Mössbauer effect in superconducting tin-europium-molybdenum-sulfides

Mössbauer spectra of¹¹⁹Sn in Sn_1–z Eu _z Mo₆S₈ (z=0, 1/3, 2/3) have been measured in the temperature range 4.9 KT293 K. The spectra consist of a quadrupole doublett with unequal intensities at all temperatures. The Debye-Waller factor, the isomeric shift and the asymmetry of the intensity of the quadrupole components show irregularities between 50 K and 110 K which are interpreted as arising from a structural phase transition. The temperature dependence of the Debye-Waller factor can be approximated by a simple phonon spectrum. No quantitative explanation can be given for the large values of the asymmetry. Evidence is presented that this behavior is connected with the properties of the librational modes of the Mo₆S₈-units in the crystal.     

Mössbauer effect in proteins

In proteins, the M?ssbauer effect and neutron scattering show a broad line and a rapid increase of the conformational mean-square displacement above about 180?K. The increase, dubbed the "dynamical transition," is controversial. We introduce a new interpretation of the M?ssbauer effect in proteins and demonstrate that no dynamical transition is required. The increase in the mean-square displacement and the broad line are caused by fluctuations in the protein's hydration shell. Using the dielectric spectrum of these fluctuations, we predict the shape of the M?ssbauer spectrum from 80 to 295?K with one dimensionless coefficient.     

Mössbauer effect in inorganic glasses

⁵⁷Fe- and¹¹⁹Sn-Mössbauer effects become a very effective tool for the local structural study of inorganic glasses. Fe³⁺ and Sn⁴⁺ occupy network former (NWF) sites in several oxide glasses, while they are present at interstitial sites as network modifier (NWM) in phosphate, germanate and sulfate glasses. Characteristic-ray or thermal neutron irradiation effect indicates the structural role of Mössbauer ions; an increase in the 4s electron density or a reduction of Fe³⁺ to Fe²⁺ is observed due to the charge transfer from oxygen to Fe³⁺ when iron occupies NWF sites. By contrast, oxidation of Fe²⁺ to Fe³⁺ takes place when iron occupies NWM sites. Debye temperatures ( _D) obtained from low-temperature Mössbauer measurements are higher than 280 K when Fe³⁺ and Sn⁴⁺ are covalently bonded to oxygen at NWF sites, while the _D is lower than 270 K when these ions are ionically bonded to oxygen or halogen at NWM sites. A linear relationship between the glass transition temperatureT _g and the quadrupole splitting () of Fe³⁺, named T_g- rule, also indicates the structural role of Fe³⁺, i.e.,T _g versus plot yields a large slope of 680°C/mm s^–1 when Fe³⁺ occupies NWF sites, whereas it is only 35°C/mm s^–1 when the iron is present at NWM sites.     

Mössbauer effect in187rhenium

The Mössbauer effect of the 134 keV transition in¹⁸⁷Re has been investigated in order to determine the Debye-Waller factors of rhenium in different environments. The Debye temperatures of Re (Re) and Re (W) were determined to be (9 K)=(291±10) K and=(298±5) K, respectively. These data are compared with results obtained by other methods.     

The Mössbauer effect and magnetism

The Mössbauer effect enabled the magnetic hyperfine splitting (hfs) in ferromagnetic solids to be observed directly for the first time. It was quickly extended to measurements on antiferromagnets, ferrimagnets and paramagnets, and is now well established as a probe for the study of magnetic materials. Applications have ranged from the old problem of the state of the iron atoms in ferromagnetic alloys to the new magnetic materials, e.g. amorphous magnets, spin glasses, fine particle magnets and multilayers. Some examples where the Mössbauer effect has made an important contribution are described.     

The Mössbauer effect in the points of phase transitions

The temperature dependence of the intensity of the Mössbauer line and the shift of its centre on passing through the points of phase transitions was studied. Both quantities exhibit discontinuity at transitions of the first kind while only the shift of the line centre does so at transitions of the second kind. The possibility of using the Mössbauer effect for the exact localization or even classification of phase transitions is shown.     

Application of M?ssbauer spectroscopy in magnetism

An overview is provided on our recent work that applies ⁵⁷Fe M?ssbauer spectroscopy to specific problems in nanomagnetism. ⁵⁷Fe conversion electron M?ssbauer spectroscopy (CEMS) in conjunction with the ⁵⁷Fe probe layer technique as well as ⁵⁷Fe nuclear resonant scattering (NRS) were employed for the study of various nanoscale layered systems: (i) metastable fct-Fe; a strongly enhanced hyperfine magnetic field B_hf of ～39?T at 25?K was observed in ultrahigh vacuum (UHV) on uncoated three-monolayers thick epitaxial face-centered tetragonal (fct) ⁵⁷Fe(110) ultrathin films grown by molecular-beam epitaxy (MBE) on vicinal Pd(110) substrates; this indicates the presence of enhanced Fe local moments, μ_Fe, as predicted theoretically; (ii) Fe spin structure; by applying magnetic fields, the Fe spin structure during magnetization reversal in layered (Sm–Co)/Fe exchange spring magnets and in exchange-biased Fe/MnF₂ bilayers was proven to be non-collinear and depth-dependent; (iii) ferromagnet/semiconductor interfaces for electrical spin injection; CEMS was used as a diagnostic tool for the investigation of magnetism at the buried interface of Fe electrical contacts on the clean surface of GaAs(001) and GaAs(001)-based spin light-emitting diodes (spin LED) with in-plane or out-of-plane Fe spin orientation; the measured rather large average hyperfine field of ～27?T at 295?K and the distribution of hyperfine magnetic fields, P(B_hf), provide evidence for the absence of magnetically “dead” layers and the existence of relatively large Fe moments (μ_Fe ～ 1.8?μ_B) at the ferromagnet/semiconductor interface. - Finally, a short outlook is given for potential applications of M?ssbauer spectroscopy on topical subjects of nanomagnetism/spintronics.     

Mössbauer effect study of the magnetic structure in δ-FeOOH

Two samples of-FeOOH with different particle sizes have been studied in an external field of 4 T and as a function of temperature. They were found to have a ferrimagnetic structure due to an unequal occupancy of antiferromagnetically coupled octahedral ferric ions. The large surface contribution, which is characterized by a canted spin structure and by highly deformed Fe³⁺ co-ordinations, strongly influences the magnetic properties observed with Mössbauer spectroscopy.     

New applications of the Mössbauer effect

Results from a Mössbauer experiment to observe acoustic oscillations induced by pulsed laser excitation in MgO:⁵⁷Fe²⁺ crystal are presented. Time-domain spectra are satsifacorily described by the theory of the frequency modulation of Mössbauer radiation transmitted through a vibrating resonance medium. It is proposed that the D _4Ω/D _2Ω ratio of the fourth and second Fourier harmonics of the modulated radiation be used to measure the amplitude of nuclear oscillations.     

A Mössbauer effect study of barium ferrite ball-milled in air

The effects of dry-milling BaFe₁₂O₁₉ in air for periods of 190, 360, 590, 690 and 1000 h have been investigated by X-ray diffraction and Mössbauer effect measurements. The sizes of the BaFe₁₂O₁₉ particles decrease on milling, as expected, although a partial decomposition of BaFe₁₂O₁₉ to -Fe₂O₃ is found to take place on extended milling (1000 h). The room temperature Mössbauer spectra are consistent with superparamagnetic relaxation associated with the fine BaFe₁₂O₁₉ and -Fe₂O₃ particles. The X-ray diffraction patterns of all the milled samples exhibit features indicative of a disordered structural state, consistent with the nanoscale particles and a nanostructured state.     

169Tm Mössbauer effect in Tm2Fe14B

The compound Tm₂Fe₁₄B was investigated by means of ¹⁶⁹Tm Mössbauer spectroscopy in the temperature range from 4.2 to 300 K. The temperature dependencies of the hyperfine fields and quadrupole splittings for the two Tm sites were determined. The data were analysed on the basis of the spin structure determined by Yamada et al., which appears to be only partially compatible with the observed spectrum.     

On Mössbauer effect in polypyrrole doped with selected metal halides

The reaction of partially oxidized polypyrrole [(C₄H₃N)₄⁺Cl₋]_n with FeCl₃ and SnCl₄ leads to the insertion of only one type metal halide species namely FeCl₄⁻ and SnCl₅⁻. The reaction with stannic halide is acid-base in nature whereas the reaction with ferric chloride may be either redox or acid-base. The use of SbCl₅ results in the insertion of two non-equivalent antimony halide species: SbCl₆⁻ and SbCl₃. Much simpler Mössbauer spectra can be obtained if in the doping reactions SbCl₅ or SbF₅ are replaced by NO₂⁺SbF₆⁻. Only one type of antimony with Mössbauer parameters characteristics of SbF₆⁻ is observed in this case. The Mössbauer lattice temperatures, θ_M, calculated from the temperature dependence of the recoil free fractions are 94 K and 105 K for FeCl₃ and SnCl₄ doped polypyrrole respectively. The obtained values are within the range typically observed for other conducting polymeric systems.     

Enhancing vibration measurements by Mössbauer effect

The measurement of the Mössbauer effect in a system excited with a periodic perturbation can provide information about it. For that purpose, the Mössbauer absorption of a source-absorber set which hyperfine parameters are well known, is measured at a constant relative velocity (i.e. at a defined spectral energy). The resulting Mössbauer absorption periodic signal provides information of the sample ac perturbation response. This approach has been used time ago to measure small tympanic vibrations (mechanical perturbations). In this work we present an extension of the vibration experiments, by measuring them at various absorber-source relative velocities within a constant-velocity strategy. As a demonstration test, the frequency response of a piezoelectric diaphragm in the 100 Hz–5 kHz range is obtained with a custom electronic counter. The experiments are performed using a ⁵⁷Co(Rh) source and a 25- $\upmu$ m-thick stainless-steel absorber fixed to a piezoelectric diaphragm. Phase shifts and amplitude vibrations with velocities in the range from 1.5 $\upmu$ m/s to 20 mm/s are well characterized, extending the linearity limit well beyond the earlier suggested one of 1 mm/s.     

The relaxation dynamics, iron valence state, and M?ssbauer effect in PFN ceramics

The dielectric properties, X-ray emission spectra, and M?ssbauer effect in ceramics made of PbFe_1/2Nb₁/₂O₃ (PFN) compound were studied. The relaxation dynamics revealed above Curie temperature TC at a frequency of 3 × 10⁻²–10⁵ Hz is described in detail. Analysis of the X-ray emission and M?ssbauer spectra showed that at room temperature (T = 300 K), the iron ions in PFN are mainly in the high-spin valence state Fe³⁺. The M?ssbauer spectral parameters obtained at T = (300, 353, and 393 K) indicate an octahedral environment for Fe³⁺ in both the ferroelectric and paraelectric phases.     

A M?ssbauer study of the magneto-structural coupling effect in SrFe2As2 and SrFeAsF

In the present paper, we report a comparison study of SrFe₂As₂ and SrFeAsF using M?ssbauer spectroscopy. The temperature dependence of the magnetic hyperfine field is fitted with a modified Bean–Rodbell model. The results give much smaller magnetic moment and magneto-structural coupling effect for SrFeAsF, which may be understood as due to different inter-layer properties of the two compounds.     

Mössbauer effect diffraction from polycrystalline57Fe

We built a Debye-Scherrer type powder diffractometer with a⁵⁷Co radiation source and a large-angle position-sensitive detector. We tuned the incident -ray beam on and off the Mössbauer effect resonance, and measured diffraction patterns from our textured samples of polycrystalline⁵⁷Fe. Mössbauer effect diffraction dominated over X-ray diffraction for the higher order Bragg peaks.     

An in situ M?ssbauer study using synchrotron radiation

We have been developing a system for in situ M?ssbauer studies using synchrotron radiation (SR) to elucidate the mechanism of hydrogenation processes. In the system, samples reacts in a pressure-temperature chamber and SR-based M?ssbauer spectra using variable-frequency nuclear monochromator and energy spectra of inelastic nuclear resonant scattering (NRS) of SR are measured. As a feasibility study, the temperature dependence of the M?ssbauer and inelastic NRS spectra of ⁵⁷Fe in c-GdFe₂H₃ under vacuum were measured. In both spectra, clear differences were observed between 373?K and 573?K. These differences can be interpreted by the change of microscopic environment around Fe at the dehydrogenation. Thus, it is confirmed that this system works well enough to perform the in-situ M?ssbauer study on the dehydrogenation of c-GdFe₂H₃.     

Mössbauer effect study of Ni bearing aluminous goethites

Five synthetic oxyhydroxides of iron with different contents of Al and an approximately fixed content of Ni have been studied using⁵⁷Fe Mössbauer spectroscopy in conjunction with X-ray diffraction, infrared spectroscopy, transmission electron microscopy and chemical analysis. It has been shown that, besides alumogoethite, some amorphous Al-bearing phase of iron exists, Ni being preferentially associated with this phase. Not only the effective magnetic field, but also the line width of the Mössbauer spectra at 77 K may be a measure of Al content in both phases. Doublet to sextet area ratio provides a measure of the relative concentration of the amorphous and crystalline phases.