Increase In Isomer Shift Research Articles

Structural transformations of low-nickel Fe-Ni alloys probed by Mössbauer spectroscopy

The structure and magnetic state of Fe100-xNix alloys (x ≤ 20) quenched from 1100 °С have been studied by means of Mössbauer spectroscopy, X-ray diffraction, transmission electron microscopy, and magnetization measurements. The concentration dependences of the lattice parameter of bcc structure and of spontaneous magnetic moment were measured at room temperature. From the analysis of Mössbauer spectra, concentration dependences of average hyperfine parameters have been determined. Both isomer shift and hyperfine field increase with increasing Ni content in the alloy. Fitting of the spectra with a set of subspectra gives grounds to conclude that the structure of quenched Fe-Ni samples represents a system of bcc regions of varying content, which is formed as a result of separation of the alloy composition. This conclusion is supported by the data of transmission electron microscopy.

Study on variable temperature Mössbauer spectra of GdFe0.5Cr0.5O3 perovskite

ABSTRACT The synthesis of GdFe0.5Cr0.5O3 by sol-gel method was reported, and the cooling process of the sample was studied by Mössbauer spectroscopy. The Mössbauer spectra at room temperature reflect the characteristics of GdFe0.5Cr0.5O3 with orthogonally distorted perovskite structure, indicating that its Fe3+ is a triangular or tetragonal distorted octahedron symmetric structure. With the decrease of temperature, the isomer shift and quadrupole splitting increase. The spectral transition from the doublet state at 250 K to the sextet state at 120 K reflects the magnetic phase transition in this temperature range. The observed magnetic field is a super-transferred hyperfine field (STHF) formed by the superexchange mechanism of oblique antiferromagnetic ordered Cr atoms.

Study of Structural, Magnetic, and Mossbauer Properties of Dy2Fe16Ga1−xNbx (0.0 ≤ x ≤ 1.0) Prepared via Arc Melting Process

Intermetallic compounds of Dy2Fe16Ga1−xNbx (x = 0.0 to 1.00) were synthesized by arc melting. Samples were investigated for structural, magnetic, and hyperfine properties using X-ray diffraction, vibration sample magnetometer, and Mossbauer spectrometer, respectively. The Rietveld analysis of room temperature X-ray diffraction data shows that all the samples were crystallized in Th2Fe17 structure. The unit cell volume of alloys increased linearly with an increase in Nb content. The maximum Curie temperature Tc ~523 K for x = 0.6 sample is higher than Tc = 153 K of Dy2Fe17. The saturation magnetization decreased linearly with increasing Nb content from 61.57 emu/g for x = 0.0 to 42.46 emu/g for x = 1.0. The Mössbauer spectra and Rietveld analysis showed a small amount of DyFe3 and NbFe2 secondary phases at x = 1.0. The hyperfine field of Dy2Fe16Ga1−xNbx decreased while the isomer shift values increased with the Nb content. The observed increase in isomer shift may have resulted from the decrease in s electron density due to the unit cell volume expansion. The substantial increase in Tc of thus prepared intermetallic compounds is expected to have implications in magnets used for high-temperature applications.

Preparation and Characterization of Amorphous Fe-B-P Ultrafine Particles

Soft magnetic Fe-B-P ultrafine particles with amorphous structure were prepared by aqueous reduction method with variable pH values. The microstructure, compositions and the soft magnetic properties of Fe-B-P submicron particles were investigated by using field emission scanning electron microscope (FE-SEM), X-Ray Diffraction (XRD), mössbauer spectrum (MS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), vibrating sample magnetometer (VSM). The results showed that a series of Fe-B-P amorphous particles with the average particle size of 0.26~0.42 μm were obtained depending on the pH value of solution. As pH value increased, the contents of B and P increased, and the specific saturation magnetization of the Fe-B-P submicron particles decreased. Some electrons, transferred from p-shell of B atoms and P atoms to 3d-shell of Fe atom, resulted in the decrease of the number of unpaired electron, which in turn led to reduction of the average hyperfine magnetic field of the Fe-B-P particles and the decrease of the magnetic torque of Fe atoms. The relation between hyperfine magnetic field (Hhf) and the specific saturation magnetization was in accord with the Marshall’s equation. At the same time, the enhancement of the shielding effect on 4s electron orbital, i.e., the density of 4s electron reduced, leading to the increase of Isomer Shift.

Structure and hyperfine interactions in Bi1−xNdxFeO3 solid solutions prepared by solid-state sintering

The series of polycrystalline ceramic powders, Bi1−xNdxFeO3 (x=0.1−1) was successfully synthesized by mixed oxide method followed by pressureless sintering. X-ray diffraction and Mössbauer spectroscopy were used as complementary methods to study structure and hyperfine interactions of the samples. It was found that with an increase of Nd content, within the range of x=0.2−0.3 a structural phase transition from rhombohedral to orthorhombic system occurs. The lattice parameters and unit-cell volume decrease with an increase of Nd concentration. The Mössbauer spectra registered for Bi1−xNdxFeO3 are characterized by sextets with slightly broadened lines and reveal distributions of hyperfine magnetic fields. It was found that the value of the hyperfine magnetic field induction monotonically increases with Nd content. The mean values of isomer shift are typical for Fe3+ ions in a high spin state and slightly decrease from 0.40mms−1 for x=0.3 to 0.37mms−1 for x=0.9. The small value of quadrupole shift confirms octahedral coordination of the iron ions. The structural transition is accompanied by an increase of isomer shift and a change of sign of quadrupole shift.

Low-temperature Crystal Structure and 57Fe Mössbauer Spectroscopy of Sr3Sc2O5Fe2As2

The crystal structure of the layered iron arsenide Sr3Sc2O5Fe2As2 was determined between 300 and 10 K. The lattice parameters of the tetragonal cell decrease anisotropically according to Δc/c : Δa/a ≈ 4.2, which results in a slight flattening of the As-Fe-As bond angle within the FeAs layers. No indication of a structural instability could be detected. 57Fe Mössbauer spectroscopic data show only a single signal at 4.2, 77, and 298 K, subjected to quadrupole splitting. The isomer shift increases from 0.36(1) mms−1 at 298 K to 0.49(1) mms−1 at 4.2 K. No indication for magnetic ordering was found.

Spin- and charge density around Rh impurity in α-Fe studied by 57Fe Mössbauer spectroscopy

Random solution of rhodium in ferromagnetic α-Fe of the BCC structure has been investigated by means of the 14.4-keV Mössbauer transition in 57Fe at room temperature. Rhodium atoms have been randomly substituted on the iron sites with the concentration up to 15 at.%. Contributions to the hyperfine field and isomer shift on the iron nuclei have been determined as the function of the distance between iron nucleus and rhodium impurity up to the third co-ordination shell. Rhodium atom as the nearest iron neighbor changes iron hyperfine field by +0.73 T, as the second neighbor makes change by +0.70 T and finally as the third neighbor changes the field by +0.43 T. Corresponding changes in the isomer shift are as follows: −0.015 mm/s, −0.005 mm/s and +0.039 mm/s. The average hyperfine field and the isomer shift increase linearly versus rhodium concentration at rates 0.156 T/at.% and 4.42 × 10 −3 mm/(s at.%), respectively. Hence, addition of rhodium lowers the electron density ρ on the iron nucleus at the rate ∂ ρ/∂ c = −1.52 × 10 −2 electron a.u. −3 (at.%) −1.

Electron transfer rate and structure of [Fe2(bpmp)(ppa)2](BF4)2

Mixed-valence μ-phenolate-bis(μ-carboxylate) diiron(II, III) complex [Fe2(bpmp)(ppa)2](BF4)2 (1) is prepared. The temperature dependence of the Mossbauer spectra of the complex 1 is measured. The Mossbauer parameters of the complex 1 show the unusual increase of isomer shift and quadrupole splitting with an increase in temperature. The spectra are explained by the dynamic electronic states in which the valence states of two iron atoms in a molecule are 2.2 and 2.8 on the Mossbauer time scale at 293 K. The single crystal X-ray structure of the complex 1 is determined at 133 K. The complex 1 crystallizes in the triclinic space group \(\bar P\)1. Two iron atoms in the complex 1 are crystallographycally inequivalent.

Relationship between structure and intrinsic magnetic parameters of nanocrystalline Sm 2(Fe,Ga) 17C 2

The nanocrystalline carbides Sm 2Fe 17− x Ga x C 2 ( x ≤ 2) were prepared with a two-step method: high-energy milling with annealing at 1050 °C and carbonation at 420 °C by a solid–solid reaction with heavy hydrocarbon. The Rietveld analysis reveals an anisotropic volume expansion of the R 3 ¯ m unit cell with modifications of the atomic positions compared to those of the non-carbonated alloys. The average iron magnetic moment, for carbonated alloys, deduced from high magnetic field investigations at 4 K, decreases slightly with x. The Curie temperature T C enhancement after carbonation is reduced by the presence of Ga. T C follows the reduction of the Wigner–Seitz cell (WSC) volume of the carbon site affected also by the presence of Ga. A detailed analysis of the Mössbauer spectra has been carried out with a model based on the WSC volume correlation with isomer shift and the statistical distribution of Ga simulated by the multinomial law. The isomer shift sequence of each Fe non-equivalent site, δ{6c} > δ{18h} > δ{18f} > δ{9d} for x < 1 and δ{18h} > δ{6c} > δ{18f} > δ{9d} for x ≥ 1, reflects the perturbation brought about by C and Ga, in perfect agreement with the structural analysis. Upon carbonation the isomer shift increases for 18f and 18h results not only from the volume effect but also from the 4s(Fe)–2p(C) inter-atomic charge transfer, modulated by the 4s(Fe)–4p(Ga) transfer. The hyperfine field sequence H{6c} > H{9d} > H{18f} > H{18h}, deduced from the isomer shift assignment, is in agreement with the number of Fe neighbors.

Combined effect of gallium and carbon on the structure and magnetic properties of nanocrystallineSmFe9

SmFe9−yGaxC carbides(0≤x≤1) were prepared by a two-step method of powder high-energy milling with annealing between873 and 1200 K and a subsequent carbonation with heavy hydrocarbon at 693 K. The x-raydiffraction analysis with the Rietveld technique indicates an anisotropic volume expansionunder carbonation slackened by the gallium substitution. The average saturation magneticmoment per Fe atom, measured with an applied field up to 200 kOe, increases slightly to2.00 µB with Ga content. The Curie temperature, always 15 K above that of the homologous2/17 carbides, is reduced by a dilution effect with Ga substitution and a concomitant increase of thenegative Fe–Fe interactions. The Mössbauer spectra have been analysed on the basis of thebinomial law related to the different Fe environments. The hyperfine parameter assignmentto each individual crystallographic site was performed according to the correlation betweenisomer shift and Wigner–Seitz cell volumes calculated with a specific code in theP 1 space group allowing the splitting of the partially occupied atomic positions. The role ofthe C insertion is shown, on the one hand, by the increase of isomer shift dueto the volume expansion with an enhancement of the charge transfer from therare-earth atoms and, on the other hand, by the hyperfine field increase, pointing outthe predominance of the negative core electron polarization term. The coercivefield of 27 kOe, combined with a Curie temperature of 680 K, makes the alloySmFe8.75Ga0.25C promising for further applications in the field of high-performance permanent magnets.

High-Pressure Mössbauer Measurements on Nanocrystalline Perovskite (La,Sr)(Mn,Fe)O3

We report here the Mossbauer measurements on nanocrystalline perovskite structured manganite La0.8Sr0.2Mn0.8Fe0.1957Fe0.01O3 as a function of pressure up to 10 GPa at room temperature. The nanocrystalline sample, prepared by sol–gel technique found to have crystallite sizes of ∼138 ± 10 A. Zero-field electrical resistivity measurements with temperature support the nanocrystalline nature. At ambient pressure, Fe3+ as well as Fe4+ ions are distributed in two different environments – Fe3+ in low symmetric site surrounded by Mn3+ ions only while Fe4+ in high symmetric site with at least one Mn3+ ion. Pressure seems to affect the higher symmetric site. A sudden increase in isomer shift at 0.52 GPa indicates the first order phase transition representing the transformation of Fe4+ to Fe3+. Another transition at 3.7 GPa, represents the presence of Fe3+ in single kind of environment. Pressure dependence of electrical resistivity measurements verifies the transitions attributing the first order transition to the cross over of localized-electron to band magnetism.

Atomic Arrangement in B2 FeAl Prepared by Self-Propagated High-Temperature Synthesis at Varying Al Content and Annealing

Atomic arrangement in B2 FeAl prepared by self-propagated high temperature synthesis (SHS) as a function of Al concentration and annealing temperature has been studied by 57Fe Mossbauer spectroscopy (MS) and X-ray diffraction (XRD). The increase of B2 FeAl isomer shift (IS) and lattice parameter (a) with Al concentration in the whole concentration range has been detected. This may originate from the formation of Al antisite atoms. Calculation of s-electron populations against number of Al antisites using a cluster approach and MO LCAO method supported this assumption. Annealing resulted in atomic rearrangements both in near-stoichiometric and Al-rich B2 FeAl.

Hard magneticSm(Fe,Si)9carbides: Structured and magnetic properties

The structural and magnetic properties of the metastable hexagonal ${\mathrm{SmFe}}_{9\ensuremath{-}x}{\mathrm{Si}}_{x}\mathrm{C}$ compounds, where x is 0.25, 0.5, 0.75, and 1, have been investigated by means of powder x-ray diffraction, Curie temperature, magnetic moment and coercivity measurements, iron-57 M\"ossbauer spectroscopy, and high-resolution transmission electron microscopy. The Rietveld analysis points out a lattice expansion after carbon insertion. Upon carbonation the Curie temperatures are systematically 26 to 70 K higher than those of the homologous ${\mathrm{Sm}}_{2}(\mathrm{Fe},\mathrm{Si}{)}_{17}{\mathrm{C}}_{2}.$ The magnetic moment per iron atom increases with x but remains below that of the non-carbonated alloys. The statistical occupation of silicon in $3g$ site and the random distribution of the $2e$ dumbbell iron atoms have been taken into account to calculate the Wigner-Seitz cell volumes, which rule the hyperfine parameter assignment. The following sequence of isomer shift $\ensuremath{\delta}{2e}&gt;\ensuremath{\delta}{3g}&gt;\ensuremath{\delta}{6l}$ is used to deduce the following sequence of hyperfine field ${H}_{\mathrm{HF}}{2e}&gt;{H}_{\mathrm{HF}}{6l}&gt;{H}_{\mathrm{HF}}{3g}.$ From $\mathrm{Sm}(\mathrm{Fe},\mathrm{Si}{)}_{9}$ to their carbides, the increase of isomer shift is less pronounced for the $2e$ site with no carbon neighbor. The small volume effect on the weighted average isomer shift may indicate hybridization between $2p$ of carbon and $3d$ of iron stronger than that of nitrides. The best coercivity of 15 kOe is obtained for ${\mathrm{SmFe}}_{8.75}{\mathrm{Si}}_{0.25}\mathrm{C}$ with annealing temperature of the noncarbonated powder at $750\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ However, ${\mathrm{SmFe}}_{8.5}{\mathrm{Si}}_{0.5}\mathrm{C},$ with 13 kOe and an optimal grain size around 22 nm, presents a better thermal stability and might be suitable for permanent-magnet applications.

Synthesis and Mössbauer Study of Maghemite Nanowire Arrays

Arrays of γ-Fe2O3 nanowire were synthesized in anodic aluminum oxide templates. The structure, morphology and magnetic property at room temperature were characterized. Temperature-dependent Mossbauer spectra was collected and the superparamagnetic relaxation was clearly observed. Both hyperfine field and isomer shift increase with decreasing temperature. The anisotropy energy constant is determined from the reduction of the hyperfine field relative to the saturation value caused by the collective magnetic excitations.

57FeMössbauer spectroscopy ofHoErFe17−xGaxCycompounds

${}^{57}\mathrm{Fe}$ M\ossbauer studies were carried out on the phases ${\mathrm{HoErFe}}_{17\ensuremath{-}x}{\mathrm{Ga}}_{x}$ $(x=0\char21{}7)$ and ${\mathrm{HoErFe}}_{17\ensuremath{-}x}{\mathrm{Ga}}_{x}\mathrm{C}$ $(x=0,$ 1, and 2) at 25 K and room temperature. The weighted average hyperfine field at room temperature is found to increase initially for low Ga content and then decreases for phases with higher Ga concentration. The hyperfine fields at Fe sites follow the sequence $4f(6c)g6g(9d)g12j(18f)g12k(18h),$ which is determined mostly by the number of Fe and R nearest neighbors. The variation of the weighted average hyperfine field with temperature is found to obey ${T}^{2}$ behavior, suggesting the presence of single-particle excitations. Insertion of carbon at interstitial sites is found to increase the individual site hyperfine fields and hence an increase in the weighted average hyperfine field. The $12j$ and $12k$ sites of Fe show significant increase in isomer shift compared to other Fe sites.

A magnetic and conversion electron Mossbauer spectral study of amorphous Dy/sub 20/Fe/sub 80-y/-Co/sub y/ thin films

Amorphous thin films of Dy/sub 20/Fe/sub 80-y/Co/sub y/, with 0<y<20 and of ca. 40 nm thickness, have been prepared by sputtering on polyimide substrates. The 295 K conversion electron Mossbauer spectra (CEMS) of these films consist of broadened sextets which have been analyzed with a distribution of hyperfine fields in which the iron moments are oriented perpendicular to the plane of the film, an orientation which is in agreement with the perpendicular magnetic anisotropy observed herein at 295 K by vibrating sample magnetometry. The average hyperfine field and isomer shift increase linearly with increasing Co content. Both increases arise from an increase in the electron occupation of the 3d states as the cobalt content increases.

Structural and magnetic studies of sputtered Fe1−xCrx thin films

X-ray diffraction, magnetization, and Mössbauer effect investigations have been performed for sputtered Fe1−xCrx (0⩽x⩽0.54) thin films. The body-centered-cubic (bcc) phase appears for x&amp;lt;0.32, while the σ phase is formed for 0.38⩽x⩽0.44. For 0.32⩽x&amp;lt;0.38 and 0.44&amp;lt;x⩽0.54, the samples are composed of both bcc and σ phases. As the Cr concentration increases, the ferromagnetic fraction, magnetization, and magnitude of the hyperfine field decrease, whereas the magnitude of the isomer shift increases. The Fe and Cr magnetic moments, isomer shift, and hyperfine field of the bcc Fe–Cr phase have been deduced and are discussed consistently in terms of charge and spin distributions, as well as magnetic valence.

A Mössbauer spectral study of Tb2Fe17 and the Tb2Fe17−xSix solid solutions

The Mössbauer spectra of a series of rhombohedral Tb2Fe17−xSix solid solutions, with x equal to 0, 1, 2, and 3, have been measured as a function of temperature. Although the spectra of Tb2Fe17 change substantially upon cooling from 295 to 85 K, it has been possible to fit them with a consistent seven sextet model corresponding to a basal magnetization. The spectral analysis yields reasonable hyperfine parameters and the expected changes with temperature. The resulting weighted average effective iron recoil mass of 66 g/mol and the Mössbauer temperature of 395 K are typical of this type of intermetallic compound. In addition, the isomer shifts and the hyperfine fields observed for the crystallographically distinct iron sites in Tb2Fe17 agree well with those expected from the differences in the Wigner–Seitz cell volumes and the near-neighbor environments of the four sites. The spectra of the silicon substituted solid solutions have been fit with the same model and similar hyperfine parameters, but with a binomial distribution of near-neighbor environments. The weighted average hyperfine field decreases slowly with increasing silicon content, whereas the isomer shift increases.

Magnetic and Mossbauer studies in Fe/Au multilayers

A series of Fe/Au multilayers have been sputter deposited and their magnetic and Mossbauer properties studied. The magnetization shows a clear tendency to increase for Fe layers t(Fe) thinner than 12 /spl Aring/. For these samples the magnetic moment of Fe at 5 K is calculated to be 2.64 /spl mu/B which is about 20% higher than that of bulk Fe. The surface anisotropy at 295 K is found to be +0.38 erg.cm/sup -2/. Due to large demagnetization effect all samples show in-plane magnetization. The hyperfine field (32.6-32.8 T) for all the samples studied is close to that of bulk Fe. For samples with t(Fe) /spl les/15 /spl Aring/, the line width and the isomer shift increase showing the presence of at least two types of Fe sites, one in the bulk of the layer and the other near the interface.

A magnetic and Mössbauer spectral study of the Tb 2Fe 17− xAl x solid solutions

The magnetic properties of a series of Tb 2Fe 17− x Al x solid solutions, with x equal to 0.00, 1.00, 1.98, 3.14, 4.08, 5.10, 6.06, 7.16, and 8.12, have been studied by magnetic measurements and Mössbauer spectroscopy. Magnetization studies indicate that the Curie temperature increases from 420 K in Tb 2Fe 17 to a maximum of 475 K in Tb 2Fe 13.86Al 3.14. The magnetization results indicate an antiferromagnetic coupling of the terbium sublattice with the iron sublattice at low temperature. The magnetically ordered Mössbauer spectra have been fit with a binomial distribution of near neigbors. The weighted average maximum hyperfine field, H max, decreases by 12 kOe per aluminum at 85 K. The decrease in hyperfine field is due to the dilution of the magnetic moments by aluminum. The weighted average decremental field, Δ H, decreases by 2 and 1 kOe per aluminum at 85 and 295 K, respectively. The compositional dependence of the decremental field indicates the presence of RKKY type spatial spin oscillation in the 4s conduction band, an oscillation which is modified by the presence of aluminum. The weighted average isomer shift increases by 0.016 and 0.010 mm/s per aluminum at 85 and 295 K, respectively, because of both the screening of the 4s electrons by the 3d electrons due to interband mixing of the 3d band with the valence band of aluminum, and the unit cell volume expansion upon aluminum substitution.