Understanding the ultrafast demagnetization of transition metals requires pump-probe experiments sensitive to the time evolution of the electronic, spin, and lattice thermodynamic baths. By means of time-resolved photoelectron energy and spin-polarization measurements in the low-pump-fluence regime on iron, we disentangle the different dynamics of hot electrons and demagnetization in the subpicosecond and picosecond time range. We observe a broadening of the Fermi-Dirac distribution, following the excitation of nonthermal electrons at specific region of the iron valence band. The corresponding reduction of the spin polarization is remarkably delayed with respect to the dynamics of electronic temperature. The experimental results are corroborated with a microscopic 3-temperature model highlighting the role of thermal disorder in the quenching of the average spin magnetic moment, and indicating Elliot-Yafet type spin-flip scattering as the main mediation mechanism, with a spin-flip probability of 0.1 and a rate of energy exchange between electrons and lattice of 2.5Kfs−1.
AgCrSe2 exhibits remarkably high ionic conduction, an inversion symmetry-breaking structural transition, and is host to complex non-colinear magnetic orders. Despite its attractive physical and chemical properties and its potential for technological applications, studies of this compound to date are focused almost exclusively on bulk samples. Here, we report the growth of AgCrSe2 thin films via molecular beam epitaxy. Single-orientated epitaxial growth was confirmed by x-ray diffraction, while resonant photoemission spectroscopy measurements indicate a consistent electronic structure as compared to bulk single crystals. We further demonstrate significant flexibility of the grain morphology and cation stoichiometry of this compound via control of the growth parameters, paving the way for the targeted engineering of the electronic and chemical properties of AgCrSe2 in thin-film form.
Mn3Si2Te6 is a rare example of a layered ferrimagnet. It has recently been shown to host a colossal angular magnetoresistance as the spin orientation is rotated from the in- to out-of-plane direction, proposed to be underpinned by a topological nodal-line degeneracy in its electronic structure. Nonetheless, the origins of its ferrimagnetic structure remain controversial, while its experimental electronic structure, and the role of correlations in shaping this, are little explored to date. Here, we combine x-ray and photoemission-based spectroscopies with first-principles calculations to probe the elemental-selective electronic structure and magnetic order in Mn3Si2Te6. Through these, we identify a marked Mn-Te hybridization, which weakens the electronic correlations and enhances the magnetic anisotropy. We demonstrate how this strengthens the magnetic frustration in Mn3Si2Te6, which is key to stabilizing its ferrimagnetic order, and find a crucial role of both exchange interactions extending beyond nearest-neighbors and antisymmetric exchange in dictating its ordering temperature. Together, our results demonstrate a powerful methodology of using experimental electronic structure probes to constrain the parameter space for first-principles calculations of magnetic materials, and through this approach, reveal a pivotal role played by covalency in stabilizing the ferrimagnetic order in Mn3Si2Te6.
Infrared scattering-type scanning near-field optical microscopy (IR s-SNOM) and imaging is here exploited together with attenuated total reflection (ATR) IR imaging and scanning electron microscopy (SEM) to depict the chemical composition of fibers in hybrid electrospun meshes. The focus is on a recently developed bio-hybrid material for vascular tissue engineering applications, named Silkothane®, obtained in the form of nanofibrous matrices from the processing of a silk fibroin-polyurethane (SFPU) blend via electrospinning. Morphology and chemistry of single fibers, at both surface and subsurface level, have been successfully characterized with nanoscale resolution, taking advantage of the IR s-SNOM capability to portray the nanoscale depth profile of this modern material working at diverse harmonics of the signal. The applied methodology allowed to describe the superficial characteristics of the mesh up to a depth of about 100 nm, showing that SF and PU do not tend to co-aggregate to form hybrid fibers, at least at the length scale of hundreds of nanometers, and that subdomains other than the fibrillar ones can be present. More generally, in the present contribution, the depth profiling capabilities of IR s-SNOM, so far theoretically predicted and experimentally proven only on model systems, have been corroborated on a real material in its natural conditions with respect to production, opening the room for the exploitation of IR s-SNOM as valuable technique to support the production and the engineering of nanostructured materials by the precise understanding of their chemistry at the interface with the environment.
In the last decade, reducing the dimensionality of materials to few atomic layers thickness has allowed exploring new physical properties and functionalities otherwise absent out of the two dimensional limit. In this regime, interfaces and interlayers play a crucial role. Here, we investigate their influence on the electronic properties and structural quality of ultrathin Cr2O3 on Pt(111), in presence of a multidomain graphene intralayer. Specifically, by combining Low-Energy Electron Diffraction, X-ray Photoelectron Spectroscopy and X-ray Absorption Spectroscopy, we confirm the growth of high-quality ultrathin Cr2O3 on bare Pt, with sharp surface reconstructions, proper stoichiometry and good electronic quality. Once a multidomain graphene intralayer is included at the metal/oxide interface, the Cr2O3 maintained its correct stoichiometry and a comparable electronic quality, even at the very first monolayers, despite the partially lost of the morphological long-range order. These results show how ultrathin Cr2O3 films are slightly affected by the interfacial epitaxial quality from the electronic point of view, making them potential candidates for graphene-integrated heterostructures.
V2O3 presents a complex interrelationship between the metal–insulator transition and the structural rhombohedral-monoclinic one in temperature, as a function of sample thickness. Whilst in bulk V2O3 the two transitions coincide on the temperature scale, at 15 nm thickness a fully independent Mott-like transition occurs at lower temperature, with no corresponding structural changes perhaps related to epitaxial strain. It is therefore of relevance to investigate the thin and ultrathin film growth to pinpoint the chemical, electronic and structural phase phenomenology and the role of the interface with the substrate. Here we present results on the thickness dependent properties of V2O3 from 1 nm up to 40 nm thick as grown on c-plane Al2O3 substrates by exploiting variable sampling depth probes. The surface morphology of stoichiometric ultra-thin V2O3 layers evolves from islands-like to continuous flat film with thickness, with implications on the overall properties.
The possibility of modifying the ferromagnetic response of a multiferroic heterostructure via fully optical means exploiting the photovoltaic/photostrictive properties of the ferroelectric component is an effective method for tuning the interfacial properties. In this study, the effects of 405 nm visible-light illumination on the ferroelectric and ferromagnetic responses of (001) Pb(Mg1/3Nb2/3)O3-0.4PbTiO3 (PMN-PT)/Ni heterostructures are presented. By combining electrical, structural, magnetic, and spectroscopic measurements, how light illumination above the ferroelectric bandgap energy induces a photovoltaic current and the photostrictive effect reduces the coercive field of the interfacial magnetostrictive Ni layer are shown. Firstly, a light-induced variation in the Ni orbital moment as a result of sum-rule analysis of x-ray magnetic circular dichroic measurements is reported. The reduction of orbital moment reveals a photogenerated strain field. The observed effect is strongly reduced when polarizing out-of-plane the PMN-PT substrate, showing a highly anisotropic photostrictive contribution from the in-plane ferroelectric domains. These results shed light on the delicate energy balance that leads to sizeable light-induced effects in multiferroic heterostructures, while confirming the need of spectroscopy for identifying the physical origin of interface behavior.
It is well-known that all the phases of the manufacturing influence the extraordinary aesthetic and acoustic features of Stradivari’s instruments. However, these masterpieces still keep some of their secrets hidden by the lack of documentary evidence. In particular, there is not a general consensus on the use of a protein-based ground coating directly spread on the wood surface by the Cremonese Master. The present work demonstrates that infrared scattering-type scanning near-fields optical microscopy (s-SNOM) may provide unprecedented information on very complex cross-sectioned microsamples collected from two of Stradivari’s violins, nanoresolved chemical sensitivity being the turning point for detecting minute traces of a specific compound, namely proteins, hidden by the matrix when macro or micro sampling approaches are exploited. This nanoresolved chemical-sensitive technique contributed new and robust evidence to the long-debated question about the use of proteinaceous materials by Stradivari.
The generation and control of surface acoustic waves (SAWs) in a magnetic material are objects of an intense research effort focused on magnetoelastic properties, with fruitful ramifications in spin-wave-based quantum logic and magnonics. We implement a transient grating setup to optically generate SAWs also seeding coherent spin waves via magnetoelastic coupling in ferromagnetic media. In this work we report on SAW-driven ferromagnetic resonance (FMR) experiments performed on polycrystalline Ni thin films in combination with time-resolved Faraday polarimetry, which allows extraction of the value of the effective magnetization and of the Gilbert damping. The results are in full agreement with measurements on the very same samples from standard FMR. Higher-order effects due to parametric modulation of the magnetization dynamics, such as down-conversion, up-conversion, and frequency mixing, are observed, testifying the high sensitivity of this technique.
Machine-learning techniques are revolutionizing the way to perform efficient materials modeling. We here propose a combinatorial machine-learning approach to obtain physical formulas based on simple and easily accessible ingredients, such as atomic properties. The latter are used to build materials features that are finally employed, through linear regression, to predict the energetic stability of semiconducting binary compounds with respect to zinc blende and rocksalt crystal structures. The adopted models are trained using a dataset built from first-principles calculations. Our results show that already one-dimensional (1D) formulas well describe the energetics; a simple grid-search optimization of the automatically obtained 1D-formulas enhances the prediction performance at a very small computational cost. In addition, our approach allows one to highlight the role of the different atomic properties involved in the formulas. The computed formulas clearly indicate that “spatial” atomic properties (i.e., radii indicating maximum probability densities for 𝑠,𝑝,𝑑 electronic shells) drive the stabilization of one crystal structure with respect to the other, suggesting the major relevance of the radius associated with the 𝑝-shell of the cation species.
Curved magnets attract considerable interest for their unusually rich phase diagram, often encompassing exotic (e.g., topological or chiral) spin states. Micromagnetic simulations are playing a central role in the theoretical understanding of such phenomena; their predictive power, however, rests on the availability of reliable model parameters to describe a given material or nanostructure. Here we demonstrate how noncollinear-spin polarized density-functional theory can be used to determine the flexomagnetic coupling coefficients in real systems. By focusing on monolayer CrI3, we find a crossover as a function of curvature between a magnetization normal to the surface to a cycloidal state, which we rationalize in terms of effective anisotropy and Dzyaloshinskii-Moriya contributions to the magnetic energy. Our results reveal an unexpectedly large impact of spin-orbit interactions on the curvature-induced anisotropy, which we discuss in the context of existing phenomenological models
Two-dimensional (2D) van der Waals (vdW) magnets provide an ideal platform for exploring, on the fundamental side, new microscopic mechanisms and for developing, on the technological side, ultracompact spintronic applications. So far, bilinear spin Hamiltonians have been commonly adopted to investigate the magnetic properties of 2D magnets, neglecting higher order magnetic interactions. However, we here provide quantitative evidence of giant biquadratic exchange interactions in monolayer NiX2 (X=Cl, Br and I), by combining first-principles calculations and the newly developed machine learning method for constructing Hamiltonian. Interestingly, we show that the ferromagnetic ground state within NiCl2 single layers cannot be explained by means of the bilinear Heisenberg Hamiltonian; rather, the nearest-neighbor biquadratic interaction is found to be crucial. Furthermore, using a three-orbitals Hubbard model, we propose that the giant biquadratic exchange interaction originates from large hopping between unoccupied and occupied orbitals on neighboring magnetic ions. On a general framework, our work suggests biquadratic exchange interactions to be important in 2D magnets with edge-shared octahedra.
The effects of competing magnetic interactions in stabilizing different spin configurations are drawing renewed attention in order to unveil emerging topological spin textures and to highlight microscopic mechanisms leading to their stabilization. The possible key role of the two-site exchange anisotropy in selecting specific helicity and vorticity of skyrmionic lattices has only recently been proposed. In this work, we explore the phase diagram of a frustrated localized magnet characterized by a two-dimensional centrosymmetric triangular lattice, focusing on the interplay between the two-ion anisotropy and the single-ion anisotropy. The effects of an external magnetic field applied perpendicularly to the magnetic layer, are also investigated. By means of Monte Carlo simulations, we find an abundance of different spin configurations, going from trivial to high-order Q skyrmionic and meronic lattices. In closer detail, we find that a dominant role is played by the two-ion over the single-ion anisotropy in determining the planar spin texture; the strength and the sign of single ion anisotropy, together with the magnitude of the magnetic field, tune the perpendicular spin components, mostly affecting the polarity (and, in turn, the topology) of the spin texture. Our analysis confirms the crucial role of the anisotropic symmetric exchange in systems with dominant short-range interactions; at the same time, we predict a rich variety of complex magnetic textures, which may arise from a fine tuning of competing anisotropic mechanisms.
The magnetic properties of the two-dimensional VI3 bilayer are the focus of our first-principles analysis, highlighting the role of t2g orbital splitting and carried out in comparison with the CrI3 prototypical case, where the splitting is negligible. In VI3 bilayers, the empty a1g state is found to play a crucial role in both stabilizing the insulating state and in determining the interlayer magnetic interaction. Indeed, an analysis based on maximally localized Wannier functions allows one to evaluate the interlayer exchange interactions in two different VI3 stackings (labeled AB and AB′), to interpret the results in terms of the virtual-hopping mechanism, and to highlight the strongest hopping channels underlying the magnetic interlayer coupling. Upon application of electric fields perpendicular to the slab, we find that the magnetic ground state in the AB′ stacking can be switched from antiferromagnetic to ferromagnetic, suggesting the VI3 bilayer as an appealing candidate for electric-field-driven miniaturized spintronic devices.
The growing demand for innovative means in biomedical, therapeutic and diagnostic sciences has led to the development of nanomedicine. In this context, naturally occurring tubular nanostructures composed of rolled sheets of alumino-silicates, known as halloysite nanotubes, have found wide application. Halloysite nanotubes indeed have surface properties that favor the selective loading of biomolecules. Here, we present the first, to our knowledge, structural study of DNA-decorated halloysite nanotubes, carried out with nanometric spatially-resolved infrared spectroscopy. Single nanotube absorption measurements indicate a partial covering of halloysite by DNA molecules, which show significant structural modifications taking place upon loading. The present study highlights the constraints for the use of nanostructured clays as DNA carriers and demonstrates the power of super-resolved infrared spectroscopy as an effective and versatile tool for the evaluation of immobilization processes in the context of drug delivery and gene transfer.
A ferromagnetic (FM) thin film deposited on a substrate of Pb(Mg1/3Nb2/3)O3−PbTiO3 (PMN-PT) is an appealing heterostructure for the electrical control of magnetism, which would enable nonvolatile memories with ultralow-power consumption. Reversible and electrically controlled morphological changes at the surface of PMN-PT suggest that the magnetoelectric effects are more complex than the commonly used “strain-mediated” description. Here we show that changes in substrate morphology intervene in magnetoelectric coupling as a key parameter interplaying with strain. Magnetic-sensitive microscopy techniques are used to study magnetoelectric coupling in Fe/PMN-PT at different length scales, and compare different substrate cuts. The observed rotation of the magnetic anisotropy is connected to the changes in morphology, and mapped in the crack pattern at the mesoscopic scale. Ferroelectric polarization switching induces a magnetic field-free rotation of the magnetic domains at micrometer scale, with a wide distribution of rotation angles. Our results show that the relationship between the rotation of the magnetic easy axis and the rotation of the in-plane component of the electric polarization is not straightforward, as well as the relationship between ferroelectric domains and crack pattern. The understanding and control of this phenomenon is crucial to develop functional devices based on FM/PMN-PT heterostructures.
Topological spin structures, such as magnetic skyrmions, hold great promises for data storage applications, thanks to their inherent stability. In most cases, skyrmions are stabilized by magnetic fields in non-centrosymmetric systems displaying the chiral Dzyaloshinskii-Moriya exchange interaction, while spontaneous skyrmion lattices have been reported in centrosymmetric itinerant magnets with long-range interactions. Here, a spontaneous anti-biskyrmion lattice with unique topology and chirality is predicted in the monolayer of a semiconducting and centrosymmetric metal halide, NiI2. Our first-principles and Monte Carlo simulations reveal that the anisotropies of the short-range symmetric exchange, when combined with magnetic frustration, can lead to an emergent chiral interaction that is responsible for the predicted topological spin structures. The proposed mechanism finds a prototypical manifestation in two-dimensional magnets, thus broadening the class of materials that can host spontaneous skyrmionic states. Skyrmions, topological spin textures, are typically stabilized by the Dzyaloshinskii-Moriya interaction and an applied magnetic field. In this theoretical study, by analysing monolayer NiI2, the authors suggest that two-site anisotropy with magnetic frustration can stabilize a skyrmion lattice.
Here, we present an integrated ultra-high vacuum apparatus—named MBE-Cluster —dedicated to the growth and in situ structural, spectroscopic, and magnetic characterization of complex materials. Molecular Beam Epitaxy (MBE) growth of metal oxides, e.g., manganites, and deposition of the patterned metallic layers can be fabricated and in situ characterized by reflection high-energy electron diffraction, low-energy electron diffraction, Auger electron spectroscopy, x-ray photoemission spectroscopy, and azimuthal longitudinal magneto-optic Kerr effect. The temperature can be controlled in the range from 5 K to 580 K, with the possibility of application of magnetic fields H up to ±7 kOe and electric fields E for voltages up to ±500 V. The MBE-Cluster operates for in-house research as well as user facility in combination with the APE beamlines at Sincrotrone-Trieste and the high harmonic generator facility for time-resolved spectroscopy.
Implementation of in-situ and operando experimental set-ups for bridging the pressure gap in characterization techniques based on monitoring of photoelectron emission has made significant achievements at several beamlines at Elettra synchrotron facility. These set-ups are now operational and have been successfully used to address unsolved issues exploring events occurring at solid–gas, solid–liquid and solid-solid interfaces of functional materials. The sections in the article communicate the research opportunities offered by the current set-ups at APE, BACH, ESCAmicroscopy and Nanospectroscopy beamlines and outline the next steps to overcome the present limits.
The redox process of pretreated Co3O4 thin film coatings has been studied by ambient pressure soft X-ray absorption spectroscopy. The Co3O4 coatings were composed of nanoparticles of about 10 nm in size as prepared by pulsed laser deposition. The thin film coatings were pretreated in He or in H2 up to 150 °C prior to exposure to the reactive gases. The reactivity toward carbon monoxide and oxygen was monitored by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy during gas exposures. The results indicate that the samples pretreated in He show reactivity only at high temperature, while the samples pretreated in H2 are reactive also at room temperature. X-ray photoemission spectroscopy measurements in ultra-high vacuum and NEXAFS simulations with the CTM4XAS code further specify the results.
Converse magnetoelectric coupling in artificial multiferroics is generally modeled through three possible mechanisms: charge transfer, strain mediated effects or ion migration. Here the role played by electrically controlled morphological modifications on the ferromagnetic response of a multiferroic heterostructure, specifically FexMn1−x ferromagnetic films on piezoferroelectric PMN‐PT [001] substrates, is discussed. The substrates present, in correspondence to electrical switching, fully reversible morphological changes at the surface, to which correspond reproducible modifications of the ferromagnetic response of the FexMn1−x films. Topographic analysis by atomic force microscopy shows the formation of surface cracks (up to 100 nm in height) upon application of a sufficiently high positive electric field (up to 6 kV cm−1). The cracks disappear after application of negative electric field of the same magnitude. Correspondingly, in operando X‐ray magnetic circular dichroic spectroscopy at Fe edge in FexMn1−x layers and micro‐MOKE measurements show local variations in the intensity of the dichroic signal and in the magnetic anisotropy as a function of the electrically driven morphological state. This morphologic parameter, rarely explored in literature, directly affects the ferromagnetic response of the system. Its proof of electrically reversible modification of the magnetic response adds a new possibility in the design of electrically controlled magnetic devices.
Le proprietà ottiche, elettroniche e magnetiche dei solidi e delle loro superfici dipendono dalla struttura degli stati elettronici entro alcuni eV dal livello di Fermi. I calcoli della struttura elettronica a bande sono efficaci solo nel caso di materiali a bassa interazione elettrone-elettrone (correlazione). L'esperimento e la guida necessaria per lo studio delle proprietà elettroniche dei solidi e delle loro superfici, ed in particolare la spettroscopia di fotoemissione (photoemission spectroscopy - PES) che si basa sulla misura dello spettro energetico degli elettroni emessi da un solido eccitato da un fascio di fotoni monocromatici di energia eccedente la funzione lavoro. La risoluzione dell'angolo di emissione (Angle-resolved photemission spectroscopy - ARPES) permette di avere informazioni sulla legge di dispersione En(k) dello stato elettronico iniziale, mentre la misura del grado di polarizzazione in spin del fascio di elettroni completa il set di numeri quantici, fornendo un dato molto importante per lo studio delle correlazioni elettroniche.
We present a new experimental setup for performing X-ray Absorption Spectroscopy (XAS) in the soft X-ray range at ambient pressure. The ambient pressure XAS setup is fully compatible with the ultra high vacuum environment of a synchrotron radiation spectroscopy beamline end station by means of ultrathin Si3N4 membranes acting as windows for the X-ray beam and seal of the atmospheric sample environment. The XAS detection is performed in total electron yield (TEY) mode by probing the drain current from the sample with a picoammeter. The high signal/noise ratio achievable in the TEY mode, combined with a continuous scanning of the X-ray energies, makes it possible recording XAS spectra in a few seconds. The first results show the performance of this setup to record fast XAS spectra from sample surfaces exposed at atmospheric pressure, even in the case of highly insulating samples. The use of a permanent magnet inside the reaction cell enables the measurement of X-ray magnetic circular dichroism at ambient pressure.
The spin-spin correlations in hollow (H) and full (F) maghemite nanoparticles (NPs) have been studied by X-ray magnetic circular dichroism (XMCD). An unexpected XMCD signal was detected and analyzed under the application of a small field (μ0H = 160 Oe) and at remanence for both F and H NPs. Clear differences in the magnitude and in the lineshape of the XMCD spectra between F and H NPs emerged. By comparing XMCD measurements performed with a variable degree of surface sensitivity, we were able to address the specific role played by the surface spins in the magnetism of the NPs.
In this work, we studied the influence of the buffer layer composition on the IrMn thickness threshold for the onset of exchange bias in IrMn/Co bilayers. By means of magnetometry, x-ray absorption and x-ray photoelectron spectroscopy, we investigated the magnetic and chemical properties of the stacks. We demonstrated a higher diffusion of Mn through the Co layer in the case of a Cu buffer layer. This is consistent with the observation of larger IrMn thickness threshold for the onset of exchange bias.
Infrared spectroscopy and spectromicroscopy have rapidly flourished using the advantages of InfraRed Synchrotron Radiation (IRSR), namely high brightness, broadband emission, linear and circular polarization and pulsed structure. InfraRed (IR) beamlines constructed at all synchrotron facilities provide a unique opportunity for a new class of experiments with significant multidisciplinary impact inaccessible to experimental equipment employing black body (globar) sources.