Magnetic properties of OH--intercalated Ni2+-Fe3+ layered double hydroxides (LDH) with a nickel-to-iron ratio (n) of 2 and 3, prepared using two independent synthesis methods, have been studied using the macroscopic magnetic method (SQUID magnetometry) and local-scale element-specific techniques (synchrotron-radiation-based soft X-ray XAS and XMCD). It was revealed that synthesis of NinFe-OH LDH using either coprecipitation or urea-based hydrothermal methods was accompanied by formation of small amounts of nanosized (on the order of 4-5 nm) particles of Ni2FeO4 and α-Fe2O3, respectively. Comparative analysis of static and dynamic magnetic response allowed us to extract the contribution of the impurity phases to the total magnetic response of the prepared samples and refine the intrinsic magnetic behavior of the studied NinFe-OH LDH. The extracted true field-dependent magnetic behavior of Ni2Fe-OH LDH prepared using two independent synthesis routes was revealed to differ, indicating possible clusters of long-range Ni/Fe cation ordering in material produced via the urea-based method. In all studied compositions, the features that indicate a glassy magnetic state at low temperatures were observed. This behavior is suggested to result from competing antiferromagnetic and ferromagnetic metal-oxygen-metal interactions with Ni2+ and Fe3+, which occur in nanosized and morphologically anisotropic LDH crystallites.

Maghemite ( γ $\gamma$ - Fe 2 ${\rm Fe}_2$ O 3 ${\rm O}_{3}$ ) is a promising non-precious-metal-containing photocatalyst for water oxidation (OER). Despite being less studied than Hematite, it offers similar corrosion resistance and a favorable band structure, along with higher conductivity and the advantage of an adaptable spinel structure. Its catalytic performance can be optimized by tuning the concentration of a donor dopant. In this work, we synthesized via molecular beam epitaxy ultra-thin (6 nm) Ni-doped γ $\gamma$ - Fe 2 ${\rm Fe}_2$ O 3 ${\rm O}_3$ (111) films with different Ni concentrations. We present a comprehensive study that sheds light on the structural reconfigurations induced by tuning the Ni concentration and the resulting effects on the OER onset and photoconversion efficiency in alkaline media. Samples' electronic reconfiguration under operando conditions and the role of Ni dopant on the catalytic mechanism are studied with X-ray Absorption Spectroscopy. Light-driven surface chemistry modifications during OER are probed with a novel X-ray fluorescence pump-and-probe approach: Fixed energy X-ray absorption PhotoVoltammetry.

The reversible oxidation of magnetite into maghemite at the nanoscale is a key challenge for various applications. While the metastability of maghemite (γ-Fe2O3) has been extensively studied in nanoparticles, its behavior in ultra-thin films remains unexplored. Hence, in this work, we investigate the temperature-dependent oxidation of epitaxial magnetite (Fe3O4) films, 7 and 21 nm thick, grown on Pt(111) using atomic oxygen plasma-assited molecular beam epitaxy (OA-MBE). This study focuses on planar films to determine whether the metastability of maghemite is unique to nanoparticles or also applies to nanofilms. Oxidation experiments conducted in dry air up to 275 °C and monitored by angle-resolved in situ X-ray Photoemission Spectroscopy (AR-XPS), reveal two distinct oxidation stages: superficial oxidation occurring around 50°C, confined to the topmost atomic layers, and deeper oxidation above 200°C, leading to a complete transformation into maghemite, as confirmed by X-Ray Diffraction (XRD). Annealing obtained maghemite films under ultra-high vacuum (UHV) at 400°C successfully reverses the oxidation, restoring stoichiometric magnetite. Finally, multiple oxidation–reduction cycles are achieved without structural degradation, demonstrating the reversibility and epitaxial stability of the Fe3O4/γ-Fe2O3 system in thin-films. The reversible redox behavior of these films holds promise for sustainable catalytic applications where precise control of iron oxidation states is essential.

Rare-earth iron garnet (RE3Fe5O12) films are promising insulating ferrimagnets. They can show low magnetic damping, perpendicular magnetic anisotropy, and ultrafast spin dynamics, which makes them ideal for spin transport applications. In this work, we investigate the interaction between the magnetic sublattices in Er3Fe5O12 thin films grown by pulsed laser deposition on a Gd3Ga5O12 substrate. Structural and magnetic characterization reveals high-quality single-crystal growth, with a compensation temperature close to the reported bulk value (similar to 80 K). Magnetic phase diagrams based on element-specific measurements map out the regions where ferrimagnetic, canted, and aligned phases are stable across the compensation temperature. The micromagnetic dynamics resulting from perpendicular magnetic pulse perturbation of an in-plane magnetized layer was investigated at room temperature and revealed complex configurations. These results are key features for modulating magnetization dynamics through the compensation phenomenon, which is essential for spin-based devices operating in a low-temperature regime.

In this work, we present a comprehensive and comparative study on the growth and magnetic properties of CoFe2O4 layers deposited on both undoped and self-oxidized N-doped BaTiO3 films grown on La2/3Sr1/3MnO3/ SrTiO3 (001) substrates, a prototypical magneto-electric multiferroic system. The oxide layers were grown by plasma-assisted molecular beam epitaxy. We show that N doping, even at a low level of 1 %, induces substantial changes in growth modes and strain relaxation in BaTiO3 and consequently in the CoFe2O4 top layer, which exhibits enhanced magnetization and reduced magnetocrystalline anisotropy. The magnetic properties were determined from extensive element-specific magnetic X-ray dichroic measurements, which were simulated through crystal field multiplet calculations. The structural properties of the samples were investigated using grazing incidence X-ray diffraction and high-resolution electron microscopy. Our observations provide evidence that the plastic relaxation of the underlying ferroelectric N-doped BaTiO3 layer affects the magnetic properties of the top CoFe2O4 layer and can serve as an additional tuning parameter for manipulating the expected properties of multiferroics.

In this work, we determine the electronic structure and charge carrier dynamics of α-Fe2O3(0001) ultrathin film deposited on Pt(111) as a function of water pressure by combined near ambient pressure-time-resolved photoelectron spectroscopy (NAP-TR-PES) measurements and DFT calculations. Under ultrahigh vacuum (UHV) α-Fe2O3 exhibits the expected typical n-type semiconductor behavior with a surface photovoltage (SPV) shift of 31 meV. Surprisingly, when exposed to water a completely different comportment appears. At a partial pressure of water of PH2O = 0.02 hPa, a much smaller SPV (7 meV) appears that is shifted in the opposite direction, which is thus characteristic of a p-type semiconductor. Finally, at PH2O > 0.1 hPa, SPV is canceled, which is consistent with a layer of metallic nature. DFT calculations explain these experimental findings well as originating from a modification of the surface structure and electronic properties of the layer consecutive to water adsorption. It is shown that the Fermi level position in the band gap of hematite depends on the quantity of adsorbed water, giving rise to the ambipolar behavior of the surface, which is able to transport both negative and positive charges when exposed to the appropriate water pressure.

Epitaxial oxynitride films have promising genuine electronic properties but are very challenging to engineer due to a detrimental imbalance between nitriding and oxidation. The crystalline growth of BaTiO3 thin films doped by atomic and ion-nitrogen plasma-assisted molecular beam epitaxy has been studied on SrTiO3(001) substrates. Several conditions for nitrogen insertion in the perovskite lattice of BaTiO3 were considered. The N-doped BaTiO3 layers are compared to undoped BaTiO3 films produced with an atomic oxygen plasma source only. Oxynitride layers were elaborated on two different perovskite surfaces: SrTiO3(001) single crystal substrates and a La2/3Sr1/3MnO3 back electrode on SrTiO3(001). This approach permits an in-depth study of the films' specific properties including crystalline structure, chemical composition, ferroelectric behavior, and optical response. The chemistry and crystalline structure of the films are found to depend modestly on the substrate nature, while the growth is strongly dominated by self-oxidation and the presence of the ca. 1% substitutional nitrogen (N) in the oxynitride films. Structural and ferroelectric properties are similar for N-doped and undoped BaTiO3 films, while significant changes in optical absorption are observed upon N-doping, confirming recent theoretical predictions. This new class of compounds is expected to be very well suited for novel applications based on band engineering in multifunctional materials.

In this study, we investigate the properties, with a special focus on the magnetic attributes, of NiFe2O4 (NFO)/BaTiO3 (BTO) multiferroic heterostructures, examining individual layer thicknesses ranging from 3 to 12 nm. X-ray diffraction reveals that as BTO thickness increases, NFO transitions from a nonstrained cubic lattice to a compressively strained tetragonal lattice. In contrast, thicker NFO layers introduce enhanced tensile stresses on the BTO layer, counteracting the compressive strain originating from the SrTiO3 (STO) substrate. Piezoresponse force microscopy demonstrates that the polarization switching voltage escalates with increasing BTO layer thickness. Through X-ray magnetic circular dichroism measurements coupled with multiplet theory, we elucidate variations in the magnetic moments and ionic distributions within the NFO layers. Remarkably, a larger BTO thickness is associated with a chemical reduction of Fe ions in the NFO layer, indicative of increased oxygen vacancies, which are induced by the increasing compressive strain as evidenced by first-principles calculations. Thinner NFO layers showed increased tetrahedral (T-d) site vacancies and oxygen vacancies concomitant with reduced magnetic moments, which can be optimized by either increasing NFO thickness or through air annealing at 450 degrees C. Combining the ionic distribution variation with in-plane lattice parameter evolution during growth, we postulated that the reduced magnetic moments originate from a 2-3 nm antiferromagnetic rock salt NiO/FeO formed at the beginning of NFO growth, while the air annealing restores the magnetism by oxidizing and arranging the NiO/FeO into spinel NFO under the synergistic effect of oxygen and heat. Air annealing at moderate temperature appears as a very efficient method to restore the magnetization of ultrathin layers of NFO, allowing us to overcome a major drawback for these compounds, which hampers their utilization in spintronics applications so far.

Crystal structures of the high-pressure synthesized perovskite phases of the (1-x)BiFe0.5Sc0.5O3-xLaFe0.5Sc0.5O3 (0 = x <= 1) system and their temperature and pressure behaviours were studied using laboratory and synchrotron X-ray diffractions as well as neutron diffraction. At room temperature, the as-prepared phases with x <= 0.05 have an antipolar structure with the Pnma symmetry and with the V2ap x 4ap x 2V2ap superstructure (where ap is the pseudocubic perovskite unit-cell parameter). An incommensurately modulated phase with the Imma(00 gamma)s00 superspace group is observed for 0.10 = x <= 0.33, while a non-polar Pnma phase (V2ap x 2ap x V2ap) is stable when x >= 0.34. The antipolar Pnma phase in the as-prepared samples with composition corresponding to x = 0 transforms into the polar Ima2 one via irreversible annealing-caused transformation accompanied by a formation of a high-temperature intermediate polar R3c polymorph, while the antipolar Pnma phase in samples with x = 0.05 is stable until the decomposition temperature. In the solid solutions with 0.10 = x <= 0.33, increasing temperature was found to result in a reversible transformation of the Imma(00 gamma)s00 phase into a non-polar Pnma one. The transition temperature decreases with increasing x. A hydrostatic pressure of few GPa was also shown to induce a reversible Imma(00 gamma)s00 -> Pnma transformation.

Tuning magnetic and electronic transport properties in spinel oxides requires a faithful description between chemical composition and cation site-occupation. Here this challenge is addressed using Fe3-xCrxO4 thin films grown by oxygen-assisted molecular-beam epitaxy within a wide range of composition (0.0 <= x <= 1.2). Spec-troscopic measurements (e.g., X-ray magnetic circular dichroism), refined by theoretical simulations (e.g., crystal field multiplet), are performed to establish a quantitative link between chromium content, Fe2+/Fe3+ site-occupation and macroscopic physical properties of the layers. It is found that Fe3-xCrxO4 thin films (i) delay the transition from inverse to normal spinel configuration with increasing chromium content and (ii) promote collinear spin structure, at odds with bulk material. As a result, strong antiferromagnetic interactions are pre-served between spins in tetrahedral and octahedral spinel sublattices, so that chromium-rich thin films exhibit Curie temperatures above room temperature and higher magnetization. Electron hopping is also favored by this singular cation distribution and electronic band gap is smaller than expected for these thin films. The cation site-occupation is therefore a key feature to consider for applications of Fe3-xCrxO4 thin films in spintronics and photocatalysis, as it enables manipulation of magnetic properties (Curie temperature and magnetization) and band gap engineering.

We predict by density functional theory (DFT) calculations structural modifications specific of the paraelectric/ferroelectric transition and demonstrate that they can be tracked experimentally by means of photoelectron spectroscopy (PES). For BaTiO3, we evidence that the Ba 4d core-level binding energy shifts conveniently allow tracking of the transition for both approaches. DFT calculations reveal different temperature behaviors for bulk and surface features, which can be experimentally followed by PES thanks to specific core-level shifts. Consistently, our PES data show that such modifications occur at a temperature close to the ferroelectric (P4mm) -> paraelectric (Pm3m) phase transition of BaTiO3. We thus demonstrate that a fine PES investigation of the electronic structure can be used to evidence a ferroelectric-to-paraelectric phase transition. These results are believed to be general for ferroelectric surfaces, and it is shown that this approach is applicable to other ferroelectric titanate compounds.

The development of pixel arrays close to the interaction point in large detector systems used in high energy physics require high radiation hardness for the pixels and their readout. A pixel device based on a quantum well, called the DoTPiX uses a sensing n-channel MOS device with a control gate. A buried Ge layer acts as a current modulation gate, which localize holes generated by impinging particles. The DotPIX buried Ge gate is obtained by low temperature epitaxial growth of Ge on Si. We have started to study the different ways to achieve these prerequisites: the need for a low temperature budget to reduce the Ge and Si intermixing, which may be detrimental to the DoTPiX operation. The use of Si thermal oxide is investigated together with that of deposited oxide (Hafnium Oxide for example), which differs from the Silicon Dioxide. The possibility of a combination of thermal silicon dioxide and deposited oxides opens another possibility in this study.

In this paper, synchrotron radiation photoemission techniques are used to unravel the pending nature of the two Ba 4d components, also observed on Ba 3d lines, and commonly named Ba(alpha) and Ba(beta). Investigations were carried out on the (001) surface of epitaxial very thin (<= 20 nm) films combined with DFT calculations. Photoelectron diffraction experiments reveal different behaviors for Ba(alpha) and Ba(beta) components. While six well-defined diffraction features corresponding to the BaTiO3 bulk structure are observed for the Ba(alpha) component, no feature really appears in the diffraction pattern of Ba(beta). This observations reveal the absence of forward scattering processes, hence this component comes mostly from surface BaO plane. Supporting these results, DFT calculations demonstrate a shift to lower binding energy (BE) only for the Ba 4d peak from the topmost BaO layer with respect to the photoelectron peak stemming from bulk layers. Hence, only one peak (the Ba(alpha) one) should be observed for a sample with a complete TiO2 termination. This hypothesis is supported by photoemission measurements carried out at low photon energy in grazing angle emission, in order to enhance the signal from topmost layers with respect to the bulk. For each sample, the Ba(beta)/ Ba(alpha) intensity ratio is well reproduced by a model where the Ba(beta) signal is only coming from the topmost surface BaO plane.

CoFe2O4 and NiFe2O4 are well-known insulating and ferrimagnetic spinel ferrites with high Curie tempera-tures, an important characteristic for electronic and spintronic applications. We used first-principles calculations to investigate how their electronic and magnetic properties can be altered or tuned by the presence of structural point defects. We considered successively the effects of cation distribution in the spinel lattice for stoichiometric compounds and of atom substitutions or vacancies. Our calculations demonstrate that a deviation from the perfectly inverse distribution of cations increases the magnetization and decreases the width of the band gap at the Fermi level. In contrast to cation vacancies, oxygen vacancies are not expected to strongly affect the magnetization. We show that NiFe2O4 crystals with an excess of Ni cations can display a spin-polarized hole conductivity. We finally calculated the formation energy of the different defects and we give details on their gap states.

Tracking detectors are of vital importance for collider-based high energy physics (HEP) experiments. The primary purpose of tracking detectors is the precise reconstruction of charged particle trajectories and the reconstruction of secondary vertices. The performance requirements from the community posed by the future collider experiments require an evolution of tracking systems, necessitating the development of new techniques, materials and technologies in order to fully exploit their physics potential. In this article we summarize the discussions and conclusions of the 2022 Snowmass Instrumentation Frontier subgroup on Solid State and Tracking Detectors (Snowmass IF03).

Epitaxial perovskite self-oxidized oxynitride N : BaTiO3 thin films, deposited on 1% Nb-doped SrTiO3(001) single crystals, were obtained by atomic nitrogen assisted molecular beam epitaxy without supplying additional oxygen gas.

${\mathrm{CoFe}}_{2}{\mathrm{O}}_{4}$ and ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$ are well-known insulating and ferrimagnetic spinel ferrites with high Curie temperatures, an important characteristic for electronic and spintronic applications. We used first-principles calculations to investigate how their electronic and magnetic properties can be altered or tuned by the presence of structural point defects. We considered successively the effects of cation distribution in the spinel lattice for stoichiometric compounds and of atom substitutions or vacancies. Our calculations demonstrate that a deviation from the perfectly inverse distribution of cations increases the magnetization and decreases the width of the band gap at the Fermi level. In contrast to cation vacancies, oxygen vacancies are not expected to strongly affect the magnetization. We show that ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$ crystals with an excess of Ni cations can display a spin-polarized hole conductivity. We finally calculated the formation energy of the different defects and we give details on their gap states.

Nanometric laminar two-dimensional artificial multiferroic oxide thin films can be elaborated using spinel ferrites and perovskite ferroelectrics like CoFe2O4 and BaTiO3. Such materials can retain their individual ferromagnetic or ferroelectric properties. In the thin epitaxial film regime a cross coupling of these properties is possible thanks to strain engineering. After introducing the concepts supporting artificial multiferroic laminar structures, the growth of strained BaTiO3 thin films and the growth of subsequent Co-ferrites layers will be detailed. With respect to the relative film thickness, a detailed understanding of the elastic behavior of these films will be proposed based on the characterization using several synchrotron radiation techniques including x-ray specular and off-specular diffraction, x-ray absorption spectroscopy, as well as x-ray magnetic circular dichroism.

Five contemporary technologies are discussed in the context of their potential roles in particle tracking for future high energy physics applications. These include sensors of the 3D configuration, in both diamond and silicon, submicron-dimension pixels, thin film detectors, and scintillating quantum dots in gallium arsenide. Drivers of the technologies include radiation hardness, excellent position, vertex, and timing resolution, simplified integration, and optimized power, cost, and material.