The design of heterostructures offers opportunities for developing new materials with emergent functionalities and tunable physical properties through layer engineering. However, the underlying mechanisms of interlayer interactions that influence the intrinsic dielectric response in promising multiferroic heterostructures remain largely unexplored. In this study, we investigate the well-defined ferroelectric soft mode in SrTiO3 (STO), which serves as a benchmark for comparing the intrinsic dielectric properties of STO layers embedded in the potential multiferroic heterostructure of 5 × (50 + 50) nm STO/AlFeO3 (AFO) with single-layer STO films of similar individual (50 nm) and total (250 nm) thickness. Our findings reveal that increasing the thickness of the STO thin films from 50 nm to 250 nm leads to significant changes in the film microstructure and the degradation of dielectric properties. Notably, the average dielectric response and structural characteristics of the 5 × 50 nm STO layers in the heterostructure remain comparable to those of the single-layer 50 nm STO film, suggesting that the incorporation of intermediate AFO layers effectively mitigates the deterioration of the intrinsic dielectric characteristics of the STO layers. The pronounced temperature-dependent variations in soft mode frequency allow us to propose two potential mechanisms for this effect, which are based on the thermal strain build-up and the formation of defect-induced polar nanoregions. Our study demonstrates that intermediate AFO layers contribute not only magnetic properties to potential multiferroic heterostructures but also enhance the dielectric response, thereby expanding the possibilities of layer engineering.

Synchrotron method of resonant X-ray reflectivity 2D mapping has been applied to study ultrathin epitaxial layers of WS 2 grown by pulsed laser deposition on Al 2 O 3 （0001） substrates. The measurements were carried out across the L absorption edge of tungsten to perform depth-dependent element-selective analysis sensitive to potential chemical modification of the WS 2 layer in ambient conditions. Despite the few monolayer thickness of the studied film, the experimentally measured maps of reflectance as a function of incident angle and photon energy turned out to be quite informative showing well-pronounced interference effects near W absorption edge at 10 210 eV. The synchrotron studies were complemented with conventional non-resonant reflectance measurements carried out in the laboratory at a fixed photon energy corresponding to Cu Kα emission. The reconstruction of the depth and energy dependent scattering length density within the studied multilayers was carried out using the OpenCL empowered fitting software utilizing spectral shaping algorithm which does not rely on the pre-measured reference absorption spectra. A thin WOx layer has been revealed at the surface of the WS 2 layer pointing out to the effect of water assisted photo-oxidation reported in a number of works related to ultrathin layers of transition metal dichalcogenides.

The present work describes an intellectual computational approach to X-ray resonant reflectometry - a synchrotron-based method aimed at non-destructive characterization of epitaxial nanoscale multilayers with low optical contrast between the sublayer materials. Resonant reflectance from specially designed bilayer structures was measured as a function of grazing angle and photon energy across the L absorption edges of rare earth elements and then modeled with the aid of a specially developed software utilizing OpenCL high speed computations performed on a graphical processing unit. The map fitting was carried out in the blind mode in which the spectral shapes of the optical constants of the sublayers were reconstructed by the fitting routine without initial knowledge of the chemical composition and without using any reference spectra. The model uses stepped Lorentzian line shape for the imaginary part of the scattering length density and the Kramers-Kronig consistent line shape for the corresponding real part. Epitaxially grown Eu2O3 / Gd3Ga5O12 bilayers were used as a test bench system to demonstrate the benefits of the proposed 2D mapping technique over conventional X-ray reflectometry. To increase reliability, the map fitting was carried out simultaneously at the absorption edges of two different chemical elements. The derived shapes and positions of the absorption peaks were used to uniquely identify rare earth elements within the corresponding sublayers and provide information on their oxidation states. The method was experimentally tested on the bilayers having different material order to show that not only the chemical composition but also the layer sequence can be effectively reconstructed in the automatic way from the 2D resonant reflectance maps.

In the present paper, we apply a blind fitting algorithm to model two-dimensional energy-incidence angle maps of soft x-ray resonant reflectivity measured in nanoscale epitaxial multilayers of multiferroic ε-Fe2O3 grown by pulsed laser deposition. The possibility of fitting the spectral shape of the complex refraction index across the Fe L3 absorption edge without having an initial guess about the optical constants has been demonstrated. The spectral shape fitting of the real (f1) and imaginary (f2) parts of the atomic scattering factor has been carried out by using a specially designed software utilizing OpenCL fast calculations on graphical processing units. The Kramers-Kronig relations between f1 and f2 are automatically taken into account by the fitting algorithm. A reasonable agreement between blindly generated and reference scattering factor spectra derived independently from an x-ray absorption spectroscopy experiment has been demonstrated. The blind fitting has been compared with the alternative refinement routines, in which small free variations of the reference spectral shapes (or no variations at all) have been allowed. The presented approach to the resonant x-ray reflectometry modeling makes it unnecessary to acquire the optical constant spectral shapes in advance and, thus, is especially helpful when such spectra cannot be physically measured, e.g., for a buried layer of a multilayer system.

In the present paper we discuss applicability of the synchrotron method of resonant x-ray reflectometry to non-destructive depth profiling of multilayer heterostructures with low optical contrast between the sublayers. The two-dimensional mapping approach comprising evaluation and measurement of reflectance as a function of photon energy and grazing angle is shown to provide a convenient way to effectively utilize the enhancement of optical contrast between the sublayers that exists at the absorption edges of chemical elements due to particular spectral shape peculiarities related to oxidation state, crystallographic environment and magnetization. In the present study the evaluation of the resonant X-ray reflectivity maps is performed using a specially developed modeling and fitting software. Estimation of the photon energy / grazing angle combinations at which the reflectance is most sensitive to the minor changes in the physical properties of the sublayers is illustrated by the example of ferroic-on-semiconductor nanoscale heterostructures composed of nanoscale film of metallic iron oxidized from either top or bottom side. The subtle differences in resonant soft x-ray reflectance caused by variation of the oxide film thickness, oxidation state and crystallographic environment are discussed. The presented approach is applicable to a large class of heterostructures composed of sublayers that can be distinguished only by the differences in their near edge X-ray absorption fine structure.

In the present work a computational approach is applied to model and predict the results of X-ray resonant magnetic reflectometry – a non-destructive synchrotron-based technique to probe chemical composition, crystallographic environment and magnetization in multilayer epitaxial heterostructures with nanoscale depth resolution. The discussed 2D mapping approach is a step forward with respect to conventional resonant X-ray reflectometry and consists of collecting a fine step array of reflected intensity as a function of grazing angle and photon energy across the absorption edge of a particular chemical element. With the use of circularly polarized photons the method can be extended to magnetic systems to produce a map of dichroic reflectance directly related to the magnetization profile of the heterostructure. Studying the magnetic field dependence of dichroic reflectance maps can provide valuable information on the magnetization reversal of individual sublayers of a multilayer heterostructure. In the present paper modeling is performed for a bilayer system mimicking the behavior of a 30 nm ɛ-Fe 2 O 3 thin film that is known to exhibit a pronounced two-component magnetic hysteresis. A technique to find optimal energy/angle combinations in order to sense magnetization of individual sublayers is proposed. Also discussed is the advantage of heavy-element capping, which leads to a substantial increase of the dichroic intensity oscillation contrast in the pre-edge region where the sensitivity to the magnetic behavior of the deeply buried interfaces is most pronounced.

Epitaxial calcium fluoride (CaF2) layers with a nominal thickness up to 10 nm on the (111)-oriented Silicon (Si) are obtained. Surface topographies of the fluoride films are recorded and the current-voltage characteristics of the Au/CaF2/Si structures are studied. On a qualitative level, these structures exhibited all the features usual for metal-insulator-semiconductor systems. The current-voltage curves of the samples were reproduced by modeling considering a finite (0.1-1 nm) value of the standard thickness deviation of the dielectric CaF2 film. Keywords: calcium fluoride, thin films, MIS structure, leakage current.

Epitaxial strain in multilayered heterostructures (thin films and core-shell nanostructures) has been intensely investigated as a fundamental way to tailor the properties of various functional materials, such as ferroelectrics, magnetics, superconductors as well as to create new multifunctional materials. Upconversion is a strain-sensitive luminescence process, in which the strain plays a critical role through distorting the local crystallographic environment of the luminescent atom. However, the majority of the research on the effect of stain on upconversion focuses on the core-shell nanostructures synthesized by wet-chemical methods, where the strain imposed by misfit of core and shell compositions is generally low (<1%). The Core-shell structure becomes unstable under greater strain. To investigate the effect of a significantly higher strain in upconversion materials, we grew epitaxial Y2O3:Yb,Er films, in which the out-of-plane strain could reach similar to 1.3% even with the thickness above 300 nm, without breaking the epitaxial ordering. The upconversion spectra reveal that the highly strained thin films of upconversion materials have the tendency to facilitate the multi-photon excitation, illuminating a novel method for the design of upconversion materials with large anti-Stokes shift.

Single-crystal 50 and 300 nm thick layers of BaM hexaferrite BaFe12O19 were syn-thesized by laser molecular beam epitaxy method on alpha-Al2O3 (0001) substrates. Crystallization process was monitored in situ by RHEED and crystal structure was analyzed using three-di-mensional mapping of diffraction patterns. The film surface morphology was investigated by atomic force microscopy. It was shown that the "as grown" structures exhibit hexaferrite structure but reveal violation of the long-range order, which is highly improved by the post growth annealing. Magnetic properties were studied by magnetooptical polar Kerr effect. Both the "as grown" and annealed structures were examined. After annealing, BaM hexaferrite films demonstrate square-type out-of-plane magnetic hysteresis loops indicating the presence of strong uniaxial magnetic anisotropy and remarkable pinning of domain walls.

Thin films of BaM hexaferrite (BaFe12O19) were grown on α-Al2O3(0001) substrates by laser molecular beam epitaxy. Structural, magnetic, and magneto-optical properties were studied using medium-energy ion scattering, energy dispersive X-ray spectroscopy, atomic force microscopy, X-ray diffraction, magneto-optical spectroscopy, and magnetometric techniques, and the dynamics of magnetization by ferromagnetic resonance method. It was shown that even a short time annealing drastically changes the structural and magnetic properties of films. Only annealed films demonstrate magnetic hysteresis loops in PMOKE and VSM experiments. The shape of hysteresis loops depends on thickness of films showing practically rectangular loops and high value of remnant magnetization (Mr/Ms~99%) for thin films (50 nm) and much broader and sloped loops in thick (350–500 nm) films. The magnitude of magnetization 4πMs ≈ 4.3 kG in thin films corresponds to that in bulk BaM hexaferrite. Photon energy and sign of bands in magneto-optical spectra of thin films correspond to ones observed earlier in bulk samples and films of BaM hexaferrite. FMR spectra of 50 nm films at 50 GHz consist of a number of narrow lines. The width of main line ΔH~20 Oe is lower than has been reported up to now.

We have studied the nature of optically induced short-range magnetic fluctuations occurring at 10-nm length scale during ultrafast demagnetization in ferromagnetic Co/Pt multilayers. The time resolved probing of magnetization dynamics was performed with femtosecond soft x-ray pulses at the European x-ray free-electron laser. A transient high -q magnetic scattering accompanying and directly correlated to the destruction of the maze domain network has been observed at picosecond time scale in the wave vector region of 0.2-0.8 nm-1. This high-q scattering has a purely magnetic nature and is ascribed to the optically induced short-range magnetic fluctuations developing in the disturbed but not fully destroyed magnetic domain network. Finally, we have simulated the optically induced response of the domain system using a two-temperature atomistic spin model and have concluded that the reason behind the high-q scattering is the laser-driven heating inducing thermal fluctuations of the domain magnetic structure.

The European X-ray Free-Electron Laser (EuXFEL) Facility is the leading international scientific center for studying the structure and properties of materials using coherent X-rays with high temporal and spatial resolution. The results of the collaboration of the EuXFEL experts and the researchers of the ITMO University in 2015–2022 are briefly described. The unique possibilities of the EuXFEL are demonstrated by an example of studying the ultrafast magnetic dynamics by the researchers of the ITMO University in 2019.

Thin films of calcium fluoride with a nominal thickness from 2 up to 20 nm were grown on silicon-(111) surface by means of molecular beam epitaxy. The interest to fluorite insulating films is stipulated by the potential applications in field-effect transistors with a 2D channel. The studied Au/CaF2/Si heterostructures exhibited behavior which is typical for a metal-insulator-semiconductor system. In order to attain a quantitative agreement between the measured current-voltage curves and the theoretical model, spatial fluctuations of the fluoride layer thickness had to be accounted for. The standard deviation of the CaF2 layer thickness was found to be between 0.2 and 1.2 nm tending to increase with the nominal thickness. These values are in agreement with root mean square surface roughness measured by atomic force microscopy. A satisfactory reproducibility of the electric characteristics of the studied films makes CaF2 a promising insulating material for electronic devices.

The present paper is dedicated to the structural study of crystallographic peculiarities appearing in epitaxial films of metastable epsilon iron oxide (ε-Fe2O3) grown by pulsed laser deposition onto a semiconductor GaN (0001) substrate.The columnar structure of the nanoscale ε-Fe2O3 films has been for the first time investigated using high resolution electron microscopy (HRTEM) direct space technique complemented by reciprocal space methods of high-energy electron diffraction and color-enhanced HRTEM image Fourier filtering.The study of ε-Fe2O3 / GaN interface formation has been further expanded by carrying out a depth resolved analysis of density and chemical composition by neutron reflectometry and energy-dispersive X-ray spectroscopy.The obtained results shed light onto the properties and the origin of the enigmatic few-nanometer thick low density transition layer residing at the ε-Fe2O3 / GaN interface.A detailed knowledge of the properties of this layer is believed to be highly important for the development of ε-Fe2O3 / GaN heterostructures that can potentially become part of the iron-oxide based ferroicon-semiconductor devices with room temperature magneto-electric coupling.

Magnetization reversal in as-grown and annealed NiFe2O4 / SrTiO3(0 0 1) epitaxial heterostructures, prepared by laser molecular beam epitaxy (LMBE), was studied using magneto-optical technique in geometry of polar (PMOKE) and longitidual (LMOKE) Kerr effect. It was found that the hysteresis loops of polarization plane rotation and ellipticity measured in LMOKE geometry are combinations of symmetric (even in magnetic field) SPart and antisymmetric (odd in magnetic field) AsPart parts, caused by quadratic (QMOKE ∼ MiMj) and linear (LMOKE ∼ Mi) in magnetization M contributions, correspondingly. The angular dependences of SPart demonstrate in-plane biaxial magnetic anisotropy (BMA) and show that magnetization reversal in as-grown structures occurs by one jump (1j) process, in contrast to the annealed structures, in which two jumps (2j) process takes place when the deviation of magnetic field from the hard axis is less than ∼20°. Analysis within the framework of the Stoner-Wohlfarth (SW) model with account of domain mechanism of magnetization jumps shows that the first jump occurs due to formation and movement of domain walls. Second jump is remarkably diffused, that is associated with the dispersion of the biaxial anisotropy field Ha and misorientations of the magnetic easy axes in different regions of the film. Narrow and strong FMR lines are observed in the most perfect structures in which the diffusion of the second jump is small. The degree of second jump diffusion can be used to assess the structural perfection of the films.

Epitaxial calcium fluoride (CaF2) layers with a nominal thickness up to 10 nm on the (111)-oriented Silicon (Si) are obtained. Surface topographies of the fluoride films are recorded and the current-voltage characteristics of the Au/CaF2/Si structures are studied. On a qualitative level, these structures exhibited all the features usual for metal-insulator-semiconductor systems. The current-voltage curves of the samples were reproduced by modeling considering a finite (0.1–1 nm) value of the standard thickness deviation of the dielectric CaF2 film.

Periodic multilayers with perpendicular magnetic anisotropy (PMA) attract a lot of attention nowadays being fundamentally interesting from the point of view of interface magnetism. They also provide a suitable test bench for the studies of spatially resolved magnetization dynamics at picosecond time scale conducted at x-ray free electron laser (XFEL) facilities. In the present paper we have developed a novel laser molecular beam epitaxy (LMBE) approach to grow Co/Pt multilayers on crystalline Al2O3 (0001) substrates. By applying Kerr-effect magnetometry to the samples fabricated at different conditions, we have found it essential for the appearance of PMA, to perform metal deposition in 0.03 mbar of argon and to keep Co layer thickness below 4 angstrom. Along with magnetic studies, we have investigated crystal structure and depth resolved layer composition of the fabricated Co/Pt multilayers by applying electron diffraction, x-ray diffraction and x-ray reflectometry. The combined atomic-force/magnetic-force microscopy studies have revealed a very low surface roughness of down to 1 angstrom (for the samples grown at properly optimized growth conditions) and a pronounced labyrinth pattern of magnetic domains with a periodicity of 250 nm appropriate for XFEL studies of ultrafast optical magnetization. The presented LMBE approach to fabricate magnetically ordered heterostructures with a pronounced PMA is supposed to be an important milestone on the way of facilitating future design of nanomaterials for applications in magnetic memories and for fundamental studies of ultrafast magnetization dynamics.

The present paper describes the technological peculiarities relevant to the nucleation and further epitaxial growth of the metastable epsilon phase of iron oxide by means of pulsed laser deposition (PLD). The orthorhombic epsilon ferrite ε-Fe2O3 is an exotic member of a large family of iron oxide polymorphs, which attracts extensive attention nowadays due to its ultra-high magneto-crystalline anisotropy and room temperature multiferroic properties. Continuing the series of previous publications dedicated to the fabrication of ε-Fe2O3 films on GaN, this present work addresses a number of important requirements for the growing conditions of these films. Among the most sensitive technological parameters, the growth temperature must be high enough to aid the nucleation of the orthorhombic phase and, at the same time, low enough to prevent the thermal degradation of an overheated ε-Fe2O3/GaN interface. Overcoming the contradicting growth temperature requirements, an alternative substrate-independent technique to stabilize the orthorhombic phase by mild aluminum substitution is proposed. The advantages of this technique are demonstrated by the example of ε-Fe2O3 films PLD growth carried out on sapphire—the substrate that possesses a trigonal lattice structure and would normally drive the nucleation of the isostructural and energetically more favorable trigonal α-Fe2O3 phase. The real-time profiling of high-energy electron diffraction patterns has been extensively utilized throughout this work to keep track of the orthorhombic-to-trigonal balance being the most important feed-back parameter at the growth optimization stage.

The spectral and magnetic field dependences of the polar magneto-optical Kerr effect (PMOKE) were studied in heterostructures formed by 10 - 40 nm Y3Fe5O12 (YIG) layer grown by laser molecular beam epitaxy on Nd3Ga5O12 (NdGG) and Gd3Ga5O12 (GGG) substrates. The PMOKE spectra can be decomposed into two components, one of which, caused by the Faraday effect in the substrate, is linear in magnetic field H, and the second is saturated at H similar to 0,04 - 0,056 MA/m. In YIG / NdGG, the spectral dependence of the substrate-caused PMOKE clearly reveals a series of narrow bands corresponding to electronic transitions (absorption bands) in the Nd3+ ion. The saturable component of PMOKE, which reflects the behavior of the magnetization of the YIG film, is similar to that observed in YIG / GGG, but at the same time clearly reveals anomalies at the photon energies, corresponding to electronic transitions in the Nd3+ ion. This indicates the presence of magnetization of Nd3+ ions in the interface region of the YIG / NdGG structure, induced by the exchange interaction Nd3+ - Fe3+. The signs of the PMOKE anomalies and possible influence of NdGG substrate complex refractive index on PMOKE caused by YIG film are discussed.