Synchrotron Mössbauer reflectometry using stroboscopic detection

The concept of the heterodyne/stroboscopic detection of nuclear resonance scattering of synchrotron radiation is extended to the grazing incidence geometry. Model calculations for an antiferromagnetic [⁵⁷Fe/Cr]₂₀ multilayer are shown and discussed. Principles and methodological aspects of stroboscopic synchrotron Mössbauer reflectometry are briefly reviewed.     

A Mössbauer study of the silica-like structures of FePO4 and (Al0.67Fe0.33)PO4

Summary Samples of FePO₄ and (Al_0.67Fe_0.33)PO₄ have been investigated by M?ssbauer spectroscopy. The data obtained on quadruple splittings are in agreement with neutron scattering and XRD data on (Al, Fe)O₄ tetrahedra structures.     

New in-beam Mössbauer spectroscopy station at the Budapest Research Reactor

We report about a new in-beam Mössbauer station which is intended to utilize the cold neutron source and guide system installed on the 10th beam line of the Budapest Research Reactor. This new in-beam facility enables us to broaden the number of nuclides accessible for Mössbauer studies in various materials. In this article we describe our new system and summarize the possibilities of its application to Mössbauer spectroscopy.     

Off-Specular Synchrotron Mössbauer Reflectometry: A Novel Tool for Studying the Domain Structure in Antiferromagnetic Multilayers

The off-specular (diffuse) nuclear resonant reflectivity of synchrotron radiation is a sensitive measure of the lateral autocorrelation of the magnetisation in thin films and multilayers. The width of the diffuse scattering peak measured at an electronically forbidden reflection is inversely proportional to the in-plane correlation length of the magnetisation direction. The average size of the in-plane antiferromagnetic domains is determined in different states of the same Fe/Cr superlattice. The hyperfine magnetic fields in coexisting small and large domains are measured independently. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nuclear Resonance Vibrational Spectroscopy (NRVS) of Fe–S model compounds, Fe–S proteins, and nitrogenase

We have used nuclear resonance vibrational spectroscopy (NRVS) to examine the nature of the Fe–S unit. Specifically, vibrational characteristics have been determined, and through incremental steps in model system complexity, applied to analysis of the enzyme nitrogenase. This stepwise strategy demonstrates NRVS as a viable bioinorganic tool, and will undoubtedly increase the application of synchrotron spectroscopy to biological problems.     

Generation of long-lived isomeric states via bremsstrahlung irradiation

A method to generate long-lived isomeric states effectively for Mössbauer applications is reported. We demonstrate that this method is better and easier to provide highly sensitive Mössbauer effect of long-lived isomers (>1 ms) such as ¹⁰³Rh. Excitation of (γ,γ) process by synchrotron radiation is painful due mainly to their limited linewidth. Instead, (γ,γ’) process of bremsstrahlung excitation is applied to create these long-lived isomers. Isomers of ⁴⁵Sc, ¹⁰⁷Ag, ¹⁰⁹Ag, and ¹⁰³Rh have been generated from this method. Among them, ¹⁰³Rh is the only one that we have obtained the gravitational effect at room temperature.     

Determination of Spin State in Dinuclear Iron(II) Coordination Compounds Using Applied Field Mössbauer Spectroscopy

So far there has been no direct method to determine the spin state of molecules in dinuclear iron(II) compounds. The molecular fractions of high spin (HS) and low spin (LS) species have been deduced from magnetic susceptibility and zero field Mössbauer spectroscopy data irrespective of whether they belong to LS-LS, LS-HS and HS-HS pairs. However, the distinction of pairs becomes possible if Mössbauer measurements are carried out in an external magnetic field. The proposed method opens new possibilities in the study of spin crossover phenomena in dinuclear compounds.

     

Mössbauer In Situ Studies of the Surface of Mars

For the first time in history, a Mössbauer spectrometer was placed on the surface of another planet. Our miniaturized Mössbauer spectrometer MIMOS II [1–4] (Figure 2) is part of the instrument payload of NASA’s twin Mars Exploration Rovers (MER) “Spirit” and “Opportunity” (see Figure 1), which in January 2004 successfully landed at the Gusev crater and the Meridiani Planum landing sites, respectively. MIMOS II determines the Fe-bearing mineralogy of Martian soils and rocks at the Rovers’ respective landing sites [5]. The main goals of this planetary twin mission are to: (1) identify hydrologic, hydrothermal, and other processes that have operated and affected materials at the landing sites; (2) identify and investigate the rocks and soils at both landing sites, as there is a possible chance that they may preserve evidence of ancient environmental conditions and possible prebiotic or biotic activities. With MIMOS II, besides other minerals the Fe silicate olivine has been identified in both soil and rocks at both landing sites. At the Meridiani site the Fe sulfate jarosite has been identified by MIMOS II which is definitive mineralogical proof of the presence of water at this site in the past.     

Effect of hydrogen on interatomic bonds in austenitic steels

Mössbauer spectroscopy was used to investigate the influence of dissolved hydrogen on the interatomic bonds in austenitic steels. It was carried out to prove the decohesion mechanism of hydrogen embrittlement (HE). It is shown that hydrogen increases Debye temperature, i.e., the interatomic bonds in hydrogenated austenitic steel becomes stronger.     

Mössbauer study of iron oxide modified montmorillonite

Montmorillonite particles were modified by iron oxides using the precipitation process with the aim to monitor the differences in the structural and magnetic properties of intercalated and adsorbed Fe³⁺. The Mössbauer spectra recorded at 5 K in zero and 6 T external fields, IR spectra and TG curves measured in zero and 32 mT fields identified the ferrihydrite pillars in an interlayer space of the montmorillonite structure and γ-Fe₂O₃ nanoparticles adsorbed on the mineral surface. The temperature dependent Mössbauer spectra (25–300 K) reflect the superparamagnetic behaviour of maghemite nanoparticles and ferrihydrite pillars with the blocking temperatures of about 80 and 25 K, respectively.     

Surface properties of Fe76Mo8Cu1B15 alloy after annealing

NANOPERM-type alloy Fe₇₆Mo₈Cu₁B₁₅ is investigated in amorphous and in partially crystallized state. Samples were prepared by 1 h isothermal annealing in vacuum at temperatures ranging from 330°C up to 700°C. Bulk and surface microstructural characteristics were studied using transmission and conversion electron Mössbauer effect techniques, respectively. Surface features were checked by the help of atomic force microscopy. Presence of nanocrystalline bcc-Fe phase was detected during the first crystallization stage. The crystallization process starts at 450°C and it is more pronounced in surface regions than in the bulk. With progressing crystallization, hyperfine parameters especially of the amorphous residual phase are altered. Distinctions in surface morphology are revealed between wheel and air side of the ribbon-shaped samples.     

La–Zn Substituted Hexaferrites Prepared by Chemical Method

La–Zn substituted M-type Ba hexaferrite powders were prepared by sol-gel (Mx) and organometallic precursor (Sk) methods with Fe/Ba ratio of 11.6 and 10.8, respectively. The compositions (LaZn) _x Ba_{1 ? x} Fe_{12 ? x} O₁₉ with 0.0 ≤ x ≤ 0.6 were annealed at 975°C/2 h. The cationic site preferences of nonmagnetic La³⁺ instead of Ba²⁺ ions and Zn²⁺ instead of Fe³⁺ ions were determined by Mössbauer spectroscopy. The La³⁺ ions substitute the large Ba²⁺ ions at 2a site and for x ≥ 0.4 also at 4f₂ site. The nearly all Zn²⁺ ions are placed at the 4f₁ sites. The thermomagnetic analysis of χ(?) confirms that only the small substitutions for x ≤ 0.4 can be taken as a single-phase hexaferrites. The coercivity H _c almost does not change at x = 0.2 for (Mx) samples and further decrease up to x = 0.6. For (Sk) samples at substitution x = 0.2 the values of H _c are decreasing and at higher x the values nearly do not change. The Curie points, T _c, slowly decrease with x for both (Mx) and (Sk) samples.     

Dynamic Jahn-teller effect in a Fe-N6 cage molecule

Mössbauer spectroscopic studies of [Fe(di(amH)-sar)](NO₃)_4·H₂O where di(amH)-sar represents 1,8 diamino 3,6,10,13,16,19-hexaza bicyclo [6,6,6] icosane in the temperature range of 4.2 to 300 K suggest that it undergoes a dynamic Jahn-Teller effect as revealed from the plots of temperature dependent quadrupole coupling constant, chemical shift and line width values. The spectrum down to 4.2 K, shows a quadrupole doublet with no magnetic hyperfine splitting.     

Cationic Order in Double Perovskite Oxide, Sr2Fe1−x Sc x ReO6 (x = 0.05, 0.1)

We have synthesized by sol–gel method the following polycrystalline double perovskite samples: Sr₂Fe_1?x Sc _x ReO₆ (x = 0, 0.05, 0.1). The results of the Rietveld refinements presented single double perovskite phases with orthorhombic symmetry for the system Sr₂Fe_1?x Sc _x ReO₆, the differences in atomic radii between Fe³⁺ and Sc³⁺ cause a lowering in symmetry with respect to the parent Sr₂FeReO₆ tetragonal compound. The Curie temperatures are found at about 426 and 436 (±5) K for Sr₂Fe_0.9Sc_0.1ReO₆ and Sr₂Fe_0.9Sc_0.05ReO₆, respectively. The Mössbauer spectra measured at 77 K show complex hyperfine structures resulting from different magnetic contributions at Fe³⁺ sites; the average hyperfine field is estimated 50 T and the isomer shift at 0.5 mm/s. At room temperature an intermediate valence state for Fe is also observed.     

57Fe Mössbauer and magnetic studies of DyFe12−x Ta x compounds

Refinements of the X-ray diffraction patterns show that DyFe_12?x Ta _x compounds with x = 0.5–0.7 crystallise in the ThMn₁₂-type structure and that the Ta atoms occupy the 8i sites. Spin reorientations have been detected by ac magnetic susceptibility for all compounds below room temperature. First the moments shift direction from easy axis to easy cone at T _sr1, then to easy plane at T _sr2. Both T _sr1 and T _sr2 increase with increasing Ta content up to x = 0.65 before decreasing with further increase in Ta content. Analyses of the Mössbauer spectra indicate that the individual Fe site hyperfine fields derived at 4.5 K for DyFe_11.35Ta_0.65 are B _hf = 37.4 T, 32.2 T and 27.6 T for the 8i, 8j and 8f sites, respectively.     

The Current Status of Gamma-Ray Lasers Based on the Mössbauer Effect

Hyperfine Interactions - The gamma-laser has not been created because ways to blind alleys were chosen. Now the perspective way in truth is revealed in order to create γ-laser. That way uses...     

Isotope shift measurement at 244fAm *

Strips of Fe_62.5Co₆Ni_7.5Zr₆Cu₁Nb₂B₁₅, prepared by rapid quenching from the melt, were annealed at different temperatures and studied performing Mössbauer and magnetoelastic measurements. By increasing annealing temperature, the long range order increases strengthening the exchange interactions. The maximum of magnetoelastic response corresponds to the maximum of the out of plane magnetization. A possible explanation for the onset of the perpendicular magnetic anisotropy is proposed.     

γ-Ray Holography – Three-Dimensional Imaging of a Local Atomic Structure

γ-ray holography is a novel method for three-dimensional (3D) imaging of a local atomic structure, which utilizes the internal reference idea of holography. The principles of this method will be presented with emphasis on the phase information gained by means of complex holograms. This revised version was published online in August 2006 with corrections to the Cover Date.     

57Fe and 151Eu Mössbauer studies of magnetoresistive Europium based cobalt perovskites

Eu_0.8Sr_0.2Fe _x Co_1?x O_3?z CMR perovskites with different iron concentrations (x?=?0, 0.025, 0.075, 0.15, 0.3) were investigated by X-ray diffraction, AC magnetic susceptibility, magnetotransport, as well as ⁵⁷Fe and ¹⁵¹Eu Mössbauer spectrometry. The valence state of europium ions was found to be trivalent, independently of the iron concentration. ⁵⁷Fe Mössbauer spectra and magnetic susceptibility of the investigated perovskites presented complementary results for the magnetic transitions.