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Engineering hard ferrite composites by combining nanostructuring and Substitution: From nano to dense bulk magnets
No full textWe have investigated the bottom-up sol-gel synthesis of nanocomposite powders comprising two magnetic phases (hexagonal Sr ferrite and spinel Co ferrite) in order to outline a strategy to obtain permanent magnets with large coercivities via low-cost and scalable syntheses. The correlation between morphological, structural and macroscopic magnetic properties of Al-substituted SrFe12O19 and SrFe12O19/CoFe2O4 nanocomposites was analyzed in detail. The hysteretic behavior can be tuned by cation substitution and/or modulation of the super-exchange coupling at the interface of the constituting phases. The magnetic data, supported by Monte Carlo simulations, indicates enhanced magnetic coupling within the composite: this observation underscores the significance of soft crystallite size and epitaxial growth quality at the interface as key factors influencing super-exchange coupling strength, ranging from fully coupled to essentially decoupled composites. Bulk magnets with high density were manufactured by compacting these nanostructured phases using spark plasma sintering, without an applied magnetic field. Consolidation of powders significantly impacted magnetic properties, by increasing remanent magnetization and decreasing coercivity due to enhanced super-exchange coupling. The presence of two phases hindered reciprocal growth, influencing coercivity differently in various compositions. Overall, the compaction enhanced magnet performance through improved particle alignment and super-exchange coupling, offering the potential for optimized magnet design
Bounding elastic photon-photon scattering at √s ≈ 1 MeV using a laser-plasma platform
No full textWe report on a direct search for elastic photon-photon scattering using x-ray and photons from a laser-plasma based experiment. A photon beam produced by a laser wakfield accelerator provided a broadband spectrum extending to above = 200 MeV. These were collided with a dense x-ray field produced by the emission from a laser heated germanium foil at ≈ 1.4 keV, corresponding to an invariant mass of √ = 1.22 ± 0.22 MeV. In these asymmetric collisions elastic scattering removes one x-ray and one high-energy photon and outputs two lower energy photons. No changes in the photon spectrum were observed as a result of the collisions allowing us to place a 95% upper bound on the cross section of 1.5 × 1015 μb. Although far from the QED prediction, this represents the lowest upper limit obtained so far for √ ≲ 1 MeV
The Forbush decrease observed by the SEVAN particle detector network in the 25th solar activity cycle
No full textThe temporal variations of cosmic-ray intensity, measured by ground-based detectors at various latitudes, longitudes, and altitudes, are related to the geophysical and solar phenomena. The latter are interplanetary coronalmass ejections and fast solar wind from coronal holes, which cause interplanetary magnetic field (IMF) abruptvariations near Earth. Interacting with the magnetosphere, they cause worldwide sudden decreases (Forbushdecreases, FDs) of intensity followed by gradual recovery. The amplitude of the flux depletion depends on thetype and energy of the registered particle, which in turn depends on geographical coordinates and the detector’senergy threshold and selective power. SEVAN particle detector network with nodes in Europe and Armeniaselects three types of particles that demonstrate coherent depletion and recovery and correspond to differentenergy galactic protons interacting with disturbed magnetospheric plasmas.On November 3–4, 2021, an interplanetary coronal mass injection (ICME) hit the magnetosphere, sparking astrong G3-class geomagnetic storm and auroras as far south as California and New Mexico. All detectors of theSEVAN network have registered an (FD) of ≈5% depletion in a 1-min time series of count rates. Approaching themaximum solar activity cycle, large variations of the particle flux intensity were registered on February 27,March 23, 2023, and March 24, 2024.In this work, we present measurements of these FDs performed on mountain altitudes on Aragats (Armenia),Lomnicky Stit (Slovakia), Mileshovka (Czechia), and at sea level DESY (Hamburg, Germany). We compared FDmeasurements made by SEVAN detectors and neutron monitors located on Aragats and Lomnicky Stit and made acorrelation analysis of FD registration at different locations
Elpasolite-type superstructures in inverse perovskite nitrides
No full textWe present a range of inverse perovskite nitrides with an elpasolite-type superstructure. (CaN)Sn and (CaN)Pb are variants of the previously described (CaN)Sn and (CaN)Pb which contain less nitrogen and crystallize in . (BaN)Sn and (BaN)Pb resemble the previously reported perovskites (BaN)Sn and (BaN)Pb, but with both the superstructure and octahedral tilting, resulting in space group . (CaN)Si, (CaN)Ge, (SrN)Ge and (BaN)Ge all crystallize in P2/. Among these, only (CaN)Ge has been previously described as (CaN)Ge. (CaN)Si is notably the first compound in which mutually isolated N and Si ions coexist. There also exists a version with composition (CaN)Si, which crystallizes in the cubic perovskite aristotype structure with space group . Similarly, there are versions of (SrN)Ge, (BaN)Sn and (BaN)Pb with elevated nitrogen contents, less strongly tilted octahedra and no apparent superstructure. Electronic structure calculations indicate a metallic nature of the title compounds, with rather narrow improper band gaps for the strontium and barium compounds
Gas tungsten arc welding of CoCrFeMnNi high entropy alloy to 316 stainless steel
No full textIn this study, an as-annealed CoCrFeMnNi high entropy alloy (HEA) was successfully gas tungsten arc-welded(GTA-welded) to 316 stainless steel, resulting in a fully penetrated joint and free of visible macroscopic de-fects. In the heat-affected zone (HAZ) on the 316 stainless steel side, a phase transformation from δ-BCC to σphase was detected. On the CoCrFeMnNi side, the base material (BM) primarily exhibited an FCC matrix phase,with a slight increase in Cr-Mn oxide content upon entering the HAZ. The formation of the BCC phase in thefusion zone (FZ) was mainly attributed to the mixing of Fe from the 316 stainless steel. This region exhibited thehighest hardness, resulting from a combination of factors, including the BCC precipitates and minor incorpo-ration of carbon introduced within the FZ, further aided by solid solution strengthening due to BMs mixing, butsolid solution strengthening is the key factor. The BMs and the adjacent HAZs displayed highly similar equiaxedgrain structure and size, and the hardness distributions, making it difficult to distinguish each region. However,the evolution of interplanar spacing obtained through synchrotron X-ray radiation effectively differentiated thevarious regions of the joint. Tensile testing revealed that the strength and hardness of this dissimilar joint werelower than those of both BMs, yet it still shows potential for structural applications. The fracture site of the jointoccurred in the HAZ on the 316 stainless steel side, primarily due to the combined effects of localized largeplastic deformation in the soft region and stress concentration caused by the presence of σ phas
Influence of Severe Plastic Deformation on the Magnetic Properties of Sm–Co Permanent Magnets
No full textHigh pressure torsion (HPT) is presented as a new fabrication route to produce bulk Sm–Co magnets with a strongly refined microstructure down to the nanometer regime. The initial powders, based on the compositions SmCo5, Sm2Co7 and Sm2Co17, are compacted and subsequently deformed by HPT. The microstructural evolution in dependence on the applied deformation parameters is characterized by electron microscopy and the effect of HPT on the phase stability is monitored by synchrotron X-ray diffraction. An increasing amount of applied strain leads to a stronger reduction in grain size while strain localization counteracts a homogeneous microstructural refinement. The positive effect of elevated deformation temperatures is demonstrated for Sm2Co17, which promotes homogeneous grain refinement, but causes strain-induced phase transformations at the same time, strongly affecting the magnetic behavior. Superconducting quantum interference device magnetometry is used to characterize the magnetic properties after HPT deformation, which indicates the formation of a magnetic texture depending on the respective phase
Berührungslose Lichtkontrolle in Luft
No full textMithilfe hochintensiver Ultraschallwellen, die sich in gasförmigenMedien ausbreiten, ist es gelungen, ultrakurze Laserpulse mit einerbisher unerreichten Spitzenleistung von zwanzig Gigawatt in Luft abzulenken und dabei gleichzeitig eine herausragende Strahlqualität beizubehalten. Das könnte in Zukunft neue, äußerst robuste optische Bauteile wie beispielsweise Linsen oder Wellenleiter in bislang unerschlossenen spektralen Bereichen ermöglichen
NiFe‐NO Layered Double Hydroxide as a Novel Anode for Sodium Ion Batteries
No full text2D materials are emerging materials for energy storage and among these layered double hydroxides (LDHs) seem particularly promising due to their structure, easily adjustable composition, and cheapness. This study marks the first reported application of an LDH, specifically NiFe-NO3 LDH, as conversion anode material in a sodium half-cell, to the best of our knowledge. Despite an initial loss in capacity, the material demonstrates notable stability, retains a high specific capacity even after 50 discharge/charge cycles (~500 mAh/g). The intricate reaction mechanism was explored using various ex-situ techniques such as DC magnetometry and FTIR, as well as in-operando X-ray Absorption Spectroscopy (XAS). The proposed Na-storage mechanism in NiFe-NO3 LDH involves an initial irreversible “activation” during the first sodiation, characterized by a phase change reaction that leads to the formation of NiOx and Fe3O4, followed by a reversible mechanism involving both intercalation and conversion in subsequent cycles
Marginal Role of the Electrostatic Instability in the GeV-scale Cascade Flux from 1ES 0229+200
No full textRelativistic pair beams produced in the intergalactic medium by TeV gamma rays from blazars are expected to generate a detectable GeV-scale electromagnetic cascade, yet this cascade is absent in the observed spectra of hard-spectrum TeV emitting blazars. This suppression is often attributed to weak intergalactic magnetic fields (IGMF) deflecting electron-positron pairs out of the line of sight. Alternatively, it has been proposed that beam-plasma instabilities could drain the energy of the beam before they produce the secondary cascades. Recent studies suggest that the modification of beam distribution due to these instabilities is primarily driven by particle scattering, rather than energy loss. In this paper, we quantitatively assess, for the blazar 1ES 0229+200, the arrival time of secondary gamma rays at Earth from the beam scattering by the electrostatic instability. We first computed the production rates of electron-positron pairs at various distances using the Monte Carlo simulation CRPropa. We then simulated the feedback of the plasma instability on the beam, incorporating production rates and inverse Compton cooling, to determine the steady-state distribution function. Our findings reveal that the time delay of the GeV secondary cascade arrival due to instability broadening is on the order of a few months. This delay is insufficient to account for the missing cascade emission in blazar spectra, suggesting that plasma instabilities do not significantly affect IGMF constraints
Iodine K- and L-edge X-ray absorption spectra of HI: The effect of molecular orbitals and core subshells
No full textAnalysis of the recently measured absorption spectra of molecular HI at K and L edges of iodine, in parallel withpreviously measured spectra of noble gas Xe and the K edge spectrum of atomic I, is presented. A strongdependence of some valence multielectron photoexcitation features on the orbital momentum of the core vacancyis found, attributed to the change of the symmetry of the HI molecule: the shake-up coexcitation of avalence electron to a free molecular orbital is much stronger at L3 than L1 edge. The effect of angular momentumof the core hole on the shake processes of deeper multielectron photoexcitations is found negligible. Both HI andXe exhibit a much weaker one-electron transition [1s]6p than monatomic I. At the K edge, the strength ofcoexcitations of 4d, 4p and 3d subshells in atomic I is close to the HI and Xe. The same is found for HI and Xe atthe L edges, due to a weak contribution of the additional free molecular orbital in HI