arXiv subjects

Ilya Sergeev

Publications and source records attributed to Ilya Sergeev.

15 recordsLinked to original sources

arXiv2026

Customized spin spirals in ferromagnetic thin films

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.

cond-mat.mtrl-sci↗

arXiv2026

Phonons and magnetism of kagome FeGe probed by nuclear resonant scattering

Kagome FeGe hosts a $2\times2\times2$ charge-density wave (CDW) that strongly interplays with antiferromagnetic order. Here, we report $^{57}$Fe 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 M\"{o}ssbauer 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.

cond-mat.str-el↗

arXiv2026

Single-shot sorting of Mössbauer time-domain data at X-ray free-electron lasers

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.

quant-ph↗

arXiv2025

Dark-fringe interferometer with dynamic phase control for Mössbauer science

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 $\sim 10\,μs$. 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.

quant-ph↗

arXiv2025

Probing the Linewidth of the 12.4-keV Solid-State $^{45}$Sc Isomeric Resonance

The $^{45}$Sc 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 $\simeq 10^{19}$ -- surpassing those of the most precise atomic clocks -- making $^{45}$Sc a compelling platform for advanced metrology and nuclear clocks. Here we investigate how closely the spectral width and quality factor of the solid-state $^{45}$Sc 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.

quant-ph↗

arXiv2025

Energy Time Ptychography for one-dimensional phase retrieval

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 Mössbauer 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 with modern X-ray sources and Mössbauer isotopes.

physics.optics↗

arXiv2025

Collective nuclear excitation and pulse propagation in single-mode x-ray waveguides

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.

quant-ph↗

arXiv2025

Investigation of Fe-Ag and Ag-Fe Interfaces in Ag-57Fe-Ag trilayer Using Nuclear Resonance Scattering under X-ray Standing Wave Conditions

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.

cond-mat.mtrl-sci↗

arXiv2024

Buried Interfaces and Spin Orientation in [Co/Pt]10 /Fe multilayer with Orthogonal Magnetic Anisotropy: Effect of Fe Thickness

In the present work, spin orientation and variation of the strength of coupling in [Co/Pt]ML/Fe multilayer have been investigated as a function of the thickness of the Fe layer. [Co/Pt]ML/Fe multilayer has orthogonal anisotropy with the Fe layer, and [Co/Pt]ML has in-plane magnetic anisotropy and perpendicular anisotropy, respectively. Measurements are performed using in-situ magneto-optical Kerr effect (MOKE) and isotope-sensitive depth-resolved nuclear resonance scattering (NRS) technique. Real-time in-situ MOKE measurement during Fe growth reveals that with an increase in thickness of the Fe layer, moments of Fe layer reorientation from out of the plane to in-plane direction. This is attributed to the decrease in the coupling between the [Co/Pt]ML and Fe layer. For the depth-dependent study, two [Co/Pt]ML/Fe multilayers having the same thickness but different positions of the Fe57 marker layer ([Co/Pt]MLFe/Fe57) and [Co/Pt]MLFe57/Fe ) were studied using the NRS technique. Films with varying external magnetic fields were also studied to investigate coupling strength. Measurements were performed under the x-ray standing wave conditions to enhance resonance yield. It is observed that for the 75 Å Fe in [Co/Pt]ML/Fe multilayer, the coupling varies along the depth of the Fe layer. The coupling is strong at [Co/Pt]ML and Fe interface with spins of the Fe layer aligned in the out-of-plane direction, whereas moments away from the interface are weakly coupled and aligned in-plane along the magnetic easy axis. Due to this gradient in strength of coupling along the depth, a large magnetic field is required to reorient spins at the interface along the magnetic hard axis of the Fe layer; however, spins away from the interface can rotate freely even in the low magnetic field.

cond-mat.mtrl-sci↗

arXiv2024

Resolving the pressure induced 'self-insertion' in skutterudite CoSb3

CoSb3, a skutterudite compound, is key in studying thermoelectric materials. Under compression, it undergoes a 'self-insertion' isostructural transition, redistributing large Sb atoms among crystallographic sites. We investigated CoSb3's structural stability up to 70 GPa using single crystal X-ray diffraction and high-resolution X-ray scattering, including Bragg Coherent Diffraction Imaging. We examined the material in three pressure transmitting media (PTMs), exploring how PTMs and nonhydrostatic stresses affect CoSb3. Notably, the 'self-insertion' transition may reduce or even make compressibility negative. Additionally, we report a previously unknown phase transformation from cubic Im-3 to trigonal R-3 above 40 GPa and discuss the phases' distinctive behaviors.

cond-mat.mtrl-sci↗

arXiv2024

TEMPUS, a Timepix4-based system for event-based X-rays detection

A readout system based on the Timepix4 ASIC, is being developed for photon science. The TEMPUS detector 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, we introduce the initial prototype, a single-chip system, and present the first results obtained at PETRA III and ESRF.

physics.ins-det↗

arXiv2023

Study of magnetism in MgO/FeCoB/MgO trilayers using x-ray standing wave techniques

Interfaces in the MgO-FeCoB-MgO trilayer have been studied with grazing incident nuclear resonance scattering (GINRS) using the x-ray standing waves (XSW) technique. High depth selectivity of the present method allows one to measure magnetism and structure at the two interfaces of FeCoB, namely, FeCoB-on-MgO and MgO-on-FeCoB, independently, yielding an intriguing result that both interfaces are not symmetric. A high-density layer with an increased magnetic hyperfine field at the FeCoB-on-MgO interface suggests different growth mechanisms at the two interfaces. The azimuthal angle-dependent magneto-optic Kerr effect measurements reveal the presence of unusual uniaxial magnetic anisotropy (UMA) in the trilayer. An in-situ temperature-dependent study discovered that this UMA systematically reduces with temperature. The trilayer becomes isotropic at 450C with an order-of-magnitude increase in coercivity. The asymmetry at the interfaces is, in turn, explained by boron diffusion from the FeCoB interface layer into the nearby MgO layer. Stress-induced UMA is observed in the boron-deficient FeCoB layer, superimposed with the bulk FeCoB layer, and found to be responsible for unusual UMA. The temperature-dependent variation in the UMA and coercivity can be understood in terms of variations in the internal stresses and coupling between FeCoB bulk and the interface layer.

cond-mat.mtrl-sci↗

arXiv2022

Origin of exchange bias in [Co/Pt]ML/Fe multilayer with orthogonal magnetic anisotropies

Magnetization reversal of soft ferromagnetic Fe layer, coupled to [Co/Pt]ML multilayer [ML] with perpendicular magnetic anisotropy (PMA), has been studied in-situ with an aim to understand the origin of exchange bias (EB) in orthogonal magnetic anisotropic systems. The interface remanant state of the ML is modified by magnetic field annealing, and the effect of the same on the soft Fe layer is monitored using the in-situ magneto-optical Kerr effect (MOKE). A considerable shift in the Fe layer hysteresis loop from the centre and an unusual increase in the coercivity, similar to exchange bias phenomena, is attributed to the exchange coupling at the [Co/Pt]ML and Fe interface. The effect of the coupling on spin orientation at the interface is further explored precisely by performing an isotope selective grazing incident nuclear resonance scattering (GINRS) technique. Here, the interface selectivity is achieved by introducing a 2 nm thick Fe57 marker between [Co/Pt]ML and Fe layers. Interface sensitivity is further enhanced by performing measurements under the x-ray standing wave conditions. The combined MOKE and GINRS analysis revealed the unidirectional pinning of the Fe layer due to the net in-plane magnetic spin at the interface caused by magnetic field annealing. Unidirectional exchange coupling or pinning at the interface, which may be due to the formation of asymmetrical closure domains, is found responsible for the origin of EB with an unusual increase in coercivity.

cond-mat.mtrl-sci↗

arXiv2022

Magnetism in four-layered Aurivillius Bi$_5$FeTi$_3$O$_{15}$ at high pressures : A nuclear forward scattering study

We report the structural and magnetic properties of four-layer Aurivillius compound Bi$_5$FeTi$_3$O$_{15}$ (BFTO) at high hydrostatic pressure conditions. The high-pressure XRD data does not explicitly show structural phase transitions with hydrostatic pressure, however the observed changes in lattice parameters indicate structural modifications at different pressure values. In the initial pressure region values, the lattice parameters $\textit{a}$- and $\textit{b}$- are nearly equal implying a quasi-tetragonal structure, however as the pressure increases $\textit{a}$- and $\textit{b}$- diverges apart and exhibits complete orthorhombic phase at pressure values of about $\geq$8 GPa. Principal component analysis of high pressure Raman measurements point out an evident change in the local structure at about 5.5 GPa indicating that the evolution of the local structure under applied pressure seems to not follow crystallographic changes (long range order). Nuclear forward scattering (NFS) measurement reveal the development of magnetic ordering in BFTO at 5K with high pressures. A progressive increase in magnetic order is observed with increase in pressure at 5K. Further, NFS measurements carried out at constant pressure (6.4GPa) and different temperatures indicate that the developed magnetism disappears at higher temperatures (20K). It is attempted to explain these observations in terms of the observed structural parameter variation with pressure.

cond-mat.mtrl-sci↗

arXiv2016

A medium-resolution monochromator for 73 keV x-rays - Nuclear resonant scattering of synchrotron radiation from 193-Ir

We report on the development and characterization of a medium resolution monochromator for synchrotron-based hyperfine spectroscopy at the 73 keV nuclear resonance of 193-Ir. The device provides high throughput of 6*10^8 ph/s in an energy bandwidth of 300(20) meV. We excited the nuclear resonance in 193-Ir at 73.04 keV and observed nuclear fluorescence of 193-Ir in Iridium metal. The monochromator allows for Nuclear Forward Scattering spectroscopy on Ir and its compounds.

physics.ins-det↗