121Sb Mössbauer spectroscopy of Mn1+x Sb

¹²¹Sb Mössbauer spectra for ferromagnets, Mn_1+x Sb (0.00≤x≤0.22) in ε phase were measured at 77 K. The large unique hyperfine magnetic field (H _hf=361 kOc) atx=0 tends to decrease and to have a wide distribution with increase ofx. The decreasing ratio of the mean value ofH _hf for the nonstoichiometric samples is in accordance with that of their magnetizationM _S(x), i.c., the relation \(\overline {H_{hf} } (x)/H_{hf} (0) = M_S (x)/M_S (0)\) holds in this system.     

Mössbauer spectroscopy of lanthanum and holmium ferrites

PrRh₂Si₂ is highly anisotropic Ising-type antiferromagnetic system. The ordering temperature (T _N~ 68 K) of PrRh₂Si₂ is exceptionally high on the de-Gennes scale in the family of RRh₂Si₂ (R = rare earths). The reason for this anomalous behaviour is not clear. It is believed that the presence of uniaxial anisotropy assists in enhancing the T _N. The Mössbauer study was performed on a 10% Fe-doped PrRh₂Si₂ sample to understand the magnetic coupling between different sites of PrRh₂Si₂. The Mössbauer spectra together with the magnetic susceptibility data suggest that the magnetic coupling in PrRh₂Si₂ is provided mainly by the RKKY interaction between Pr-moments.     

Mössbauer spectroscopy studies of the magnetic properties of ferrite nanoparticles

Studies of the ferrite nanoparticles prepared by the chemical decomposition of iron chlorides with a various ratio ξ = Fe³⁺/Fe²⁺ are herein presented. The microstructure and the magnetic properties have been studied by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Mössbauer spectroscopy (MS). The TEM studies show that the nanoparticles have almost a spherical shape with the diameter of (12 ± 2) nm for all samples. The measured XRD pattern was mainly composed of lines which were indexed with a cubic spinel structure. The analysis of the Mössbauer data shows that the microstructure of the nanoparticles consists of the core formed by nonstoichiometric magnetite and maghemite shell. A small amount of hematite, probably on the surface of the nanoparticles with ξ = 1.75, 2.0, was detected. At temperatures T ≤ 150 K the spin canting of surface maghemite with ξ = 2.25 was observed while for the samples with ξ = 1.75, 2.0 such effect was suppressed by the presence of hematite on the surface of the nanoparticles. Infield Mössbauer spectra with ξ = 1.75, 2.0 show that magnetic moments of the magnetite/maghemite core are parallel while magnetic moments of the surface hematite are perpendicular to the direction of the external magnetic field.     

Dynamic properties of polymers bases on vinylferrocene from Mössbauer spectroscopy

Mössbauer absorption experiments on polyvinylferrocene, on a copolymer of acrylonitrile and vinylferrocene and on the swollen copolymer reveal an anomalous decrease of the recoilfree fraction f between 80 K and 130 K accompanied by line broadening and subsequent motional narrowing. The data are interpreted in terms of an anharmonic, hindered motion of the ferrocene sidegroup. In polyvinylferrocene this activation can be described by an Arrhenius law with barriers corresponding to about 15 kJ/mole separating at least three conformational states characterized by different hyperfine interactions. The anomalous change of f arises from a phase incoherence during gamma-ray absorption due to hopping of the iron between different positions. The maximum distance between these iron sites is estimated to be about 0.44 Å. A further decrease of f near the glass transition (190 K) of the swollen copolymer indicates a widening of the outer dimension of the anharmonic conformational potential to about 0.95 Å.Work supported by Bundesministerium für Forschung und Technologie.     

Effect of Zn–Ti double substitution on the electrical properties of Bi4V2O11

New samples of the Bi₂Zn_0.1–xTi_xV_0.9O_5.35+x; 0.02 ≤ x ≤ 0.08 system have been synthesized through a standard solid-state reaction route. XRD analysis and differential thermal analysis have been used to characterize the phase structure of samples. The γ′ phase is stabilized to room temperature in all investigated samples. The electrical properties of the BIZNTIVOX system have been studied by using AC impedance spectroscopy. An AC impedance response as a function of frequency (20 Hz–1 MHz) has been used to investigate the electrical conductivity and the dielectric permittivity in the temperature range of 150 °C–700 °C. In this temperature range, the phase transition γ′ to γ has been observed in all the compositions studied. AC impedance spectroscopy indicates that the resistance of samples decreases with increase of temperature. The ionic conductivity of samples appeared as a two-line region in Arrhenius dependence. At 300 °C, the highest ionic conductivity is shown by the composition x = 0.05 (σ₃₀₀ = 1.35 × 10^?4 S cm^?1).     

Effect of pressure on Néel temperature and hyperfine interactions in EuTe studied by Mössbauer spectroscopy

The pressure dependence of the Néel temperature T_N, the isomer shift S and the magnetic hyperfine field B_hf were measured in EuTe up to 60 kbar by means of the 22 keV Mössbauer resonance of ¹⁵¹Eu. A smooth increase of T_N was found, which yields λ_m = − dlnT_N/dlnV = 0.6(3). The variation of T_N and B_hf with pressure are discussed in terms of the exchange parameter J₂.     

Effect of random magnetic fields on the hyperfine structure of Mössbauer spectra in paramagnetic single crystals

Under certain conditions, the⁵⁷Fe Mössbauer spectra of Al(NO₃)₃·9H₂OFe³⁺ single crystals exhibit new lines [1,2] (so-called z-lines) at an unusually high velocity of about 10.5 mm/s. The dependence of the positions and intensities of these z-lines on the magnitude and direction of an external magnetic field has been investigated. The appearance of the z-lines is explained by small random magnetic fields arising from magnetic nuclei in the crystal.     

Effect of vanadium neighbors on the hyperfine properties of iron–vanadium alloys

The electronic and magnetic structures of Fe–V alloys are calculated using the discrete-variational and full-potential linearized-augmented-plane wave methods. The derived hyperfine properties at Fe sites are studied against the number of Fe atoms in the neighbouring shells. As expected the magnetic hyperfine field depends strongly on the number of Fe atoms in the first and second shells of neighbours while its dependence on the variation of atoms in the third shell is weak. The calculated distribution of the magnetic hyperfine fields at the Fe sites, are compared to the experimental data of Krause et al. (Phys Rev B 61:6196–6204, 2000). The contact charge densities and the magnetic moments are also calculated. It was found that the contact charge density increases with increasing V contents and this leads to negative isomer shift on addition of V.     

Studies on spintronics-related thin films using synchrotron-radiation-based Mössbauer spectroscopy

Applications of synchrotron-radiation-based Mössbauer spectroscopy in the “energy domain” to the studies on magnetism of thin films are introduced on the basis of the experiments recently performed at SPring-8, Japan. The measured samples are spintronics-related thin films, such as Co₂MnSn films, layered Fe/Cr films, layered Fe/Fe₃O₄ films, and Fe₄N films. The validity of the energy domain measurements is demonstrated in the light of industrial applications of magnetic thin films.     

Study on chemical reactions of isolated Mössbauer probes in solid gas matrices using in-beam Mössbauer spectroscopy

In-beam Mössbauer spectra of ⁵⁷Fe after ⁵⁷Mn (T _1/2= 1.45 min) implantation into a CH ₄ matrix and mixture matrices of CH ₄ and Ar at 18 K were measured. The spectrum obtained in the CH ₄ matrix was analyzed well with a doublet and a singlet peaks. These components were assigned to two constitutional isomers of Fe(CH ₄)\(_{\mathrm {2}}^{\mathrm {+}}\) as derived from density functional theory calculations and the Mössbauer parameters. In the case of the low concentration of CH ₄ with an Ar matrix, the Fe ⁺ (3d ⁷4s) in the excited state atomic configuration and Fe ⁺ (3d ⁶4s ¹) in the ground state were observed, as observed in our previous implantation experiment into Ar and Xe matrices. The formation yields of Fe(CH ₄)\(_{\mathrm {2}}^{\mathrm {+}}\) are discussed in term of the number of first-nearest-neighbor CH ₄ molecules around an Fe ⁺ ion.     

Mössbauer spectroscopy of zirconium alloys

Mössbauer investigations of zirconium alloys were examined. Data about the chemical state of iron atoms in the zirconium alloys of different composition has been provided. Mössbauer spectroscopy revealed that small quantities of iron in binary zirconium alloy are in the solid solution α-Zr (up to 0.02 wt.%). Different iron atoms concentration and thermo-mechanical treatments may lead to formation the intermetallic compounds Zr₃Fe, Zr₂Fe, ZrFe₂. Adding tin atoms does not affect the formation and shape of Mössbauer spectra of these compounds. Adding Cr and Nb atoms makes significant changes in the shape of Mössbauer spectra and leads to the formation of complex intermetallic compounds. Adding Cu and W atoms, the shape of the binary alloys spectra (Zr-Fe) remains unchanged, but a change in the temperature dependence behavior of the spectral parameters occurs and also, changes to the properties of the alloys.     

Application of Mössbauer spectroscopy on corrosion products of NPP

Steam generator (SG) is generally one of the most important components at all nuclear power plants (NPP) with close impact to safe and long-term operation. Material degradation and corrosion/erosion processes are serious risks for long-term reliable operation. Steam generators of four VVER-440 units at nuclear power plants V-1 and V-2 in Jaslovske Bohunice (Slovakia) were gradually changed by new original “Bohunice” design in period 1994–1998, in order to improve corrosion resistance of SGs. Corrosion processes before and after these design and material changes in Bohunice secondary circuit were studied using Mössbauer spectroscopy during last 25 years. Innovations in the feed water pipeline design as well as material composition improvements were evaluated positively. Mössbauer spectroscopy studies of phase composition of corrosion products were performed on real specimens scrapped from water pipelines or in form of filters deposits. Newest results in our long-term corrosion study confirm good operational experiences and suitable chemical regimes (reduction environment) which results mostly in creation of magnetite (on the level 70 % or higher) and small portions of hematite, goethite or hydrooxides. Regular observation of corrosion/erosion processes is essential for keeping NPP operation on high safety level. The output from performed material analyses influences the optimisation of operating chemical regimes and it can be used in optimisation of regimes at decontamination and passivation of pipelines or secondary circuit components. It can be concluded that a longer passivation time leads more to magnetite fraction in the corrosion products composition.     

Temperature Dependence of the Mössbauer Effect on Prussian Blue Nanowires

Highly ordered Prussian blue nanowires with diameter of about 50 nm and length up to 4 m have been fabricated by an electrodepositing technology with two-step anodizing anodic aluminum oxide films. The Mössbauer spectra taken between 15 and 300 K indicate that the hyperfine parameters decrease as the temperature increases. The temperature dependence of the quadrupole splitting, the isomer shift and the spectra area are discussed. A decrease of Debye temperature for Prussian blue nanowires was found with respect to that of Prussian blue bulk.     

Study of the FeII ion role in the magnetic properties of the AFM ring Cr7Fe using Mössbauer spectroscopy

In recent years the synthesis of antiferromagnetic rings traced a path to the observation of interesting quantum coherence phenomena in heterometallic Single Molecule Magnets. Because of the presence of different magnetic centers, it is crucial to understand the distribution of the molecular spin on all the sites. ⁵⁷Fe Mössbauer spectroscopy is an efficient and powerful technique to contribute to this knowledge in antiferromagnetic wheels containing iron ions. We analyze the Cr₇Fe^II wheel: the Mössbauer line shape evolution of low temperature spectra (2.1 K) at different external fields is illustrated and the mean spin value of the iron ion is evaluated.     

Auto-combustion synthesis,Mössbauer study and catalytic properties of copper-manganese ferrites

Spinel ferrites with nominal composition Cu _0.5Mn _0.5Fe ₂ O ₄ and different distribution of the ions are obtained by auto-combustion method. Mössbauer spectroscopy, X-ray Diffraction, Thermogravimetry-Differential Scanning Calorimetry, Scanning Electron Microscopy and catalytic test in the reaction of methanol decomposition is used for characterization of synthesized materials. The spectral results evidence that the phase composition, microstructure of the synthesized materials and the cation distribution depend on the preparation conditions. Varying the pH of the initial solution microstructure, ferrite crystallite size, cation oxidation state and distribution of ions in the in the spinel structure could be controlled. The catalytic behaviour of ferrites in the reaction of methanol decomposition also depends on the pH of the initial solution. Reduction transformations of mixed ferrites accompanied with the formation of Hägg carbide χ-Fe ₅ C ₂ were observed by the influence of the reaction medium.     

Mössbauer spectroscopy in the investigation of new mineral–related materials

New materials based on the composition of the mineral schafarzikite, FeSb $_{2}\textit {O}_{4}$ , have been synthesised. $^{57}$ Fe- and $^{121}$ Sb- Mössbauer spectroscopy shows that iron is present as Fe $^{2+}$ and that antimony is present as Sb $^{3+}$ . The presence of Pb $^{2+}$ on the antimony sites in materials of composition FeSb $_{1.5}$ Pb $_{0.5}\textit {O}_{4}$ induces partial oxidation of Fe $^{2+}_{}$ to Fe $^{3+}$ . The quasi-one-dimensional magnetic structure of schafarzikite is retained in FeSb $_{1.5}$ Pb $_{0.5}\textit {O}_{4}$ and gives rise to weakly coupled non-magnetic Fe $^{2+}$ ions coexisting with Fe $^{3+}$ ions in a magnetically ordered state. A similar model can be applied to account for the spectra recorded from the compound Co $_{0.5}$ Fe $_{0.5}$ Sb $_{1.5}$ Pb $_{0.5}\textit {O}_{4}$ .