Search

We studied the structural and magnetic properties of ${\mathrm{Fe}}_{0.8}{\mathrm{C}}_{0.2}$ thin films deposited by co-sputtering of Fe and C targets in a direct current magnetron sputtering (dcMS) process at a substrate temperature (${T}_{s}$) of 300, 523, and 773 K. The structure and morphology were measured using x-ray diffraction (XRD), x-ray absorption near-edge spectroscopy (XANES) at Fe $L$ and C $K$ edges and atomic/magnetic force microscopy (AFM, MFM). An ultrathin (3-nm) $^{57}\mathrm{Fe}_{0.8}\mathrm{C}{}_{0.2}$ layer, placed between relatively thick ${\mathrm{Fe}}_{0.8}{\mathrm{C}}_{0.2}$ layers was used to estimate Fe self-diffusion taking place during growth at different ${T}_{s}$ using depth profiling measurements. Such $^{57}\mathrm{Fe}_{0.8}\mathrm{C}{}_{0.2}$ layer was also used for $^{57}\mathrm{Fe}$ conversion electron M\"ossbauer spectroscopy (CEMS) and nuclear resonance scattering (NRS) measurements, yielding the magnetic structure of this ultrathin layer. We found from XRD measurements that the structure formed at low ${T}_{s}$ (300 K) is analogous to Fe-based amorphous alloy and at high ${T}_{s}$ (773 K), predominantly a ${\mathrm{Fe}}_{3}\mathrm{C}$ phase has been formed. Interestingly, at an intermediate ${T}_{s}$ (523 K), a clear presence of ${\mathrm{Fe}}_{4}\mathrm{C}$ (along with ${\mathrm{Fe}}_{3}\mathrm{C}$ and Fe) can be seen from the NRS spectra. The microstructure obtained from AFM images was found to be in agreement with XRD results. MFM results also agree well with NRS as the presence of multi-magnetic components can be clearly seen in the sample grown at ${T}_{s}$ = 523 K. The information about the hybridization between Fe and C, obtained from Fe $L$- and C $K$-edge XANES also supports the results obtained from other measurements. In essence, from this work, a possibility for experimental realization of ${\mathrm{Fe}}_{4}\mathrm{C}$ has been demonstrated. It can be anticipated that by further fine-tuning of the deposition conditions, even single-phase ${\mathrm{Fe}}_{4}\mathrm{C}$ can be realized which hitherto remains an experimental challenge.

The structure and dynamical properties of the ${\mathrm{Fe}}_{3}\mathrm{Si}/\mathrm{GaAs}(001)$ interface are investigated by density functional theory and nuclear inelastic scattering measurements. The stability of four different atomic configurations of the ${\mathrm{Fe}}_{3}\mathrm{Si}/\mathrm{GaAs}$ multilayers is analyzed by calculating the formation energies and phonon dispersion curves. The differences in charge density, magnetization, and electronic density of states between the configurations are examined. Our calculations unveil that magnetic moments of the Fe atoms tend to align in a plane parallel to the interface, along the [110] direction of the ${\mathrm{Fe}}_{3}\mathrm{Si}$ crystallographic unit cell. In some configurations, the spin polarization of interface layers is larger than that of bulk ${\mathrm{Fe}}_{3}\mathrm{Si}$. The effect of the interface on element-specific and layer-resolved phonon density of states is discussed. The Fe-partial phonon density of states measured for the ${\mathrm{Fe}}_{3}\mathrm{Si}$ layer thickness of three monolayers is compared with theoretical results obtained for each interface atomic configuration. The best agreement is found for one of the configurations with a mixed Fe-Si interface layer, which reproduces the anomalous enhancement of the phonon density of states below 10 meV.

Physical review materials 1(2), 023001(2017). doi:10.1103/PhysRevMaterials.1.023001, We present a synchrotron-based x-ray scattering technique which allows disentangling magnetic propertiesof heterogeneous systems with nanopatterned surfaces. This technique combines the nanometer-range spatialresolution of surface morphology features provided by grazing incidence small angle x-ray scattering and thehigh sensitivity of nuclear resonant scattering to magnetic order. A single experiment thus allows attributingmagnetic properties to structural features of the sample; chemical and structural properties may be correlatedanalogously. We demonstrate how this technique shows the correlation between structural growth and evolutionof magnetic properties for the case of a remarkable magnetization reversal in a structurally and magneticallynanopatterned sample system., Published by APS, College Park, MD

Growth of magnetron sputtered Fe films on clean single crystalline MgO (001) substrate has been studied using in situ grazing incidence small angle x-ray scattering (GISAXS) and grazing incidence nuclear resonant scattering (GINRS) measurements. While GISAXS provides information about morphological changes, GINRS provides information about structural and magnetic properties, thus making it possible to correlate the evolution of magnetic properties with that of morphology and structure of the film. The film exhibits a Volmer-Weber type growth, with percolation transition occurring around 2 nm film thickness. Presence of a finite quadrupole splitting, as seen in GINRS measurements, suggests a significant distortion from cubic symmetry up to a film thickness of 3.5 nm, which can be attributed to hybridization between Fe $3d$ and O $2p$ orbitals at the interface as well as in-plane tensile strain induced as a result of coalescence of islands. Initially Fe islands exhibit superparamagnetic relaxation, while finite magnetic moment appears upon formation of macroscopic percolation islands. The film exhibits a weak perpendicular magnetic anisotropy (PMA), which vanishes concurrently with disappearance of structural distortion, suggesting that the observed PMA at least partly originates from inherent strain in the film. No presence of any known oxide of Fe was detected at the interface. More precise information about topological and magnetic structure of the interfaces between Fe and MgO layers is obtained using combined x-ray reflectivity and nuclear resonance reflectivity measurements on a $^{57}\mathrm{Fe}/\mathrm{MgO}$ multilayer. Measurements show that about two monolayers of Fe at the interface have a reduced hyperfine field, providing evidence for hybridization with O atoms, as predicted by theory.

We study two fundamental transitions from the ground state $^{1}S_{0}$ to $^{1}P_{1}$ ($w$ line) and $^{3}P_{1}$ ($y$ line) in heliumlike ${\mathrm{Kr}}^{34+}$ by resonant single-photon excitation using an electron-beam ion trap and monochromatic x rays at PETRA III. Our results for the transition energies $E(w)=13\phantom{\rule{0.16em}{0ex}}114.47(14)$ eV and $E(y)=13\phantom{\rule{0.16em}{0ex}}026.15(14)$ eV are in excellent agreement with quantum electrodynamics calculations, but disagree for $w$ with the average of the hitherto reported experimental results ${\overline{E}}_{\mathrm{Lit}}(w)=13\phantom{\rule{0.16em}{0ex}}115.15(17)$ eV. Independently of any energy calibration, we obtain a value $E(w)/E(y)=1.006\phantom{\rule{0.16em}{0ex}}780(7)$, also in consistency with predictions.

Modern x-ray light sources promise access to structure and dynamics of matter in largely unexplored spectral regions. However, the desired information is encoded in the light intensity and phase, whereas detectors register only the intensity. This phase problem is ubiquitous in crystallography and imaging, and impedes the exploration of quantum effects at x-ray energies. Here, we demonstrate phase-sensitive measurements characterizing the quantum state of a nuclear two-level system at hard x-ray energies. The nuclei are initially prepared in a superposition state. Subsequently, the relative phase of this superposition is interferometrically reconstructed from the emitted x-rays. Our results form a first step towards x-ray quantum state tomography, and provide new avenues for structure determination and precision metrology via x-ray Fano interference., 5 pages, 3 figures, plus supplementary information

We present absolute measurements of radiative and Auger decay rates of $K\ensuremath{-}\text{shell}$ vacancies in highly charged iron ions. The ions were resonantly excited with monochromatic x rays from the PETRA III synchrotron source. By measuring x-ray fluorescence and Auger decay simultaneously, absolute transition rates could be determined independently of most experimental parameters. The results confirm theoretical calculations, which are essential to model the photoexcited plasmas in x-ray binary stars and active galactic nuclei.

The lattice dynamics in Bi${}_{2}$Te${}_{3}$ and Sb${}_{2}$Te${}_{3}$ were investigated both microscopically and macroscopically using ${}^{121}$Sb and ${}^{125}$Te nuclear inelastic scattering, x-ray diffraction, and heat capacity measurements. In combination with earlier inelastic neutron scattering data, the element-specific density of phonon states was obtained for both compounds and phonon polarization analysis was carried out for Bi${}_{2}$Te${}_{3}$. A prominent peak in the Te specific density of phonon states at $13\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$, that involves mainly in-plane vibrations, is mostly unaffected upon substitution of Sb with Bi revealing vibrations with essentially Te character. A significant softening is observed for the density of vibrational states of Bi with respect to Sb, consistently with the mass homology relation in the long-wavelength limit. In order to explain the energy mismatch in the optical phonon region, a $\ensuremath{\sim}$$20%$ force constant softening of the Sb-Te bond with respect to the Bi-Te bond is required. The reduced average speed of sound at $20\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in Bi${}_{2}$Te${}_{3}$, $1.75(1)\phantom{\rule{0.28em}{0ex}}\mathrm{km}/\mathrm{s}$, compared to Sb${}_{2}$Te${}_{3}$, $1.85(4)\phantom{\rule{0.28em}{0ex}}\mathrm{km}/\mathrm{s}$, is not only related to the larger mass density but also to a larger Debye level. The observed low lattice thermal conductivity at $295\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, $2.4\phantom{\rule{4.pt}{0ex}}{\text{Wm}}^{\ensuremath{-}1}{\text{K}}^{\ensuremath{-}1}$ for Sb${}_{2}$Te${}_{3}$ and $1.6\phantom{\rule{4.pt}{0ex}}{\text{Wm}}^{\ensuremath{-}1}{\text{K}}^{\ensuremath{-}1}$ for Bi${}_{2}$Te${}_{3}$, cannot be explained by anharmonicity alone given the rather modest Gr\"uneisen parameters, $1.7(1)$ for Sb${}_{2}$Te${}_{3}$ and $1.5(1)$ for Bi${}_{2}$Te${}_{3}$, without accounting for the reduced speed of sound and more importantly the low acoustic cutoff energy.

The lattice dynamics of the filled skutterudite YbFe${}_{4}$Sb${}_{12}$ was studied by resonant ultrasound spectroscopy and an anomalous softening in the temperature dependence of the elastic constants at $\ensuremath{\sim}$50 K was observed. This anomaly can not be explained by the dynamics of the filler, in contrast to other filled skutterudites. We have further investigated the origin of this anomaly using macroscopic and microscopic measurements. A rearrangement of the spectral weight of the Yb phonon states was observed in the temperature dependence of the density of phonon states, obtained by inelastic neutron scattering. We suggest that the anomaly is due to a change of the Yb valence state and that the anomaly and the phonon spectral weight rearrangement have the same origin.

The density of phonon states in the thermoelectric material Yb${}_{14}$MnSb${}_{11}$ has been studied first by inelastic neutron scattering and second in an element-specific way by nuclear inelastic x-ray scattering. The low sound velocity of 1880(50) m/s as obtained from the density of phonon states can be identified as an important reason for the low heat transport in this system. The high melting temperature of Yb${}_{14}$MnSb${}_{11}$ contrasts with the low energy of all phonons ($l$25 meV) and relates to an unusual lack of softening of phonon modes with temperature, when comparing the phonon density of states observed at ambient temperatures and at 1200 K. We have also measured the density of phonon states of the related Eu${}_{14}$MnSb${}_{11}$ compound and of the thermoelectric Zintl phase Zn${}_{4}$Sb${}_{3}$ in order to compare with related thermodynamic properties of Yb${}_{14}$MnSb${}_{11}$ and to elucidate the different mechanisms of the heat conductivity reduction in Zintl phases.

We report on the observation of perturbed angular {gamma}-{gamma} correlations with 67.41 keV nuclear resonance of {sup 61}Ni using nuclear resonance scattering of synchrotron radiation. The dependence of the correlations on the direction and the magnitude of the hyperfine magnetic field is demonstrated in the feasibility study with nickel metal. The interpretation of the experimental data is done using modification of the theory of conventional perturbed angular correlations for the case of the linearly polarized first photon.

The antimony specific lattice dynamic properties in the unfilled and filled EuFe4Sb12 skutterudites have been determined by nuclear inelastic scattering at the 121Sb nuclear resonance energy of 37.1298(2) keV with a 4.5 meV high-resolution backscattering sapphire monochromator. The Sb partial vibrational density of states, DOS, shows a maximum centered at 17 and 16 meV in CoSb3 and EuFe4Sb12, respectively. The difference in the Sb DOS of CoSb3 and EuFe4Sb12 reveals that upon filling a transfer of 10% of the vibrational states towards lower energy occurs. Further, a likely indication of the coupling between the guest and the host lattice in rattler systems is observed, a coupling that is required to reduce the lattice thermal conductivity.

It is demonstrated that nuclear resonance reflectivity from isotopic multilayers can be used as a sensitive technique to study self-diffusion of a M\"ossbauer isotope ($^{57}\mathrm{Fe}$ in the present case). In the case of isotopic multilayers, in which alternate layers have the same chemical composition and differ only in the abundance of $^{57}\mathrm{Fe}$, nuclear resonance scattering causes x-ray scattering contrast between adjacent layers, resulting in the appearance of Bragg peaks corresponding to the bilayer periodicity. Diffusion of $^{57}\mathrm{Fe}$ across the isotopic interface results in a decrease in the scattering contrast and thus a decrease in the intensity of the Bragg peak, making it possible to measure diffusion lengths of the order of $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in chemically homogeneous films. The technique has been used to study self-diffusion of Fe in amorphous FeZr and nanocrystalline FeN alloys. In $\mathrm{a}\text{\ensuremath{-}}\mathrm{Fe}\mathrm{Zr}$, measurements yield activation energy for Fe diffusion $E=0.42\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and the pre-exponent factor ${D}_{0}=\mathrm{exp}(\ensuremath{-}39\ifmmode\pm\else\textpm\fi{}1)\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}∕\mathrm{s}$. In nanocrystalline ${\mathrm{Fe}}_{60}{\mathrm{N}}_{40}$, variation in diffusivity due to structural relaxation at temperatures as low as $393\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ could be observed. Measurements in the structurally relaxed state yield $E=0.8\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and ${D}_{0}=\mathrm{exp}(\ensuremath{-}28\ifmmode\pm\else\textpm\fi{}4)\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}∕\mathrm{s}$.

We have observed coherent nuclear resonant scattering of synchrotron radiation from the 67.41-keV level of {sup 61}Ni. The time evolution of the forward scattering signal was recorded by employing the nuclear lighthouse effect. This method is used to investigate Moessbauer isotopes in a coherent scattering process with synchrotron radiation at high transition energies. The decay of the excited ensemble of nuclei in Ni metal shows quantum beats that allowed the determination of the magnetic hyperfine field at the {sup 61}Ni nucleus. Moreover, we determined the lifetime of the 67.41-keV level of {sup 61}Ni to be 7.4(1) ns.

We have measured, by x-ray backscattering, the lattice constant of four highly enriched Ge isotopes: A = 70, 73, 74, and 76 at temperatures ranging from 8 to 300 K. Comparing with {sup 70}Ge, values of {Delta}a/a ranged from -10 to -46 p.p.m. A good quantitative agreement over the whole temperature range (8-300 K) was found with values calculated from a theory that takes into account the zero point motion and the anharmonicity of the lattice.