Layer magnetization canting in 57Fe/FeSi multilayer observed by synchrotron Mössbauer reflectometry

Synchrotron Mössbauer reflectometry and CEMS results on a [⁵⁷Fe(2.55 nm)/FeSi\break(1.57 nm)]₁₀ multilayer (ML) on a Zerodur substrate are reported. CEMS spectra are satisfactorily fitted by α‐Fe and an interface layer of random α‐(Fe, Si) alloy of 20% of the ⁵⁷Fe layer thickness on both sides of the individual Fe layers. Kerr loops show a fully compensated AF magnetic layer structure. Prompt X‐ray reflectivity curves show the structural ML Bragg peak and Kiessig oscillations corresponding to a bilayer period and total film thickness of 4.12 and 41.2 nm, respectively. Grazing incidence nuclear resonant Θ–2Θ scans and time spectra (E = 14.413 keV, λ = 0.0860 nm) were recorded in different external magnetic fields (0 < B^ext < 0.95 T) perpendicular to the scattering plane. The time integral delayed nuclear Θ–2Θ scans reveal the magnetic ML period doubling. With increasing transversal external magnetic field, the antiferromagnetic ML Bragg peak disappears due to Fe layer magnetization canting, the extent of which is calculated from the fit of the time spectra and the Θ–2Θ scans using an optical approach. In a weak external field the Fe layer magnetization directions are neither parallel with nor perpendicular to the external field. We suggest that the interlayer coupling in [Fe/FeSi]₁₀ varies with the distance from the substrate and the ML consists of two magnetically distinct regions, being of ferromagnetic character near substrate and antiferromagnetic closer to the surface.     

Magnetism under high pressure studied by 57Fe and 151Eu nuclear scattering of synchrotron radiation

The nuclear forward scattering (NFS) of synchrotron radiation is especially suited for probing magnetism at very high pressure, here in the Mbar range, by the nuclear resonances of ⁵⁷Fe and ¹⁵¹Eu. We report on high pressure (h.p.) NFS studies with the 14.4 keV transition of ⁵7Fe on magnetic RFe₂ Laves phases of cubic C15 structure (YFe₂, GdFe₂) and hexagonal C14 structure (ScFe₂, TiFe₂) at pressures up to 100 GPa (=1 Mbar). We present also h.p. NFS studies performed with the 21.5 keV resonance of ¹⁵¹Eu, probing the magnetism in the CsCl-type h.p. phase of EuTe. This revised version was published online in August 2006 with corrections to the Cover Date.     

Comparison of nuclear Bragg scattering with Mössbauer spectroscopy for the measurement of hyperfine interactions

Fourier analysis of the time evolution of the nuclear Bragg scattering from the (7, 7, 7) reflection in α-hematite (⁵⁷Fe) excited by synchrotron radiation was used to extract values of Zeeman splitting of the ground and 14.413-keV states of⁵⁷Fe in the crystal magnetic field. The results so obtained apparently do not agree with the corresponding values obtained from Mössbauer spectroscopy on α-hematite. Possible reasons for the discrepancy are discussed.     

The sapphire backscattering monochromator at the Dynamics beamline P01 of PETRA III

We report on a high resolution sapphire backscattering monochromator installed at the Dynamics beamline P01 of PETRA III. The device enables nuclear resonance scattering experiments on Mössbauer isotopes with transition energies between 20 and 60 keV with sub-meV to meV resolution. In a first performance test with ¹¹⁹Sn nuclear resonance at a X-ray energy of 23.88 keV an energy resolution of 1.34 meV was achieved. The device extends the field of nuclear resonance scattering at the PETRA III synchrotron light source to many further isotopes like ¹⁵¹Eu, ¹⁴⁹Sm, ¹⁶¹Dy, ¹²⁵Te and ¹²¹Sb.     

Diffraction of X rays at a Bragg angle of π/2 (Back reflection) with consideration of multiwave effects

The energy dependence of the back reflectivity in the dynamical diffraction of x rays at a Bragg angle of π/2 (back diffraction) in perfect crystals of cubic symmetry (silicon) is investigated theoretically. In this case strict backscattering is realized only under the conditions of multiple diffraction. The features of the influence of multiple diffraction on back reflection in the energy range near the nuclear resonance radiation energy of 14.41 keV for ⁵⁷Fe nuclei, specifically in the six-wave case, including the silicon (1,9,9) reflection (with an energy of 14.57 keV), which can be investigated experimentally with high energy resolution (1 meV) using synchrotron radiation and a monochromator developed for nuclear resonant absorption, are thoroughly studied. It is shown that the back reflectivity observed under the conditions of multiple diffraction has several maxima on the plot of its energy dependence with a value at each maximum smaller than half, in contrast to two-wave diffraction, where there is one maximum with a value close to unity. Zh. éksp. Teor. Fiz. 116, 940–952 (September 1999)     

Influence of pressure on the inhomogeneous mixed-valent state in Eu3S4

The 21.7 keV nuclear gamma resonance of ¹⁵¹Eu was used for studying the inhomogeneous mixed-valent compound Eu₃S₄ in the pressure range 0–15 kbar. At room temperature the activation energy for electron hopping between the crystallographically equivalent divalent and trivalent Eu sites was found to decrease with pressure by dE_a/dP=-1.8±0.3 meV kbar^-1. No change of the mean valence of Eu with pressure was observed.     

Atomic vibrational dynamics of amorphous Fe-Mg alloy thin films

The atomic vibrational dynamics of ⁵⁷Fe in 800-Å thick amorphous Fe_xMg_1−x alloy thin films (0.3≤x≤0.7) has been investigated at room temperature by nuclear resonant inelastic X-ray scattering (NRIXS) of 14.4125 keV synchrotron radiation. The amorphous phase has been successfully stabilized by codeposition of Fe and Mg in ultrahigh vacuum onto a substrate held at −140 °C during deposition. The amorphous structure of the samples was confirmed by X-ray diffraction and conversion electron Mössbauer spectroscopy. The ⁵⁷Fe-projected partial vibrational density of states, g(E), has been obtained from the measured NRIXS vibrational excitation probability, together with thermodynamic quantities such as the probability of recoilless absorption (f-factor), the average kinetic energy per Fe atom, the average force constant, and the vibrational entropy per Fe atom. A plot of g(E)/E² versus E proves the existence of non-Debye-like vibrational excitations with a peak at E_bp∼3-5 meV (boson peak). Both the boson peak height and E_bp were found to depend linearly on the composition x. Above the boson peak, g(E)/E² exhibits an exponential decrease.     

Energy calibration issues in nuclear resonant vibrational spectroscopy: observing small spectral shifts and making fast calibrations

The conventional energy calibration for nuclear resonant vibrational spectroscopy (NRVS) is usually long. Meanwhile, taking NRVS samples out of the cryostat increases the chance of sample damage, which makes it impossible to carry out an energy calibration during one NRVS measurement. In this study, by manipulating the 14.4 keV beam through the main measurement chamber without moving out the NRVS sample, two alternative calibration procedures have been proposed and established: (i) an in situ calibration procedure, which measures the main NRVS sample at stage A and the calibration sample at stage B simultaneously, and calibrates the energies for observing extremely small spectral shifts; for example, the 0.3 meV energy shift between the 100%‐⁵⁷Fe‐enriched [Fe₄S₄Cl₄]⁼ and 10%‐⁵⁷Fe and 90%‐⁵⁴Fe labeled [Fe₄S₄Cl₄]⁼ has been well resolved; (ii) a quick‐switching energy calibration procedure, which reduces each calibration time from 3–4 h to about 30 min. Although the quick‐switching calibration is not in situ, it is suitable for normal NRVS measurements.     

Inelastic X-ray scattering investigations of lattice dynamics in SmFeAsO1−xFysuperconductors

We report measurements of the phonon density of states as probed with inelastic X-ray scattering in SmFeAsO_1−xF_y powders. An unexpected strong renormalization of phonon branches around 23 meV is observed as fluorine is substituted for oxygen. Phonon dispersion measurements on SmFeAsO_1−xF_y single crystals allow us to identify the 21 meV A_1g in-phase (Sm,As) and the 26 meV B_1g (Fe,O) modes to be responsible for this renormalization, and may reveal unusual electron-phonon coupling through the spin channel in iron-based superconductors.     

Search for resonant absorption of solar axions emitted in an <Emphasis Type="Italic">M</Emphasis>1 transition in <Superscript>57</Superscript>Fe nuclei

The resonant absorption of solar axions by ⁵⁷Fe nuclei, which is accompanied by the excitation of the first excited nuclear level: A + ⁵⁷Fe → ⁵⁷Fe* → ⁵⁷Fe + γ(14.4 keV), is sought. To seek 14.4-keV photons, a Si(Li) detector and an enriched ⁵⁷Fe target are used. The detector and target are placed in a low-background setup equipped with passive and active shieldings. As a result, a new upper limit m _A ≤ 360 eV (at 90% C.L.) has been determined for the axion mass.     

The interaction of Fe on MgO(1 0 0) surfaces

The atomic interaction and magnetic properties of ultrathin Fe films grown on cleaved and polished MgO(1 0 0) surfaces were studied by conversion electron Mössbauer spectroscopy (CEMS). ⁵⁷Fe layers were deposited as probe atoms in different layer positions in 10 ML thick Fe films. Fe layers of different thicknesses were formed on polished and cleaved substrate surfaces at RT deposition. The analysis of the spectra showed no Fe-O^2- interaction in MgO/Fe interface. FeO phase formation was excluded. The Mössbauer spectrum of 5 ML ⁵⁷Fe sample showed enhanced internal magnetic field at 80 K. No interdiffusion of ⁵⁷Fe and ⁵⁶Fe atoms was observed between the layers at room temperature.     

New limit on the mass of 14.4-keV solar axions emitted in an <Emphasis Type="Italic">M</Emphasis>1 transition in <Superscript>57</Superscript>Fe nuclei

Axions of energy 14.4 keV that originated from the M1 transition in ⁵⁷Fe nuclei in the Sun were sought by using the resonance-absorption reaction A+⁵⁷Fe → ⁵⁷Fe* → ⁵⁷Fe+γ (14.4 keV). Asectioned Si(Li) detector arranged in a low-background facility was used to record photons from this reaction. This resulted in setting a new limit on the axion couplings to nucleons, |−1.19g _AN ⁰ + g _AN ³| ≤ 3.0×10⁻⁶. Within the hadronic-axion model, the respective constraint on the axion mass is m _A ≤ 145 eV (at a 95% C.L.).     

149Sm and 57Fe nuclear resonant inelastic scattering of filled skutterudites SmFe4X12 (X: pnictogen)

¹⁴⁹Sm and ⁵⁷Fe nuclear resonant inelastic scattering (NRIS) measurements were carried out on SmFe₄P₁₂, SmFe₄As₁₂ and SmFe₄Sb₁₂. A clear dip structure of the ⁵⁷Fe NRIS spectrum was found in SmFe₄P₁₂, which was not clearly observed for SmFe₄As₁₂ and SmFe₄Sb₁₂. On the other hand, the line width of the phonon excitation in the ¹⁴⁹Sm NRIS spectrum increases with increasing the ionic radius of the pnictogen. These findings imply that the hybridization between the Sm and Fe phonon modes is correlated to changes in the ionic radius of the pnictogen.     

Fe-induced magnetic transition in YBa2(Cu1−x 57Fe x )3O6 by NQR and Mössbauer spectroscopy

Nuclear quadrupole resonance spectroscopy (around 30 MHz) on the chain site Cu(1) nuclei in oxygen deficient YBa₂(Cu_1?x Fe _x )₃O₆ doped with different amounts of⁵⁷Fe (x≤0.01) reveal an onset of magnetic order at low temperatures represented by a symmetrical doublet contribution to the nuclear quadrupole resonance (NQR) spectrum. The onset temperatureT _N2 depends on the concentration of Fe reaching 130 K forx=0.01. The splitting forx=0.01 at 100 K corresponds to a net internal field of 0.09 T with a distribution of ≈0.08 T representing about 70 percent of the Cu(1) nuclei.⁵⁷Fe and⁵⁷Co Mössbauer spectroscopy at 4.2 K with and without an external magnetic field of 5 T revealed that belowx=0.00015 Fe spins are decoupled from the Cu(2) moments in the antiferromagnetic state. Results are interpreted in terms of the magnetic model structure suggested by Kadowaki et al. [1].     

Kaonic atoms measurements at the DAΦNE accelerator

Radioactive ⁵⁷Mn⁺(T _1/2?= 1.5 min) ions have been implanted at the ISOLDE facility at CERN with 60 keV energy to fluences <10¹²/cm² into p-type Si_1???x Ge _x (x < 0.1) single crystals held at 300–600 K. The implantation and annealing processes result in the majority of the implanted Mn ions occupying substitutional lattice sites. In the subsequent ⁵⁷Mn nuclear β ^???-decay to the 14.4 keV Mössbauer state of ⁵⁷Fe (T _1/2?= 100 ns), an average recoil energy of 40 eV is imparted to the ⁵⁷Fe daughter atoms which results in a large fraction being expelled into tetrahedral interstitial sites and the creation of a vacancy. The remainder occupies substitutional sites. This technique of recoil production of ^57m Fe_I thus allows for the study of the diffusion characteristics of interstitial Fe. From the temperature dependent line broadening, the activation energies have been determined and decrease with increasing Ge concentration which contributes significantly to the increase of the jump frequency. A similar result has been obtained in n-type SiGe but there the values for the activation energies were much higher.     

The contact charge density of conduction electrons of Fe in Co metal as derived from internal conversion

From an internal-conversion measurement of the 14.4 keV transition ⁵⁷Fe, the contact 1 charge density of conduction electrons for Fe in Co metal was obtained to be ?_4s(0) = 4.6 ±0.4 a₀⁻³, which is close to theoretical values but is smaller than that for Fe in Fe metal obtained recently.     

Observation of localized modes in TbO x using the Mößbauer effect

The resonant absorption of the 58 keV Mößbauer line under excitation of one phonon states in TbO _x was investigated. Using an ultra centrifuge, theγ energy was shifted by ±25meV. Narrow absorption lines at 14.1 meV, 16.0 meV and presumably at 12.3 meV are interpreted as being caused by localized modes of the terbium ion.     

High-resolution study of E0 internal pair decay of excited 0+ states in 58,60,62Ni

The recently developed magnetic plus Si(Li)-Si(Li) sum-coincidence technique is employed to measure E0 internal-pair-formation (IPF) branching ratios of excited 0⁺ states in ^58,60,62Ni. The $\text{X(}\text{E}\text{0}\text{E}\text{2}\text{)}$ values are obtained for a new 0⁺₄ state in ⁶⁰Ni at 3588.0 keV, for the 0⁺₂ and 0⁺₃ states at 2942.3 keV and 3530.9 keV in ⁵⁸Ni, for the corresponding states at 2284.8 keV and 3318.3 keV in ⁶⁰Ni, and for the 0⁺₂ state in ⁶²Ni at 2048.4 keV. The results are combined with the available lifetimes of these states to extract the monopole strengths ?²(0⁺_i ? 0⁺₁). The results and the nature of the 0⁺ states are discussed.     

<Superscript>57</Superscript>Fe NMR study of amorphous and rapidly quenched crystalline Fe-B alloys

Amorphous and crystalline Fe-B alloys (5–25 at % B) were studied using pulsed ⁵⁷Fe nuclear magneticr esonance at 4.2 K. The alloy samples were prepared from a mixture of the ⁵⁷Fe and ¹⁰B isotopes by rapid quenching from the melt. In the microcrystalline Fe-(5–12 at %) B alloys, the resonance frequencies were measured for local states of ⁵⁷Fe nuclei in the tetragonal and orthorhombic Fe₃B phases and also in α-Fe. The resonance frequencies characteristic of ⁵⁷Fe nuclei in α-Fe crystallites with substitutional impurity boron atoms in the nearest neighborhood were also revealed. In the resonance frequency distribution P(f) in the amorphous Fe-(18–25) at % B alloys, there are frequencies corresponding to local Fe atom states with short-range order of the tetragonal and orthorhombic Fe₃B phases. As the boron content decreases below 18 at %, the P(f) distributions are shifted to higher frequencies corresponding to ⁵⁷Fe NMR for atoms exhibiting a short-range order of the α-Fe type. The local magnetic structure of the amorphous Fe-B alloys is also considered.     

Search for resonant absorption of solar axions emitted in M1 transition in <Superscript>57</Superscript>Fe nuclei

A search for resonant absorption of 14.4 keV solar axions by a ⁵⁷Fe target was performed. The Si(Li) detector placed inside the low-background setup was used to detect the γ-quanta appearing in the deexcitation of the 14.4 keV nuclear level: A+⁵⁷Fe→⁵⁷Fe^*→⁵⁷Fe+γ. The new upper limit for the hadronic axion mass has been obtained of m _A ≤159 eV (95% c.l.) (S=0.5, z=0.56).