The increasing need for environment-friendly substitutes for rare-earth-based magnets has sparked interest in materials such as the L10-ordered FeNi (tetrataenite) phase, which possesses high magnetocrystalline anisotropy and saturation magnetization. Despite being a promising candidate, preparation of this ordered phase in the laboratory remains a challenge due to the slow diffusion kinetics that prevent atomic ordering under normal conditions. From the theoretical estimations and experimental results, Cu is known for accelerating the atomic interdiffusion and promoting chemical disorder, which may facilitate the grain boundary diffusion. In the present work, chemically homogeneous multilayers of equiatomic FeNi and Cu-doped FeNi (5 at.%) were studied to investigate the correlation between self-diffusion and magnetism. Nuclear resonance reflectivity and forward scattering measurements on as-deposited and annealed samples showed that Cu doping substantially increases self-diffusion, which is in agreement with significant changes in the local magnetic environment, as supported by conversion electron Mossbauer spectroscopy. Although the net magnetic moment remained nearly unchanged, an enhancement in the coercivity at 573 K was observed in the Cu-doped sample, as quantified by SQUID-VSM. These observations highlight the potential of Cu-assisted diffusion channels to facilitate the formation of ordered phases in FeNi systems as a strategic approach to the development of rare-earth-free permanent magnets.

Here we report results of orientation dependent nuclear inelastic scattering (NIS) experiments on a single crystal of a chemical ferrous iron complex performed at T = 10 K. For that purpose, we have constructed a sample holder which is equipped with a magnetic coupling to accommodate single crystal loops as used in protein crystallography. The sample holder is compatible with the sample rod of a helium cryostat at beamline P01, PETRA III, DESY Hamburg, which allows NIS experiments down to liquid He temperatures and below. For our experiments we chose a 57Fe enriched single crystal of the compound [FeII(b(bdpa))](PF6)2, with (b(bdpa)) = N, N‘-bis(benzyl)-N, N’-bis(2-pyridylmethyl)-6,6’-bis(aminomethyl)-2,2’-bipyridine). Crystals of this complex have unit cells containing two crystallographically independent molecular sites. Our single crystal NIS experiments show that the two irons of the unit cell are in their low spin (S = 0) state at 10 K. This is in contradiction with previous findings (Brady at al. Inorg. Chem. 2004, 43, 14, 4289–4299) which show significant residual high spin (S = 2) contributions up to 25

The advancement of spintronic nanoscale devices hinges on the ability to flexibly engineer magnetic spin structures in thin-film stacks with precision and control. Meeting this demand remains a challenge for stable non-collinear spin configurations and, more specifically, vertical spin spirals in thin films. Innovative methods are required for their fabrication, stabilization and control. Here, we use oblique-incidence deposition to design and stabilize vertical spin spirals at room temperature and without an external field in magnetic thin films. We induce two crossed uniaxial magnetic anisotropies at the thin film boundaries. These anisotropies are tunable in direction and strength, thus providing control over the angular range and depth profile of the resulting spin spiral. The combination of polarized neutron reflectometry and nuclear resonant scattering enables precise and direct determination of the depth-dependent spin configurations. Our results establish a single-film design approach, in which the surface anisotropies independently serve as controllable design parameters for tailoring the vertical spin-spiral profile. Potential applications include nanoscale energy-storage devices, magnetic sensors, and ferromagnetic-resonance filters, advancing all-spin-based device engineering in general.

Nanostructuring offers unique opportunities to manipulate vibrational properties of materials through reduced dimensionality. This is widely exploited in materials science to engineer thermal conductivity in nanostructures, for example. Similarly, in quantum optics, nanoscale engineering of vibrational states can suppress thermal decoherence and enhance coherent light-matter interactions. In x-ray quantum optics, vibrational properties are especially critical because light-matter interaction often occurs via coherent excitations of ultrasharp nuclear resonances, requiring a recoilless interaction, i.e., no energy exchange between nuclei and lattice. This raises the question: can nanoscale engineering also enhance coherent light-matter interactions in x-ray quantum optics, and thereby enable the use of nuclear resonances with typically low recoilless interaction fractions, such as the resonance of . Here, we demonstrate the tunability of the vibrational properties of nanometer-thin Sn films by embedding them in multilayer structures, achieving nearly an order-of-magnitude increase in the recoilless fraction. Using nuclear inelastic x-ray scattering, we studied vibrational density of states of Sn layers with respect to interlayer diffusion, structural disorder and interfacial intermetallic compound formation. Our results show that suitable embedding materials can substantially modify Sn's vibrational behavior and thus potentially enable the nuclear resonance for x-ray quantum optics applications.

The use of oxygen and nitrate as terminal electron acceptors provides organisms with a huge amount of available energy but necessitates methods to detoxify reactive intermediates. The mechanisms of NO and O2 detoxification in many organisms involve flavodiiron proteins (FDPs). Although the proteinaceous ligands that coordinate the diiron active site of these enzymes are well established, its exact coordination environment remains under debate due to conflicting interpretations of crystallographic and spectroscopic/theoretical studies. Using 57Fe nuclear resonance vibrational spectroscopy (NRVS), complemented by Mössbauer spectroscopy and density functional theory, we elucidated the redox-linked structural changes in the FDP from Escherichia coli. The as-isolated diferric state is best described as a dihydroxo-bridged Fe(III)-(μOH-)2-Fe(III) core, which upon reduction converts to a monohydroxo Fe(II)-(μOH-)-Fe(II) center through the loss of one bridging ligand. This ligand rearrangement defines the structural basis for redox-linked reactivity in FDPs. The study further demonstrates that photoreduction of a stable metalloprotein species can occur under NRVS conditions, indicating that synchrotron-based vibrational measurements may induce subtle redox changes even under low photon flux. These findings provide a mechanistic framework for interpreting redox-linked ligand dynamics in diiron enzymes and highlight the need to collect damage-free X-ray crystal structures avoiding potential beam-induced reduction. Furthermore, diiron active sites are found in numerous other enzyme classes (e.g., methane monooxygenase), and therefore, our findings have implications way beyond the FDPs.

Fe(II) containing spin crossover (SCO) complexes can be reversibly switched between a low-spin (S = 0) and a high-spin (S = 2) state. Switching between the two spin states can be performed e.g. via change of temperature. In some SCO complexes, it is also possible to trigger a light induced excited spin state trapping (LIESST) effect by irradiating the SCO material with light. Here, we report on the vibrational properties of the LIESST state of the SCO complex Fe(PM-BiA)2(NCS)2 investigated via nuclear inelastic scattering (NIS). For our NIS experiments, we used a liquid Helium cryostat at beamline P01, PETRA III, DESY Hamburg, dedicated to NIS experiments down to LHe temperature. Using a modified sample rod which allows illumination of samples within the synchrotron beam down to LHe temperatures we could stabilize the LIESST state of Fe(PM-BiA)2(NCS)2 for nearly 100

Kagome FeGe hosts a 2×2×2 charge-density wave (CDW) that strongly interplays with antiferromagnetic order. Here, we report ^57Fe nuclear resonant scattering measurement to study FeGe across its long-range CDW and incommensurate magnetic transitions. Upon entering the CDW state, hardening of acoustic phonons and optical phonons around 22 meV, 27 meV, and 31 meV are observed in the Fe partial phonon density of states, which can be qualitatively captured by first-principle calculations. Upon entering the incommensurate magnetic phase, neither the phonon density of states nor the hyperfine interaction parameters change significantly, although a subtle feature associated with the incommensurate magnetic order or slow fluctuations is detected in the time-domain Mössbauer spectra. These findings show that the CDW in kagome FeGe significantly modifies its lattice dynamics and magnetism, evidencing an intertwined nature of the spin, charge, and lattice degrees of freedom.

Waveguides offer a means to controllably couple atomic ensembles to the electromagnetic field therein. Here, we demonstrate x-ray propagation in planar thin-film waveguides coupled to Mössbauer nuclei under collective resonant excitation by short pulses of synchrotron radiation. We record x-ray photons that have been emitted into resonant modes of the waveguide. Depending on the geometry and mode of excitation, two fundamentally different signatures of the collective emission are observed, for which we present a unifying theoretical model. Our results form a new platform for waveguide quantum electrodynamics in the hard x-ray regime with the potential to provide a coherent narrowband source of x-rays on the nanometer scale.

Mössbauer spectroscopy is widely used to study structure and dynamics of matter with remarkably high energy resolution, provided by the narrow nuclear resonance line widths. However, the narrow width implies low count rates, such that experiments commonly average over extended measurement times or many x-ray pulses (“shots”). This averaging impedes the study of non-equilibrium phenomena. It has been suggested that X-ray free-electron lasers (XFELs) could enable Mössbauer single-shot measurements without averaging, and a proof-of-principle demonstration has been reported. However, so far, only a tiny fraction of all shots resulted in signal-photon numbers which are sufficiently high for a single-shot analysis. Here, we demonstrate coherent nuclear-forward-scattering of self-seeded XFEL radiation, with up to 900 signal-photons per shot. We develop a sorting approach which allows us to include all data on a single-shot level, independent of the signal content of the individual shots. It utilizes the presence of different dynamics classes, i.e. different nuclear evolutions after each excitation. Each shot is assigned to one of the classes, which can then be analyzed separately. Our approach determines the classes from the data without requiring theory modeling nor prior knowledge on the dynamics, making it also applicable to unknown phenomena. We envision that our approach opens up new grounds for Mössbauer science, enabling the study of out-of-equilibrium transient dynamics of the nuclei or their environment.

Iron-sulfur clusters fulfill numerous roles throughout biology. The reduced [2Fe-2S] + cluster offers unique electronic and magnetic properties due to its mixed-valent nature and can serve as an essential model for understanding electron transfer, electron delocalization, and accessible spin states not only in mixed-valent dimers, but potentially larger iron sulfur clusters. Recently a series of mixed-valent diiron dichalcogenide complexes [L 2 Fe 2 Q 2 ] − (Q = S ( 1 ), Se ( 2 ), Te ( 3 ), L = 2,6-diisopropylphenyl β-diketiminate ligand) were synthesized and characterized, where complex 1 showed a typical S = 1/2 spin state, while complexes 2 and 3 exhibited intermediate S = 3/2 spin states, potentially enabled by the minimization of vibronic coupling. Here we studied the vibrational dynamics of the Fe and Te centers in these complexes using 57 Fe and 125 Te nuclear resonance vibrational spectroscopy (NRVS), coupled with DFT calculations. The findings suggest that heavy character of larger chalcogen atoms results in decreased vibronic coupling. The observation of an intermediate spin state is shown to be unattainable for lighter Fe 2 Q 2 cores. This highlights the crucial role of vibronic coupling in modulating the electronic structure of mixed-valence systems and should enhance understanding of the electronic structure in more complex biological Fe-S clusters.

Interference is a powerful tool for measuring and control. In Mössbauer science, interference effects are essential to most applications, due to the coherent scattering nature. However, Mössbauer interferometry remains challenging, due to stability requirements imposed by the short x-ray wavelength. Here, we put forward a “dark fringe” interferometer with vanishing transmission in the empty state, thereby facilitating sensitive measurements. The relative interferometer phase can dynamically be tuned by displacing a Mössbauer target. We experimentally demonstrate the tuning capabilities of this interferometer by controlling the transmitted x-ray intensity on nanosecond time scales. Then, we demonstrate sensitive measurements by observing the propagation of impulsively launched sound waves in the target over ∼ 10 μs. The interferometer concept opens avenues towards polarization-sensitive phase measurements, the generation of coherent multi-pulse sequences for controlling nuclear dynamics, and the implementation of feedback loops to adaptively optimize the interferometer, thereby fueling the further development of nuclear quantum optics.

Understanding the interfaces of layered nanostructures is key to optimizing their structural and magnetic properties for the desired functionality. In the present work, the two interfaces of a few nm thick Fe layer in Ag-57Fe-Ag trilayers are studied with a depth resolution of a fraction of a nanometer using x-ray standing waves (XSWs) generated by an underlying [W-Si]x10 multilayer (MLT) at an x-ray incident angle around the Bragg peak of the MLT. Interface selectivity in Ag-57Fe-Ag trilayers was achieved by moving XSW antinodes across the interfaces by optimizing suitable incident angles and performing depth-resolved nuclear resonance scattering (NRS) and X-ray fluorescence (XRF) measurements for magnetic and structural properties. The combined analysis revealed that the rms roughness of 57Fe-on-Ag and Ag-on-57Fe interfaces are not equal. The roughness of the 57Fe-on-Ag interface is 10 Angstrom, while that of the Ag-on-57Fe interface is 6 Angstrom. 57Fe isotope sensitive NRS revealed that hyperfine field (HFF) at both interfaces of 57Fe-on-Ag and Ag-on-57Fe interfaces are distinct, which is consistent with the difference in interface roughnesses measured as root mean square (RMS) roughness. Thermal annealing induces 57Fe diffusion into the Ag layer, and annealing at 325 C transforms the sample into a paramagnetic state. This behavior is attributed to forming 57Fe nanoparticles within the Ag matrix, exhibiting a paramagnetic nature. These findings provide deep insights into interface properties crucial for developing advanced nanostructures and spintronic devices.

CoSb3 belongs to the skutterudite family of compounds and serves as a crucial platform for the exploration of thermoelectric materials, however, its importance is equally high for studies of strong correlations at high pressures. Under compression it undergoes a 'self-insertion' isostructural transition resulting in a peculiar redistribution of large Sb atoms between different crystallographic sites. We conducted a comprehensive investigation of the structural phase stability of CoSb3 up to 70 GPa using single crystal samples characterized employing conventional single crystal X-ray diffraction and X-ray scattering focused on measuring Bragg peak at high resolution (including elements of Bragg Coherent Diffraction Imaging). We explore the compression behavior of CoSb3 in three different pressure transmitting media (PTM) and address several important, but previously unexplored topics: the influence of various PTMs and nonhydrostatic stresses on the strongly correlated system of CoSb3, including the 'self-insertion' crossover, the phase stability of CoSb3, the compound's polymorphism, its crystal chemistry, and its peculiar evolution under pressure at ambient temperature. Among other important observations, we track the population of Sb atoms within the dodecahedral sites of CoSb3 on compression, during the process of 'self-insertion', and on decompression. We detect that 'self-insertion' may not only reduce the solid's compressibility, but also make it negative. Finally, but not least, we report that the 'self- insertion' crossover is an important step preceding a previously unknown phase transformation from cubic Im3 CoSb3 into trigonal R3 occurring above 40 GPa, and discuss the distinctive behavior of CoSb3 phases and their structural frameworks.

The ^45Sc nuclear transition from the ground to the isomeric state at 12.389 keV, with a lifetime of 0.46 s, exhibits an extraordinarily narrow natural width of 1.4 feV and a quality factor ≃ 10^19 – surpassing those of the most precise atomic clocks – making ^45Sc a compelling platform for advanced metrology and nuclear clocks. Here we investigate how closely the spectral width and quality factor of the solid-state ^45Sc resonance can approach these natural limits. Using the European X-ray Free-Electron Laser, we confirm the isomer's lifetime via time-delayed incoherent K_α,β fluorescence and observe previously unreported elastic fluorescence, yielding a partial internal conversion coefficient of 390(60). The absence of a clear nuclear forward scattering signal beyond a 2-ms delay implies environmental broadening of at least 500 Γ_0 under experimental conditions, placing bounds on solid-state decoherence mechanisms. These findings set new experimental benchmarks for solid-state nuclear clock development.

Phase retrieval is at the heart of adaptive optics and modern high-resolution imaging. Without phase information, optical systems are limited to intensity-only measurements, hindering full reconstruction of object structures and wavefront dynamics essential for advanced applications. Here, we address a one-dimensional phase problem linking energy and time, which arises in X-ray scattering from ultrasharp nuclear resonances. We leverage the M & ouml;ssbauer effect, where nuclei scatter radiation without energy loss to the lattice and are sensitive to their magneto-chemical environments. Rather than using traditional spectroscopy with radioactive gamma-ray sources, we measure nuclear forward scattering of synchrotron X-ray pulses in the time domain, providing superior sensitivity and faster data acquisition. Extracting spectral information from a single measurement is challenging due to the missing phase information, typically requiring extensive modeling. Instead, we use multiple energetically overlapping measurements to retrieve both the transmission spectrum and the phase of the scattering response, similar to ptychographic phase retrieval in imaging. Our robust approach can overcome the bandwidth limitations of gamma-ray sources, opening new research directions, to the best of our knowledge, with modern X-ray sources and M & ouml;ssbauer isotopes.

TEMPUS is a new detector system being developed for photon science. It is based on the Timepix4 chip and, thus, it can be operated in two distinct modes: a photon-counting mode, which allows for conventional full-frame readout at rates up to 40 kfps; and an event-driven time-stamping mode, which allows excellent time resolution in the nanosecond regime in measurements with moderate X-ray flux. In this paper, the initial prototype, a single-chip device, is introduced, and the readout system described. Moreover, and in order to evaluate its capabilities, some tests were performed at PETRA III and ESRF for which results are also presented.

Organic spintronics has emerged as a promising field for exploring novel spin-based phenomena and devices, offering the potential for low-power, flexible, and biocompatible electronics. The interface between metallic ferromagnetic and semiconducting organic layers plays a pivotal role in spin injection, transport, and extraction processes in these devices. Therefore, achieving a comprehensive understanding of the magnetic properties at these interfaces is essential for advancing device performance and functionality. This work explores the magnetic properties at the interface between thin Fe film and the C60 layer. We employ a multi-technique approach, combining the magneto-optic Kerr effect, which provides a global assessment of magnetic properties, and depth-resolved grazing incidence nuclear resonance scattering (GINRS) under X-ray standing wave conditions, enabling us to probe magnetism with high spatial resolution within the interfacial region. GINRS measurements reveal intriguing behavior at the interface, characterized by reduced hyperfine fields in diffused 57Fe layers. This observation suggests the formation of superparamagnetic clusters, which significantly influence the magnetic properties at the interface. These findings provide valuable insights into the complex interplay between ferromagnetic materials and organic semiconductors at the nanoscale, offering potential avenues for tailoring magnetoresistance effects in organic spintronic devices and contributing to the fundamental understanding of spin-dependent phenomena in organic spintronics.

The present work reports the ferroelectric (FE) element/site specific spectroscopic measurements in BaTi0.7 119Sn0.3 O3 relaxor using 119Sn hyperfine interaction studies. The relaxor properties in the studied sample are confirmed from the macroscopic measurements such as temperature dependent dielectric constant measurements. The presence of quadrupole splitting (QS) below its dielectric maxima (Tm) is confirmed unambiguously using synchrotron radiation perturbed angular correlation experiments. Further, temperature dependent nuclear forward scattering data clearly depict the development of QS below Tm. Finite values of QS with temperature, below Tm, unambiguously indicate the development of electric field gradient around Sn/Ti sites in BaTi0.7 119Sn0.3 O3 relaxor, emphasizing the applicability of conventional FE models in explaining relaxor properties. The phonon density of states (PDOS) obtained from nuclear inelastic scattering measurements has been found to qualitatively match the results of PDOS calculated from molecular dynamics simulations and experimental Raman modes that are considered to be characteristic of Ti site in BaTiO3.