The fabrication and integration of high-quality structures of Yttrium Iron Garnet (YIG) is critical for magnonics. Films with excellent properties are obtained only on single crystal Gadolinium Gallium Garnet (GGG) substrates using high-temperature processes. The subsequent realization of magnonic structures via lithography and etching is not straightforward as it requires a tight control of the edge roughness, to avoid magnon scattering, and planarization in case of multilayer devices. In this work a different approach is described based on local laser annealing of amorphous YIG films, avoiding the need for subjecting the entire sample to high thermal budgets and for physical etching. Starting from amorphous and paramagnetic YIG films grown by pulsed laser deposition at room temperature on GGG, a 405 nm laser is used for patterning arbitrary shaped ferrimagnetic structures by local crystallization. In thick films (160 nm) the laser induced surface corrugation prevents the propagation of spin-wave modes in patterned conduits. For thinner films (80 nm) coherent propagation is observed in 1.2 µm wide conduits displaying an attenuation length of 5 µm that is compatible with a damping coefficient of ≈5 × 10−3. Possible routes to achieve damping coefficients compatible with state-of-the art epitaxial YIG films are discussed.
Polarization dependent x-ray absorption spectroscopy was used to study the magnetic ground state and the orbital occupation in bulk-phase VI3 van der Waals crystals below and above the ferromagnetic and structural transitions. X-ray natural linear dichroism and x-ray magnetic circular dichroism spectra acquired at the V $L_{2,3}$ edges are compared against multiplet cluster calculations within the frame of the ligand field theory to quantify the intra-atomic electronic interactions at play and evaluate the effects of symmetry reduction occurring in a trigonally distorted VI6 unit. We observed a non zero linear dichroism proving the presence of an anisotropic charge density distribution around the V3+ ion due to the unbalanced hybridization between the vanadium and the ligand states. Such hybridization acts as an effective trigonal crystal field, slightly lifting the degeneracy of the $t_{2g}^2$ ground state. However, the energy splitting associated to the distortion underestimates the experimental band gap, suggesting that the insulating ground state is stabilized by Mott correlation effects rather than via a Jahn–Teller mechanism. Our results clarify the role of the distortion in VI3 and establish a benchmark for the study of the spectroscopic properties of other van der Waals halides, including emerging 2D materials with mono and few-layers thickness, whose fundamental properties might be altered by reduced dimensions and interface proximity.
Space and mirror charge effects in time-resolved photoemission spectroscopy can be modeled to obtain relevant information on the recombination dynamics of charge carriers. We successfully extracted from these phenomena the reneutralization characteristic time of positive charges generated by photoexcitation in CeO2-based films. For the above-band-gap excitation, a large fraction of positive carriers with a lifetime that exceeds 100 ps are generated. Otherwise, the sub-band-gap excitation induces the formation of a significantly smaller fraction of charges with lifetimes of tens of picoseconds, ascribed to the excitation of defect sites or to multiphoton absorption. When the oxide is combined with Ag nanoparticles, the sub-band-gap excitation of localized surface plasmon resonances leads to reneutralization times longer than 300 ps. This was interpreted by considering the electronic unbalance at the surface of the nanoparticles generated by the injection of electrons, via localized surface plasmon resonance (LSPR) decay, into CeO2. This study represents an example of how to exploit the space charge effect in gaining access to the surface carrier dynamics in CeO2 within the picosecond range of time, which is fundamental to describe the photocatalytic processes.
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.
Hybridization of electronic states and orbital symmetry in transition metal oxides are generally considered key ingredients in the description of both their electronic and magnetic properties. In the prototypical case of La0.65Sr0.35MnO3 (LSMO), a landmark system for spintronics applications, a description based solely on Mn 3d and O 2p electronic states is reductive. We thus analyzed elemental and orbital distributions in the LSMO valence band through a comparison between density functional theory calculations and experimental photoelectron spectra in a photon energy range from soft to hard x rays. We reveal a number of hidden contributions, arising specifically from La 5p, Mn 4s, and O 2s orbitals, considered negligible in previous analyses; our results demonstrate that all these contributions are significant for a correct description of the valence band of LSMO and of transition metal oxides in general.
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 femtosecond evolution of the electronic temperature of laser-excited gold nanoparticles is measured, by means of ultrafast time-resolved photoemission spectroscopy induced by extreme-ultraviolet radiation pulses. The temperature of the electron gas is deduced by recording and fitting high-resolution photo emission spectra around the Fermi edge of gold nanoparticles providing a direct, unambiguous picture of the ultrafast electron-gas dynamics. These results will be instrumental to the refinement of existing models of femtosecond processes in laterally-confined and bulk condensed-matter systems, and for understanding more deeply the role of hot electrons in technological applications.
The study of ionic materials on nanometer scale is of great relevance for efficient miniaturized devices for energy applications. The epitaxial growth of thin films can be a valid route to tune the properties of the materials and thus obtain new degrees of freedom in materials design. High crystal quality SmxCe1-xO2-δ films are here reported at high doping level up to x=0.4, thanks to the good lat-tice matching with the (110) oriented NdGaO3 substrate. X-ray diffraction and transmission electron microscopy demonstrate the ordered structural quality and absence of Sm segregation at macroscopic and atomic level, respectively. Therefore, in epitaxial thin films the homogeneous doping can be obtained even with high dopant content not always approachable in bulk form, getting even an improvement of the structural properties. In situ spectroscopic measurements by x-ray photoemission and x-ray absorption show the O 2p band shift towards the Fermi level which can favor the oxygen exchange and vacancy formation on the surface when the Sm doping is increased to x=0.4. X-ray absorption spectroscopy also confirms the absence of ordered oxygen vacancy clusters and further reveals that the 5d eg and t2g states are well separated by the crystal field in the undistorted local structure even in the case of high doping level x=0.4.
Whenever one is interested in making high temperature superconductor-based devices, the goodness of the sample surface in terms of structural and electrical properties is a strong issue. In fact, it is well known that the surface of high Tc superconducting samples is not bulk-representative, due to air contamination and to the possible presence of oxygen vacancies. In addition, the quality of the surface layer results to be crucial in surface sensitive measurements as in X-ray photoelectron and Angle-resolved photoemission spectroscopy. Recently, some studies have been dedicated to the realization of devices based on electron-doped cuprates, bilayers and nanowires, showing the actual possibility to realize good quality junctions by using these cuprates. In this work, we report on the fabrication of thin films of the electron-doped Nd2−xCexCuO4±δ compound and analyze the surface natural barrier of as-grown films by means of point contact spectroscopy measurements. Suitable treatments of samples in an ozone rich atmosphere have been developed in order to improve the surface quality of the films. Auger electron spectroscopy has been used to monitor the effectiveness of these treatments.
Cu2ZnSnS4 (CZTS) nanocrystals (NCs) were produced via hot-injection from metal chloride precursors. A systematic investigation of the influence of synthesis conditions on composition, size and microstructure of CZTS NCs is presented. The results show that the solvent amount (oleylamine) is a key parameter in the synthesis of this quaternary chalcogenide: a low solvent content leads to CZTS NCs with a prominent kesterite phase with the desired composition for use as absorber material in thin film photovoltaic cells. It is also observed that lowering the injection temperature (250 °C) favours formation of CZTS NCs in the wurtzite phase. The effect of different high temperature thermal treatments on the grain growth is also shown: large crystals are obtained with annealing in inert atmosphere, whereas nanocrystalline films are obtained introducing sulphur vapour during the heat treatment. A correlation between the grain dimension and the carbonaceous residues in the final films is investigated. It is shown that the grain growth is hindered by organic residues, amount and nature of which depend on the heat treatment atmosphere. In fact, oleylamine is removed by a complex pyrolytic process, which is affected by the presence of sulphur vapour. The latter favours the stability of oleylamine residuals against its non-oxidative release.
In this paper, we present the first publicly available human-annotated dataset of images obtained by the Scanning Electron Microscopy (SEM). A total of roughly 22,000 SEM images at the nanoscale are classified into 10 categories to form 4 labeled training sets, suited for image recognition tasks. The selected categories span the range of 0D objects such as particles, 1D nanowires and fibres, 2D films and coated surfaces as well as patterned surfaces, and 3D structures such as microelectromechanical system (MEMS) devices and pillars. Additional categories such as tips and biological are also included to expand the spectrum of possible images. A preliminary degree of hierarchy is introduced, by creating a subtree structure for the categories and populating them with the available images, wherever possible.
The design and characterization of a HHG source conceived for Time and Angle Resolved PhotoElectron Spectroscopy (TR-ARPES) experiments are presented. The harmonics are selected through a grating monochromator with an innovative design able to provide XUV radiation for two distinct TR-ARPES setups.
Interfaces play a crucial role in the study of novel phenomena emerging at heterostructures comprising metals and functional oxides. For this reason, attention should be paid to the interface chemistry, which can favor the interdiffusion of atomic species and, under certain conditions, lead to the formation of radically different compounds with respect to the original constituents. In this work, we consider Cr/
BaTiO3 heterostructures grown on SrTiO3 (100) substrates. Chromium thin films (1–2 nm thickness) are deposited by molecular beam epitaxy on the
BaTiO3 layer, and subsequently annealed in vacuum at temperatures ranging from 473 to 773 K. A disordered metallic layer is detected for annealing temperatures up to 573 K, whereas, at higher temperatures, we observe a progressive oxidation of chromium, which we relate to the thermally activated migration of oxygen from the substrate. The chromium oxidation state is +3 and the film shows a defective rocksalt structure, which grows lattice matched on the underlying BaTiO3 layer. One out of every three atoms of chromium is missing, producing an uncommon tetragonal phase with Cr2O3 stoichiometry. Despite the structural difference with respect to the ordinary corundum α-Cr2O3 phase, we demonstrate both experimentally and theoretically that the electronic properties of the two phases are, to a large extent, equivalent.
We investigate the solvatochromic effect of a Fe-based spin-crossover (SCO) compound via ambient pressure soft X-ray absorption spectroscopy (AP-XAS) and atomic force microscopy (AFM). AP-XAS provides the direct evidence of the spin configuration for the Fe(II) 3d states of the SCO material upon in situ exposure to specific gas or vapor mixtures; concurrent changes in nanoscale topography and mechanical characteristics are revealed via AFM imaging and AFM-based force spectroscopy, respectively. We find that exposing the SCO material to gaseous helium promotes an effective decrease of the transition temperature of its surface layers, while the exposure to methanol vapor causes opposite surfacial and bulk solvatochromic effects. Surfacial solvatochromism is accompanied by a dramatic reduction of the surface layers stiffness. We propose a rationalization of the observed effects based on interfacial dehydration and solvation phenomena.
The electric and nonvolatile control of the spin texture in semiconductors would represent a fundamental step toward novel electronic devices combining memory and computing functionalities. Recently, GeTe has been theoretically proposed as the father compound of a new class of materials, namely ferroelectric Rashba semiconductors. They display bulk bands with giant Rashba-like splitting due to the inversion symmetry breaking arising from the ferroelectric polarization, thus allowing for the ferroelectric control of the spin. Here, we provide the experimental demonstration of the correlation between ferroelectricity and spin texture. A surface-engineering strategy is used to set two opposite predefined uniform ferroelectric polarizations, inward and outward, as monitored by piezoresponse force microscopy. Spin and angular resolved photoemission experiments show that these GeTe(111) surfaces display opposite sense of circulation of spin in bulk Rashba bands. Furthermore, we demonstrate the crafting of nonvolatile ferroelectric patterns in GeTe films at the nanoscale by using the conductive tip of an atomic force microscope. Based on the intimate link between ferroelectric polarization and spin in GeTe, ferroelectric patterning paves the way to the investigation of devices with engineered spin configurations.
Interfaces between organic semiconductors and ferromagnetic metals offer intriguing opportunities in the rapidly developing field of organic spintronics. Understanding and controlling the spin-polarized electronic states at the interface is the key toward a reliable exploitation of this kind of systems. Here we propose an approach consisting in the insertion of a two-dimensional magnetic oxide layer at the interface with the aim of both increasing the reproducibility of the interface preparation and offering a way for a further fine control over the electronic and magnetic properties. We have inserted a two-dimensional Cr4O5 layer at the C60/Fe(001) interface and have characterized the corresponding morphological, electronic, and magnetic properties. Scanning tunneling microscopy and electron diffraction show that the film grows well-ordered both in the monolayer and multilayer regimes. Electron spectroscopies confirm that hybridization of the electronic states occurs at the interface. Finally, magnetic dichroism in X-ray absorption shows an unprecedented spin-polarization of the hybridized fullerene states. The latter result is discussed also in light of an ab initio theoretical analysis.
The knowledge of the picosecond dynamics of the energy level alignment between donor and acceptor materials in organic photovoltaic devices under working conditions is a challenge for fundamental material research. We measured by means of time-resolved Resonant X-ray Photoemission Spectroscopy (RPES) the energy level alignment in ZnPc/C60 films. We employed 800 nm femtosecond laser pulses to pump the system simulating sunlight excitation and X-rays from the synchrotron as a probe. We measured changes in the valence bands due to pump induced modifications of the interface dipole. Our measurements prove the feasibility of time-resolved RPES with high repetition rate sources.
TiO2 is commonly used as the active switching layer in resistive random access memory. The electrical characteristics of these devices are directly related to the fundamental conditions inside the TiO2 layer and at the interfaces between it and the surrounding electrodes. However, it is complex to disentangle the effects of film “bulk” properties and interface phenomena. The present work uses hard X-ray photoemission spectroscopy (HAXPES) at different excitation energies to distinguish between these regimes. Changes are found to affect the entire thin film, but the most dramatic effects are confined to an interface. These changes are connected to oxygen ions moving and redistributing within the film. Based on the HAXPES results, post-deposition annealing of the TiO2 thin film was investigated as an optimisation pathway in order to reach an ideal compromise between device resistivity and lifetime. The structural and chemical changes upon annealing are investigated using X-ray absorption spectroscopy and are further supported by a range of bulk and surface sensitive characterisation methods. In summary, it is shown that the management of oxygen content and interface quality is intrinsically important to device behavior and that careful annealing procedures are a powerful device optimisation technique.
We investigated the influence of surfaces in the formation of different crystal structures of a spin crossover compound, namely [Fe(L)2] (LH: (2-(pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine), which is a neutral compound thermally switchable around room temperature. We observed that the surface induces the formation of two different crystal structures, which exhibit opposite spin transitions, i.e. on heating them up to the transition temperature, one polymorph switches from high spin to low spin and the second polymorph switches irreversibly from low spin to high spin. We attributed this inversion to the presence of water molecules H-bonded to the complex tetrazolyl moieties in the crystals. Thin deposits were investigated by means of polarized optical microscopy, atomic force microscopy, X-ray diffraction, X-ray absorption spectroscopy and micro Raman spectroscopy; moreover the analysis of the Raman spectra and the interpretation of spin inversion were supported by DFT calculations.