arXiv subjects

Stefan Stanescu

Publications and source records attributed to Stefan Stanescu.

8 recordsLinked to original sources

arXiv2026

AI-Driven Phase Identification from X-ray Hyperspectral Imaging of cycled Na-ion Cathode Materials

Na-ion batteries have emerged as viable candidates for large-scale energy storage applica- tions due to resource abundance and cost advantages. The constraints imposed on their performance and durability, for instance, by complex phase transformations in positive electrode materials during electrochemical cycling, can be addressed and are thus not detrimental to their development. However, diffusion-limited Na-ion transport can drive spatially heterogeneous phase nucleation and propagation, leading to multiphase coexis- tence and locally non-uniform electrochemical activity, generating complex reaction path- ways that challenge both mechanistic understanding and predictive material optimization. These challenges can be addressed by investigating single-crystalline regions of materials, i.e. down to the scale of individual particles, although such analyses are often constrained by energetically and/or spatially sparse hyperspectral datasets. Here, we developed an AI-driven method to process hyperspectral data under sparse sampling conditions and generate multiphase maps with nanometer-scale resolution over a micrometer-scale field of view. We applied this processing on scanning transmission X-ray microscopy (STXM) data to determine the distribution and coexistence of phases in individual particles of NaxV2(PO4)2F3 cathode materials, at different states of charge. The methodology relies on a workflow which combines a Gaussian mixture variational autoencoder (GMVAE) algorithm with the Pearson corre- lation coefficient to identify the sodium content and map their spatial distribution. Our approach reveals nanoscale phase heterogeneity and evolution within individual particles, and improves the reliability of phase detection by identifying ambiguity zones, false assign- ments, and transition phases localized at grain boundaries.

cond-mat.mtrl-sci↗

arXiv2024

Fractional Skyrmion Tubes in Chiral-Interfaced Three-Dimensional Magnetic Nanowires

Magnetic skyrmions are chiral spin textures with rich physics and great potential for unconventional computing. Typically, skyrmions form in bulk crystals with reduced symmetry or ultrathin film multilayers involving heavy metals. Here, we demonstrate the formation of fractional Bloch skyrmion tubes at room temperature by 3D printing ferromagnetic double-helix nanowires with two regions of opposite chirality. Using X-ray microscopy and micromagnetic simulations, we show that the coexistence of vortex and anti-parallel spin states induces the formation of fractional skyrmion tubes at zero magnetic fields, minimising the energy cost of breaking the coupling between geometric and magnetic chirality. We also demonstrate control over zero-field states, including pure vortex, or mixed skyrmion-vortex states, highlighting the magnetic reconfigurability of these 3D nanowires. This work shows how interfacing chiral geometries at the nanoscale can enable advanced forms of topological spintronics.

cond-mat.mes-hall↗

arXiv2024

Magnetic X-ray imaging using a single polarization and multimodal-ptychography

Polarized X-rays allow for imaging birefringent or dichroic properties of materials with nanometric resolution. To disentangle these properties from the electronic density, either a polarization analyzer or several measurements with different polarizations (typically two, or more) are needed. Here we demonstrate that ptychography can disentangle these from a single-polarization measurement by using a multimodal analysis. This new method provides an alternative to obtain polarization-resolved images of a sample when manipulating the incident polarization is not possible nor sufficient.

cond-mat.mes-hall↗

arXiv2023

Enhancement of the Solar Water Splitting Efficiency Mediated by Surface Segregation in Ti-doped Hematite Nanorods

Band engineering is employed thoroughly and targets technologically scalable photoanodes for solar water splitting applications. Complex and costly recipes are necessary, often for average performances. Here we report simple photoanode growth and thermal annealing, with effective band engineering results. By comparing Ti-doped hematite photoanodes annealed under Nitrogen to photoanodes annealed in air, we found strongly enhanced photocurrent, of more than 200 % in the first case. Using electrochemical impedance spectroscopy and synchrotron X-rays spectromicroscopies we demonstrate that oxidized surface states and increased density of charge carriers are responsible for the enhanced photoelectrochemical activity. Surface states are found to be related to the formation of pseudo-brookite clusters by surface Ti segregation. Spectro-ptychography is used for the first time at Ti L3 absorption edge to isolate Ti chemical coordination arising from pseudo-brookite clusters contribution. Correlated with electron microscopy investigation and Density Functional Theory (DFT) calculations, the synchrotron spectromicroscopy data prove unambiguously the origin of the better photoelectrochemical activity of N2- annealed Ti-doped hematite nanorods. Finally, we present here a handy and cheap surface engineering method, beyond the known oxygen vacancy doping, allowing a net gain in the photoelectrochemical activity for the hematite-based photoanodes.

cond-mat.mtrl-sci↗

arXiv2023

Soft X-ray spectro-ptychography on boron nitride nanotubes, carbon nanotubes and permalloy nanorods

Spectro-ptychography offers improved spatial resolution and additional phase spectral information relative to that provided by scanning transmission X-ray microscopes (STXM). However, carrying out ptychography at the lower range of soft X-ray energies (e.g., below 200 eV to 600 eV) on samples with weakly scattering signals can be challenging. We present soft X-ray ptychography results at energies as low as 180 eV and illustrate the capabilities with results from permalloy nanorods (Fe 2p), carbon nanotubes (C 1s), and boron nitride bamboo nanostructures (B 1s, N1s). We describe optimization of low energy X-ray spectro-ptychography and discuss important challenges associated with measurement approaches, reconstruction algorithms, and their effects on the reconstructed images. A method for evaluating the increase in radiation dose when using overlapping sampling is presented.

cond-mat.mtrl-sci↗

arXiv2021

Skyrmions in synthetic antiferromagnets and their nucleation via electrical current and ultrafast laser illumination

Magnetic skyrmions are topological spin textures that hold great promise as nanoscale information carriers in non-volatile memory and logic devices. While room-temperature magnetic skyrmions and their current-induced manipulation were recently demonstrated, the stray field resulting from their finite magnetization as well as their topological charge limit their minimum size and reliable motion in tracks. Antiferromagnetic (AF) skyrmions allow these limitations to be lifted owing to their vanishing magnetization and net zero topological charge, promising room-temperature, ultrasmall skyrmions, fast dynamics, and insensitivity to external magnetic fields. While room-temperature AF spin textures have been recently demonstrated, the observation and controlled nucleation of AF skyrmions operable at room temperature in industry-compatible synthetic antiferromagnetic (SAF) material systems is still lacking. Here we demonstrate that isolated skyrmions can be stabilized at zero field and room temperature in a fully compensated SAF. Using X-ray microscopy techniques, we are able to observe the skyrmions in the different SAF layers and demonstrate their antiparallel alignment. The results are substantiated by micromagnetic simulations and analytical models using experimental parameters, which confirm the chiral SAF skyrmion spin texture and allow the identification of the physical mechanisms that control the SAF skyrmion size and stability. We also demonstrate the local nucleation of SAF skyrmions via local current injection as well as ultrafast laser excitations at zero field. These results will enable the utilization of SAF skyrmions in skyrmion-based devices.

cond-mat.mtrl-sci↗

arXiv2018

Exploiting atomic layer deposition for fabricating sub-10 nm X-ray lenses

Moving towards significantly smaller nanostructures, direct structuring techniques such as electron beam lithography approach fundamental limitations in feature size and aspect ratios. Application of nanostructures like diffractive X-ray lenses requires feature sizes of below 10 nm to enter a new regime in high resolution X-raymicroscopy. As such dimensions are difficult to obtain using conventional electron beam lithography, we pursue a line-doubling approach. We demonstrate that thismethod yields structure sizes as small as 6.4 nm. X-ray lenses fabricated in this way are tested for their efficiency and microscopic resolution. In addition, the line-doubling technique is successfully extended to a six-fold scheme, where each line in a template structure written by electron beam lithography evolves into six metal lines.

physics.app-ph↗

arXiv2006

Structural and magnetic properties of CoPt mixed clusters

In this present work, we report a structural and magnetic study of mixed Co58Pt42 clusters. MgO, Nb and Si matrix can be used to embed clusters, avoiding any magnetic interactions between particles. Transmission Electron Microscopy (TEM) observations show that Co58Pt42 supported isolated clusters are about 2nm in diameter and crystallized in the A1 fcc chemically disordered phase. Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) reveal that buried clusters conserve these properties, interaction with matrix atoms being limited to their first atomic layers. Considering that 60% of particle atoms are located at surface, this interactions leads to a drastic change in magnetic properties which were investigated with conventional magnetometry and X-Ray Magnetic Circular Dichroïsm (XMCD). Magnetization and blocking temperature are weaker for clusters embedded in Nb than in MgO, and totally vanish in silicon as silicides are formed. Magnetic volume of clusters embedded in MgO is close to the crystallized volume determined by GIWAXS experiments. Cluster can be seen as a pure ferromagnetic CoPt crystallized core surrounded by a cluster-matrix mixed shell. The outer shell plays a predominant role in magnetic properties, especially for clusters embedded in niobium which have a blocking temperature 3 times smaller than clusters embedded in MgO.

cond-mat.other↗