface waves, supported by a gold overlayer on top of a magnetic iron garnet (IG) single-crystalline film, are demonstrated to induce a change in the magnetic anisotropy of the IG. This effect is found to be similar to the previously reported photo-induced magnetic anisotropy in this material. However, its dependence on the polarization of the light and orientation of the magnetization is found to be different. This electromagnetic surface wave control of the spins opens new interesting possibilities for all-optical ultrafast control of the magnetization at a nanometre length scale.

The influence of current pulses on the domain structure of a 2 µm wide wire composed of a soft out-of-plane magnetized magnetic material is studied by high spatial resolution nonintrusive magnetic imaging. The injection of current pulses (1012 A/m2) leads to stable magnetic states composed of two domains with opposite magnetization direction separated by a domain wall parallel to the wire. The direction of the magnetization in the domains is reversed back and forth by applying successive current pulses with opposite polarity. The formation and control of the domain states by the current is attributed to the effect of the Oersted field, which is calculated to be large enough to induce the switching.

The magneto-optical properties of surface-plasmon polaritonic crystals on ferromagnetic substrates have been studied. The resonant optical transmission of such magneto-plasmonic nanostructures can be efficiently controlled with the applied static magnetic field. The effect is explained by the influence of magneto-optical effects on surface-plasmon polariton waves supported by the metal/magnetic-dielectric interface and, in particular, on the plasmonic bandgap formation.

We demonstrate the coherent control of the surface plasmon polariton mediated optical transmission through arrays of nano-holes in a gold film on a garnet substrate by a standing surface acoustic wave (SAW) induced by a sequence of ultrashort laser pulses. The transmission of the pump pulse through the holes results in a periodic pattern of shock-waves leading, via interference, to a standing SAW. The resulting modulation frequency is determined by the periodicity of the arrays. The polarization of the transmitted light is modulated as well via a strain-induced linear dichroism.