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Symmetry is a powerful concept in physics, but its applicability to far-from-equilibrium states is still being understood. Recent attention has focused on how far-from-equilibrium states lead to spontaneous symmetry breaking. Conversely, ultrafast optical pumping can be used to drastically change the energy landscape and quench the magnetic order parameter in magnetic systems. Here, we find a distinct symmetry-dependent ultrafast behaviour by use of ultrafast x-ray scattering from magnetic patterns with varying degrees of isotropic and anisotropic symmetry. After pumping with an optical laser, the scattered intensity reveals a radial shift exclusive to the isotropic component and exhibits a faster recovery time from quenching for the anisotropic component. These features arise even when both symmetry components are concurrently measured, suggesting a correspondence between the excitation and the magnetic order symmetry. Our results underline the importance of symmetry as a critical variable to manipulate the magnetic order in the ultrafast regime.

The use of ultrashort laser pulses with long wavelengths as drivers is a relevant strategy for scaling high harmonic generation (HHG) to higher photon energies. Here, stimulated Raman scattering enhanced by the formation of multidimensional solitary states in a molecular gas‐filled hollow‐core fiber as the mechanism to produce a versatile HHG driver is reported on. This recently discovered method allows to red shift and to compress conventional subpicosecond laser pulses with a simple experimental apparatus, ultimately increasing the generated photon energy, while assuring a high photon flux. The adaptability, simplicity, and stability of this method make it attractive for tailoring HHG sources to individual applications at specific photon energies. Measurements of resonant magnetic scattering in a cobalt/platinum multilayer sample are presented as a demonstration of the relevance of this approach for photon‐hungry applications in the extreme ultraviolet.

Multidimensional solitary states generated by intermodal stimulated Raman scattering in a gas-filled hollow-core fiber form red-shifted, few-cycle pulses after propagation thought glass. These pulses, when used to drive the high harmonic generation process, allow for tailoring the extreme ultraviolet source to specific photon-demanding applications.

Structural dynamics 8(3), 034305 (1-9) (2021). doi:10.1063/4.0000109, During the last two decades, a variety of models have been developed to explain the ultrafast quenching of magnetization following femtosecond optical excitation. These models can be classified into two broad categories, relying either on a local or a non-local transfer of angular momentum. The acquisition of the magnetic depth profiles with femtosecond resolution, using time-resolved x-ray resonant magnetic reflectivity, can distinguish local and non-local effects. Here, we demonstrate the feasibility of this technique in a pump–probe geometry using a custom-built reflectometer at the FLASH2 free-electron laser (FEL). Although FLASH2 is limited to the production of photons with a fundamental wavelength of 4 nm (≃310 eV), we were able to probe close to the Fe $L_3$ edge (706.8 eV) of a magnetic thin film employing the third harmonic of the FEL. Our approach allows us to extract structural and magnetic asymmetry signals revealing two dynamics on different time scales which underpin a non-homogeneous loss of magnetization and a significant dilation of 2 Å of the layer thickness followed by oscillations. Future analysis of the data will pave the way to a full quantitative description of the transient magnetic depth profile combining femtosecond with nanometer resolution, which will provide further insight into the microscopic mechanisms underlying ultrafast demagnetization, Published by AIP Publishing LLC, Melville, NY

High harmonic generation (HHG) in gasses has become a method of choice among table-top extreme ultraviolet (XUV) sources. In order to push the harmonic cut-off towards higher photon energies, many strategies can be implemented, including red-shifting and compressing the driver pulses. Here, we propose a new approach for tuning the red-shift of sub-picosecond driver pulses and compressing them to few-cycle durations in a single stage. This method uses newly discovered multidimensional solitary states (MDSS) pulses as a spatio-temporal engineered source based on the nonlinear Raman process in gas-filled hollow-core fibres (HCF) [1] . It has a few key advantages: 1) The MDSS pulses exhibit a negative quadratic spectral phase and can be compressed by simple propagation through glass. 2) In contrast with commonly-used optical parametric amplifiers, a small redshift can be obtained, allowing to reach a target XUV photon energy while minimizing the detrimental effects of long driver wavelengths on HHG efficiency. We apply our method to X-ray Resonant Magnetic Scattering (XRMS) measurements on a cobalt/platinum ferromagnet, an application that requires a high photon flux at the M 2,3 edge of cobalt (60 eV).

Non-collinear spin textures in ferromagnetic ultrathin films are attracting a renewed interest fueled by possible fine engineering of several magnetic interactions, notably the interfacial Dzyaloshinskii-Moriya interaction. This allows the stabilization of complex chiral spin textures such as chiral magnetic domain walls (DWs), spin spirals, and magnetic skyrmions. We report here on the ultrafast behavior of chiral DWs after optical pumping in perpendicularly magnetized asymmetric multilayers, probed using time-resolved circular dichroism in x-ray resonant magnetic scattering (CD-XRMS). We observe a picosecond transient reduction of the CD-XRMS, which is attributed to the spin current-induced coherent and incoherent torques within the continuously dependent spin texture of the DWs. We argue that a specific demagnetization of the inner structure of the DW induces a flow of hot spins from the interior of the neighboring magnetic domains. We identify this time-varying change of the DW textures shortly after the laser pulse as a distortion of the homochiral N'eel shape toward a transient mixed Bloch-N\'eel-Bloch textures along a direction transverse to the DW. Our study highlights how time-resolved CD-XRMS can be a unique tool for studying the time evolution in other systems showing a non-collinear electric/magnetic ordering such as skyrmion lattices, conical/helical phases, as well as the recently observed antiskyrmion lattices, in metallic or insulating materials.

Ultrashort optical pulses can trigger a variety of non-equilibrium processes in magnetic thin films affecting electrons and spins on femtosecond timescales. In order to probe the charge and magnetic degrees of freedom simultaneously, we developed an X-ray streaking technique that has the advantage of providing a jitter-free picture of absorption cross-section changes. In this paper, we present an experiment based on this approach, which we performed using five photon probing energies at the Ni M2,3-edges. This allowed us to retrieve the absorption and magnetic circular dichroism time traces, yielding detailed information on transient modifications of electron and spin populations close to the Fermi level. Our findings suggest that the observed absorption and magnetic circular dichroism dynamics both depend on the extreme ultraviolet (XUV) probing wavelength, and can be described, at least qualitatively, by assuming ultrafast energy shifts of the electronic and magnetic elemental absorption resonances, as reported in recent work. However, our analysis also hints at more complex changes, highlighting the need for further experimental and theoretical studies in order to gain a thorough understanding of the interplay of electronic and spin degrees of freedom in optically excited magnetic thin films.

We use time-resolved x-ray resonant magnetic scattering (TR-XRMS) at the Co ${M}_{2,3}$ and Tb ${O}_{1}$ edges to study ultrafast demagnetization in an amorphous ${\mathrm{Co}}_{88}{\mathrm{Tb}}_{12}$ alloy with stripe domains. Combining the femtosecond temporal with nanometer spatial resolution of our experiment, we demonstrate that the equilibrium spin texture of the thin film remains unaltered by the optical pump pulse on ultrashort timescales ($l1\phantom{\rule{0.28em}{0ex}}\mathrm{ps}$). However, after $\ensuremath{\simeq}4$ ps, we observe the onset of a significant domain wall broadening, which we attribute to a reduction of the uniaxial magnetic anisotropy of the system, due to energy transfer to the lattice. Static temperature-dependent magnetometry measurements combined with analytical modeling of the magnetic structure of the thin film corroborate this interpretation.

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.

International audience; Laser-driven non-local electron dynamics in ultrathin magnetic samples on a sub-10 nm length scale is a key process in ultrafast magnetism. However, the experimental access has been challenging due to the nanoscopic and femtosecond nature of such transport processes. Here, we present a scattering-based experiment relying on a laser-induced electro-and magneto-optical grating in a Co/Pd ferromagnetic multilayer as a new technique to investigate non-local magnetization dynamics on nanometer length and femtosecond timescales. We induce a spatially modulated excitation pattern using tailored Al near-field masks with varying periodicities on a nanometer length scale and measure the first four diffraction orders in an x-ray scattering experiment with magnetic circular dichroism contrast at the free-electron laser facility FERMI, Trieste. The design of the periodic excitation mask leads to a strongly enhanced and characteristic transient scattering response allowing for sub-wavelength in-plane sensitivity for magnetic structures. In conjunction with scattering simulations, the experiment allows us to infer that a potential ultrafast lateral expansion of the initially excited regions of the magnetic film mediated by hot-electron transport and spin transport remains confined to below three nanometers.

Ultrafast demagnetization, a phenomenon of utmost interest in the context of optical control of magnetically recorded data, has been extensively studied in a variety of different materials. However, only a limited number of studies have investigated the impact of the pump laser wavelength on the process, and only within a narrow spectral range. Performing resonant scattering experiment at the cobalt ${M}_{2,3}$ edges, using extreme ultraviolet radiation photons from a high harmonic source, we studied the ultrafast demagnetization dynamics of Co/Pt multilayers by tuning the pump wavelength to 0.4, 0.8, and 1.8 \ensuremath{\mu}m. We show that the degree of demagnetization at short time scale (100s of fs) is stronger at longer wavelengths. This is explained by the wavelength dependence of both the laser induced heating of the electrons (${T}_{e}\ensuremath{\propto}{\ensuremath{\lambda}}^{2}$) and the spatial distribution of the electromagnetic energy deposited into the multilayer sample.

A high harmonics source is used to perform x-ray resonant magnetic scattering on magnetic samples. The dynamics of laser-induced ultrafast demagnetization are probed and longer pump wavelengths are found to increase the initial quenching levels.

Optics express 29(20), 32388-32403 (2021). doi:10.1364/OE.430828, In this paper, we present a new setup for the measurement of element-specific ultrafast magnetization dynamics in ferromagnetic thin films with a sub-15-fs time resolution. Our experiment relies on a split and delay approach which allows us to fully exploit the shortest X-rays pulses delivered by X-ray Free Electrons Lasers (close to the attosecond range), in an X-ray pump – X-ray probe geometry. The setup performance is demonstrated by measuring the ultrafast elemental response of Ni and Fe during demagnetization of ferromagnetic Ni and Ni$_{80}$Fe$_{20}$ (Permalloy) samples upon resonant excitation at the corresponding absorption edges. The transient demagnetization process is measured in both reflection and transmission geometry using, respectively, the transverse magneto-optical Kerr effect (T-MOKE) and the Faraday effect as probing mechanisms., Published by Soc., Washington, DC

© 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.

International audience; During the past few years high-order harmonic generation (HHG) has opened up the field of ultrafast spectroscopy to an ever larger community by providing a table-top and affordable femtosecond extreme ultraviolet (EUV) and soft-x-ray source. In particular, the field of femtomagnetism has largely benefited from the development of these sources. However, the use of x-ray magnetic circular dichroism (XMCD) as a probe of magnetization, the most versatile and reliable one, has been constrained by the lack of polarization control at HHG sources, so studies have relied on more specific magneto-optical effects. Even the recent developments on the generation of elliptically polarized harmonics have only resulted in a few time-resolved experiments relying on this powerful technique since they add complexity to already-difficult measurements. In this article we show how to easily probe magnetization dynamics with linearly polarized EUV or soft-x-ray light with a versatility similar to XMCD by exploiting the Faraday effect. Static and time-resolved measurements of the Faraday effect are presented around the Co M edges. Using simple theoretical considerations, we show how to retrieve the samples magnetization dynamics from the Faraday rotation and ellipticity transients. Ultrafast demagnetization dynamics of a few nanometers in Co-based samples are measured with this method in out-of-plane as well as in-plane magnetization configurations, showing its great potential for the study of femtomagnetism.

A high harmonics source is used to perform x-ray resonant magnetic scattering on a magnetic sample. The dynamics of laser-induced ultrafast demagnetization are probed with respect to the wavelength of the pump beam. Longer pump wavelengths are found to increase the initial quenching levels.

Magnetostriction, the strain induced by a change in magnetization, is a universal effect in magnetic materials. Owing to the difficulty in unraveling its microscopic origin, it has been largely treated phenomenologically. Here, we show how the source of magnetostriction—the underlying magnetoelastic stress—can be separated in the time domain, opening the door for an atomistic understanding. X-ray and electron diffraction are used to separate the sub-picosecond spin and lattice responses of FePt nanoparticles. Following excitation with a 50-fs laser pulse, time-resolved X-ray diffraction demonstrates that magnetic order is lost within the nanoparticles with a time constant of 146 fs. Ultrafast electron diffraction reveals that this demagnetization is followed by an anisotropic, three-dimensional lattice motion. Analysis of the size, speed, and symmetry of the lattice motion, together with ab initio calculations accounting for the stresses due to electrons and phonons, allow us to reveal the magnetoelastic stress generated by demagnetization., Although magnetostriction is universal in magnetic materials, understanding its microscopic origin remains challenging. Here the authors use X-ray and ultrafast electron diffraction to separate the material’s sub-picosecond spin and lattice responses and reveal the magnetoelastic stress generated by demagnetization.

Physical review letters 121(13), 137403 (2018). doi:10.1103/PhysRevLett.121.137403, Using ultrafast $≃2.5 fs$ and $≃25 fs$ self-amplified spontaneous emission pulses of increasing intensity and a novel experimental scheme, we report the concurrent increase of stimulated emission in the forward direction and loss of out-of-beam diffraction contrast for a $Co/Pd$ multilayer sample. The experimental results are quantitatively accounted for by a statistical description of the pulses in conjunction with the optical Bloch equations. The dependence of the stimulated sample response on the incident intensity, coherence time, and energy jitter of the employed pulses reveals the importance of increased control of x-ray free electron laser radiation., Published by APS, College Park, Md.

As a type of fermions without counterpart in high energy physics, triply degenerate fermions show exotic physical properties, which are represented by triply degenerate nodal points in topological semimetals. Here, based on the space group theory analysis, we propose a practical guidance for seeking a topological semimetal with triply degenerate nodal points located at a symmetric axis, which is applicable to both symmorphic and nonsymmorphic crystals. By using this guidance in combination with the first-principles electronic structure calculations, we predict a class of triply degenerate topological semimetals $R{\mathrm{Rh}}_{6}{\mathrm{Ge}}_{4}\phantom{\rule{4pt}{0ex}}(R=\text{Y},\text{La},\text{Lu})$. In these compounds, the triply degenerate nodal points are located at the $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}A$ axis and not far from the Fermi level. Especially, ${\mathrm{LaRh}}_{6}{\mathrm{Ge}}_{4}$ has a pair of triply degenerate nodal points located at just $\ensuremath{\sim}3$ meV below the Fermi level. Considering the fact that the single crystals of $R{\mathrm{Rh}}_{6}{\mathrm{Ge}}_{4}$ have been synthesized experimentally, the $R{\mathrm{Rh}}_{6}{\mathrm{Ge}}_{4}$ class of compounds will be an appropriate platform for studying exotic physical properties of triply degenerate topological semimetals.

The investigation of ultrafast dynamics, taking place on the few to sub-picosecond time scale, is today a very active research area pursued in a variety of scientific domains. With the recent advent of X-ray free-electron lasers (XFELs), providing very intense X-ray pulses of duration as short as a few femtoseconds, this research field has gained further momentum. As a consequence, the demand for access strongly exceeds the capacity of the very few XFEL facilities existing worldwide. This situation motivates the development of alternative sub-picosecond pulsed X-ray sources among which femtoslicing facilities at synchrotron radiation storage rings are standing out due to their tunability over an extended photon energy range and their high stability. Following the success of the femtoslicing installations at ALS, BESSY-II, SLS and UVSOR, SOLEIL decided to implement a femtoslicing facility. Several challenges were faced, including operation at the highest electron beam energy ever, and achievement of slice separation exclusively with the natural dispersion function of the storage ring. SOLEIL's setup also enables, for the first time, delivering sub-picosecond pulses simultaneously to several beamlines. This last feature enlarges the experimental capabilities of the facility, which covers the soft and hard X-ray photon energy range. In this paper, the commissioning of this original femtoslicing facility is reported. Furthermore, it is shown that the slicing-induced THz signal can be used to derive a quantitative estimate for the degree of energy exchange between the femtosecond infrared laser pulse and the circulating electron bunch.

Ultrafast dynamics are generally investigated using stroboscopic pump-probe measurements, which characterize the sample properties for a single, specific time delay. These measurements are then repeated for a series of discrete time delays to reconstruct the overall time trace of the process. As a consequence, this approach is limited to the investigation of fully reversible phenomena. We recently introduced an off-axis zone plate based X-ray streaking technique, which overcomes this limitation by sampling the relaxation dynamics with a single femtosecond X-ray pulse streaked over a picosecond long time window. In this article we show that the X-ray absorption cross section can be employed as the contrast mechanism in this novel technique. We show that changes of the absorption cross section on the percent level can be resolved with this method. To this end we measure the ultrafast magnetization dynamics in CoDy alloy films. Investigating different chemical compositions and infrared pump fluences, we demonstrate the routine applicability of this technique. Probing in transmission the average magnetization dynamics of the entire film, our experimental findings indicate that the demagnetization time is independent of the specific infrared laser pump fluence. These results pave the way for the investigation of irreversible phenomena in a wide variety of scientific areas., Comment: 9 pages, 5 figures

“The technical support from SLAC Accelerator Directorate, Technology Innovation Directorate, LCLS laser division and Test Facility Division is gratefully acknowledged. We thank S.P. Weathersby, R.K. Jobe, D. McCormick, A. Mitra, S. Carron and J. Corbett for their invaluable help and technical assistance. Research at SLAC was supported through the SIMES Institute which like the LCLS and SSRL user facilities is funded by the Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC02-76SF00515. The UED work was performed at SLAC MeV-UED, which is supported in part by the DOE BES SUF Division Accelerator & Detector R&D program, the LCLS Facility, and SLAC under contract Nos. DE-AC02-05-CH11231 and DE-AC02-76SF00515. Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.”and“Work at BNL was supported by DOE BES Materials Science and Engineering Division under Contract No: DE-AC02-98CH10886. J.C. would like to acknowledge the support from National Science Foundation Grant No. 1207252. E.E.F. would like to acknowledge support from the U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES) under Award No. DE-SC0003678.”This has been corrected in both the PDF and HTML versions of the Article.

The mechanism underlying femtosecond laser-pulse-induced ultrafast magnetization dynamics remains elusive, despite two decades of intense research on this phenomenon. Most experiments focused so far on characterizing magnetization and charge carrier dynamics, while the first direct measurements of structural dynamics during ultrafast demagnetization were reported only very recently. We here present our investigation of the infrared laser-pulse-induced ultrafast demagnetization process in a thin Ni film, which characterizes simultaneously magnetization and structural dynamics. This is achieved by employing femtosecond time-resolved x-ray resonant magnetic reflectivity (tr-XRMR) as the probe technique. The experimental results reveal unambiguously that the subpicosecond magnetization quenching is accompanied by strong changes in nonmagnetic x-ray reflectivity. These changes vary with reflection angle, and changes up to 30% have been observed. By modeling the x-ray reflectivity of the investigated thin film, we can reproduce these changes by a variation of the apparent Ni layer thickness of up to 1%. Extending these simulations to larger incidence angles, we show that tr-XRMR can be employed to discriminate experimentally between currently discussed models describing the ultrafast demagnetization phenomenon., Funding from the European Community's Seventh Framework Programme under Grant Agreement No. 312284 (CALIPSO Project) is gratefully acknowledged, as well as financial support received from the following agencies: (i) The French “Agence National de la Recherche” (ANR) via the projects UMAMI, ANR-11-LABX-0058_NIE and the EQUIPEX UNION (ANR-10-EQPX-52), and (ii) the CNRS-PICS program.

We demonstrate for the first time the realization of a spatial resolved two color, element-specific imaging experiment at the free-electron laser facility FERMI. Coherent imaging using Fourier transform holography was used to achieve direct real space access to the nanometer length scale of magnetic domains of Co/Pt heterostructures via the element-specific magnetic dichroism in the extreme ultraviolet spectral range. As a first step to implement this technique for studies of ultrafast phenomena we present the spatially resolved response of magnetic domains upon femtosecond laser excitation.

We present an element specific and spatially resolved view of magnetic domains in Co/Pt heterostructures in the extreme ultraviolet spectral range. Resonant small-angle scattering and coherent imaging with Fourier-transform holography reveal nanoscale magnetic domain networks via magnetic dichroism of Co at the M2,3 edges as well as via strong dichroic signals at the O2,3 and N6,7 edges of Pt. We demonstrate for the first time simultaneous, two-color coherent imaging at a free-electron laser facility paving the way for a direct real space access to ultrafast magnetization dynamics in complex multicomponent material systems, International audienceWe present an element specific and spatially resolved view of magnetic domains in Co/Pt heterostructures in the extreme ultraviolet spectral range. Resonant small-angle scattering and coherent imaging with Fourier-transform holography reveal nanoscale magnetic domain networks via magnetic dichroism of Co at the M-2,M-3 edges as well as via strong dichroic signals at the O-2,O-3 and N-6,N-7 edges of Pt. We demonstrate for the first time simultaneous, two-color coherent imaging at a free-electron laser facility paving the way for a direct real space access to ultrafast magnetization dynamics in complex multicomponent material systems.

We use femtosecond time-resolved hard x-ray scattering to detect coherent acoustic phonons generated during ultrafast laser excitation of ferromagnetic bcc Fe films grown on MgO(001). We observe the coherent longitudinal-acoustic phonons as a function of wave vector through analysis of the temporal oscillations in the x-ray scattering signal. The width of the extracted strain wave front associated with this coherent motion is $\ensuremath{\sim}100$ fs. An effective electronic Gr\"uneisen parameter is extracted within a two-temperature model. However, ab initio calculations show that the phonons are nonthermal on the time scale of the experiment, which calls into question the validity of extracting physical constants by fitting such a two-temperature model.

Scientific Reports 6: Article number: 18970; published online: 06 January 2016; updated: 04 March 2016 The original version of this Article contained a typographical error in the spelling of the author Bharati Tudu, which was incorrectly given as Bahrati Tudu. This has now been corrected in the PDF and HTML versions of the Article.

International audience; Does the excitation of ultrafast magnetization require direct interaction between the photons of the optical pump pulse and the magnetic layer? Here, we demonstrate unambiguously that this is not the case. For this we have studied the magnetization dynamics of a ferromagnetic cobalt/palladium multilayer capped by an IR-opaque aluminum layer. Upon excitation with an intense femtosecond-short IR laser pulse, the film exhibits the classical ultrafast demagnetization phenomenon although only a negligible number of IR photons penetrate the aluminum layer. In comparison with an uncapped cobalt/ palladium reference film, the initial demagnetization of the capped film occurs with a delayed onset and at a slower rate. Both observations are qualitatively in line with energy transport from the aluminum layer into the underlying magnetic film by the excited, hot electrons of the aluminum film. Our data thus confirm recent theoretical predictions. Ultrafast demagnetization is a very intriguing phenomenon. The existence and origin of this rapid loss of a thin film's magnetization on the femtosecond timescale has been controversially debated ever since its discovery in 1996 1. Recently, it has been proposed that the origin of this magnetization loss could be due to spin-dependent motion of the optically excited hot valence electrons causing a spatial redistribution of the magnetization, either to an adjacent metallic layer 2,3 or within the magnetic layer itself 4. Strong experimental evidence for this so-called superdiffusive spin transport has been found lately 5-8. However, the experiment of Eschenlohr and co-workers 8 , similar to our study reported here, has been challenged recently. Khorsand and co-workers 9 showed that the Au cap layer employed in that experiment does not yield the postulated, and for the interpretation of the data crucial, attenuation of the incident IR pump pulse. Implementing a truly IR-opaque capping layer, we evade the limitation affecting the interpretation of the results of Eschenlohr and co-workers 8. Proper characterization of the number of photons reaching the magnetic layer demonstrates that the transmitted intensity is by orders of magnitude too small to excite the ultrafast demagnet-ization process.

Understanding the correlations between magnetic nanoparticles is important for nanotechnologies, such as high-density magnetic recording and biomedical applications, where functionalized magnetic particles are used as contrast agents and for drug delivery. The ability to control the magnetic state of individual particles depends on the good knowledge of magnetic correlations between particles when assembled. Inaccessible via standard magnetometry techniques, nanoscale magnetic ordering in self-assemblies of Fe3O4 nanoparticles is here unveiled via x-ray resonant magnetic scattering (XRMS). Measured throughout the magnetization process, the XRMS signal reveals size-dependent inter-particle magnetic correlations. Smaller (5 nm) particles show little magnetic correlation, even when tightly close-packed, yielding to mostly magnetic disorder in the absence of external field, which is characteristic of superparamagnetic behavior. In contrast, larger (11 nm) particles tend to be strongly correlated, yielding a mix of magnetic orders including ferromagnetic and anti-ferromagnetic orders. These strong magnetic correlations are present even when the particles are sparsely distributed.

We model room temperature soft x-ray resonant magnetic reflectivity to determine a 24% increase of the Fe magnetic moment of the 2–3 monolayers next to both MgO interfaces in a MgO(3 nm)/Fe(12 nm)/MgO(001) heterostructure. This direct measurement of such enhanced interface magnetic moments for buried interfaces confirms theoretical predictions and highlights the importance of considering inhomogeneous in-depth magnetic profile in Fe/MgO based magnetic tunnel junctions.

We report on the local magnetic properties of ultrathin bcc Fe films grown on a Ag(1,1,6) stepped surface and covered by a Au layer. We determined the structure of the multilayer stack and the depth-resolved magnetic profile for 5.9 and 13.5 monolayer (ML) thick films. The experiments were carried out on magnetically saturated samples by soft x-ray resonant magnetic reflectivity using both circularly and linearly polarized x rays and two different acquisition configurations. Our results indicate a 20$%$ enhancement of the magnetic moment in the first 2--3 ML of Fe at both interfaces. A comparison of the magnetization profile for Fe films grown on a flat Ag(001) surface indicates a very similar effect and does not reveal any difference regarding the method's accuracy.

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

Using soft-x-ray magnetic resonant reflectivity measurements, we have studied the layer-resolved temperature dependence of the magnetic structure in a six-monolayer-thick fcc-Fe film grown on Cu(001) by pulsed laser deposition. Temperature-dependent reflectivity curves were fitted on the basis of the Heisenberg Hamiltonian involving only a small number of temperature-independent interlayer and intralayer exchange interaction parameters. Our study supports the model of the formation of the blocks of layers with robust magnetic structure whereas the interblock interactions are relatively weak. The temperature-dependent magnetization profile derived on the basis of the exchange interaction parameters reveals a strong inhomogeneity within the film, which is in agreement with the recent theoretical prediction.

International audience; A formalism to simulate the reflectivity of electromagnetic waves by magnetic materials is presented with an application. The formalism is applicable to retrieve the magnetic moment density along the depth of magnetic materials with arbitrary magnetic moment direction using matricial algebra, including roughness between layers. The reflectivity is derived following a classical description with Maxwell equations and a permittivity built from the quantum scattering amplitude. Approximations on the relative power of the Thomson scattering and the magnetic terms are trackable in order to evaluate the validity of the formalism on a case-by-case basis, from the optical light regime up to soft and hard x-rays. The originality of the present formalism lies in the use of eigen-waves throughout the treatment. An application to a Fe thin film illustrates the methodology.

The analysis of complex magnetic profiles throughout an ultrathin magnetic films by soft X-ray resonant magnetic reflectivity is discussed. Subnanometer resolution can be achieved allowing the separation of interface and inner layer magnetic contributions as well as the determination of antiferromagnetic and non-collinear spin structures. Reflectivity measurements are carried out up to large scattering angles allowing the determination of the depth-resolved profiles of the out-of-plane magnetic component.