Average Isomer Shift Research Articles

ICEMS Characterization of NixFe3-xO4 (0.7 ≤ x ≤ 1.7) thin films grown by ion beam sputtering on (0001) Al2O3 substrates

We report here on the ICEMS characterization of nickel ferrite (NixFe3−xO4) thin films having different nickel contents grown on alumina substrates by Ion Beam Sputtering. The spectra corresponding to the films with nominal x = 0.7, 1.0 and 1.2 are characteristic of compounds crystallizing in a spinel-related structure showing two different magnetic sextets associated with Fe3+ located in the tetrahedral and octahedral sites of such structure. The spectra show an additional broad third sextet with a large isomer shift which suggests the occurrence of electron hopping between Fe2+ and Fe3+ ions sitting in the octahedral sites. With increasing nickel content, the linewidth of the sextets increases and their corresponding hyperfine magnetic fields decrease. This is an indication of an increase in structural disorder in the deposited films as their nickel concentrations increase. The cation distribution of the iron ions over the tetrahedral and octahedral sites appears also to depend on the nickel content. The film with x = 1.2 shows a significant increase in the fraction of octahedral iron ions as compared with the expected nominal value suggesting that, for this composition, some Ni2+ could also occupy tetrahedral sites. The Mössbauer spectrum corresponding to the film with x = 1.7 shows a magnetic pattern with very broad lines similar to those shown by amorphous or disordered materials. The average isomer shift is quite high (around 0.40 mms− 1) and characteristic of Fe3+ in octahedral oxygen coordination. This indicates that for the largest nickel content studied (x = 1.7), the film does not contain Fe3+ in tetrahedral environments suggesting that the spinel structure is no longer present. This correlates well with the X-Ray Diffraction data which indicate a structural change from spinel to a disordered rock-salt structure for this particular film with high nickel content.

Microstructure of High‐Chromium Ferritic–Martensitic Steels for Next‐Generation Reactors

High‐chromium steels are key alloys used for the construction of technological devices in industry. Stainless steels are suitable for components that are exposed to a corrosive environment for a long time because chromium has anticorrosion properties due to segregation of chromium and the formation of a passivation layer. The physicochemical properties of the surface and the bulk of the material as well are determined by microstructure. Herein, steel NS219 is focused on, where the chromium concentration is around 13.5 wt%. In order to study the microstructure of steel, Mössbauer spectroscopy is used. Experimental results are evaluated using the binomial distribution model of the probability distribution of atoms in the nearest neighbor of the resonant atom 57Fe. Obtained spectral parameters, viz., the average magnetic hyperfine field, the average isomer shift, and the probability of the atomic configuration with no impurity atoms in the two‐shell vicinity of the iron atoms, reach saturation values from which the solubility limit of chromium in iron can be determined. On the other hand, the solubility limit of iron in Cr‐rich phase can be estimated from the value of the isomer shift of the single‐line in the spectrum annealed for the longest time.

Kinetics of Transformation, Border of Metastable Miscibility Gap in Fe–Cr Alloy and Limit of Cr Solubility in Iron at 858 K

The study was aimed at determination of the position of the Fe-rich border of the metastable miscibility gap (MMG) and of the solubility limit of Cr in iron at 858 K. Towards this end, a Fe73.7Cr26.3 alloy was isothermally annealed at 858 K in vacuum up to 8144 hours and Mössbauer spectra were recorded at room temperature after every step of the annealing. Three spectral parameters viz. the average hyperfine field, 〈B〉, the average isomer shift, 〈IS〉, and the probability of the atomic configuration with no Cr atoms in the two-shell vicinity of the probe Fe atoms, P(0,0), gave evidence that the transformation process took place in two stages. All three parameters could have been well described in terms of the Johnson–Mehl–Avrami–Kolmogorov equation, yielding kinetics parameters. The first stage, associated with the phase decomposition, proceeded much faster than the second stage, associated with the alpha-to-sigma phase transformation. The most reliable estimation of the position of the MMG and that of the value of the Cr solubility limit was obtained from the annealing time dependence of 〈B〉, namely 24.5 at. pct Cr for the former and 20.3 at. pct Cr for the latter. A comparison of these figures with the recent phase diagrams pertinent to Fe–Cr system was done.

Simple and precise method for determining relative amount of Fe3+ in Fe-gluconate

Reported is a simple and precise method for determining the relative amount of Fe3+ in a Fe-gluconate. The method is based on a linear relationship between the average isomer shift (spectrum's center of gravity) and the percentage content of the Fe3+ as found from Mössbauer spectra recorded on several samples of the Fe-gluconate and Ascofer® medicament. The new method has been tested by comparison the values of the relative abundance of the ferric ions obtained by a traditional way of the Mösbauer spectra analysis with those determined with the new method. The latter can be conveniently used in a pharmaceutical industry producing Fe-gluconate based medicaments for treating patients with iron deficiency anemia.

Correlation among local structure, magnetic, structural and electronic properties in polyol synthesized iron sulfide (FeS2) nanoparticles

A comparatively new and important cost and time effective Polyol method has been utilized for the preparation of FeS2nanoparticles (NPs), while their crystallite size, bonding as well as nearest neighbor atomic information was obtained from XRD, FTIR, Mössbauer, FE-SEM, EDX spectroscopy techniques. The average crystallite size was found to be ∼28 nm and a very small tangential strain due to the spherical shape of the particle was seen in the prepared sample. The bond length of Fe–S bond was estimated to be ∼0.29 nm and that of S–S bond was ∼0.22 nm using VESTA software confirming the Pyrite FeS2 structure in agreement with the XRD of these NPs. Also, the sample shows an octahedral geometry. Both XRD and Mössbauer spectra indicate that no impurity phase is present in these Pyrite NPs. Using Mössbauer, the average electric Quadrupole splitting (ΔEQ) and average isomer shift (δ), for Pyrite FeS2 at 300 K are found to be 0.596 ± 0.0039 mm/s and 0.311 ± 0.0023 mm/s respectively. From the Isomer shift, +2 oxidation state is confirmed. The FTIR revealed various vibrations modes of FeS2 nanoparticles (FeS, S–S and Fe–S bonds at their respective positions). Corresponding FE-SEM and EDX measurements revealed the formation of agglomerated spherical particles with the Fe:S ratio of ∼1:1.46 confirming the pyrite structure.

Synthesis and characterization of co-doped SrFe12−x(DyAl)xO19 hexaferrite

A series of (Dy-Al)3+ co-substituted SrFe12−x(DyAl)xO19 (0 ≤ x ≤ 0.9) hexaferrite were prepared via auto-combustion method with subsequent heat treatment in air at 950 °C for 10 h. The phase identification of the powders performed using x-ray diffraction show presence of single phase hexaferrite for x < 0.3 and presence of secondary phase, Fe2O3, for x ≥ 0.3. Lattice parameter of SrFe12−x(DyAl)xO19 remains unaffected upon doping mainly due to Fe3+ being replaced with a smaller size Al+3 and a larger size Dy+3 ions than Fe3+. (DyAl)+3 substitution modifies saturation magnetization, Ms, and coercivity, Hc. The room temperature Ms value decreased rather slowly with substitution while Hc increased to a maximum value of 5572 Oe at x = 0.9, which is ∼33% increase in coercivity as compare to that of pure Sr-ferrite. The Curie temperature, TC, was observed to decreases upon (DyAl)3+ substitution. Dielectric permittivity was improved with (DyAl)3+ substitution in 800 MHz to 14 GHz frequency range. The DC electrical resistivity of all samples decreased with temperature showing semiconducting behavior. The room temperature Mossbauer spectral analysis indicate that Al3+ prefers 12k, 2a, and Dy3+ prefers 4f1 sites. The weighted average hyperfine field and isomer shift decreases with (DyAl)3+ content due to magnetic dilution effect and increased charged density per unit cell, respectively. These results indicate that co-doping rare-earth and non-magnetic ion can together maintain high coercivity with fairly high magnetization value of the doped Sr-ferrite.

Magnetic properties of novel dendrimeric spin crossover iron(III) complex

The synthesis and magnetic properties of novel dendrimeric spin crossover Fe(III) complex of formula [Fe(L)2]+PF6−, where L=3,5-di(3,4,5-tris(tetradecyloxy)benzoyloxy)benzoyl-4-oxy-salicylidene-N′-ethyl-N-ethylenediamine have been studied for the first time by magnetic susceptibility, Electron Paramagnetic Resonance (EPR) and Mössbauer spectroscopy in the wide (2–300K) temperature range. EPR showed that the compound is magnetically inhomogeneous, consists of two magnetic sub-lattices, displays a partial spin crossover (S=5/2⇔1/2) of ∼25% of the Fe(III) molecules above 160K and undergoes the antiferromagnetic (AF) ordering below 10K. High-spin (HS, S=5/2) Fe(III) centers with weakly distorted octahedral environment most probably form chains in layers. The dimeric molecules, formed from low-spin (LS, S=1/2) centers and HS centers with strongly distorted octahedral environment are likely located between the layers and are involved in the spin crossover. EPR has shown the presence of AF dynamical spin clusters in the high temperature (70–300K) range, which are visible in the short time scale (10−10s) and could not be registered in the static magnetic measurements. Mössbauer spectra demonstrated in a paramagnetic state of the compound a quadrupole doublet with average isomer shift of 0.35mm/s and splitting 0.72mm/s corresponding to HS Fe(III) centers. Below 60K, the spectra displayed the appearance of magnetic hyperfine structure, whose relaxation nature testifies the collective spin flips of small clusters in the material. Mössbauer spectroscopy confirmed the existence of AF ordering in the Fe(III) dendrimeric complex at 5K.

Magnetic nanoparticles for in vivo use: a critical assessment of their composition.

Three different magnetic samples with particle sizes ranging from 10 to 30 nm were prepared by wet chemical methods. The powders were heated at 100, 150, 200, and 250 °C during 30 min under air. Ferrous and total iron contents were determined immediately after the synthesis and after the thermal treatments. All samples were characterized by X-ray diffraction, transmission and integral low-energy electron Mössbauer spectroscopy (ILEEMS) at 298 K. These samples are composed of a mixture of individual particles of maghemite and magnetite, which implies that once oxidation starts in this kind of material, it occurs throughout the entire particle volume. The existence of a maghemite/magnetite core-shell model was ruled out. A linear correlation between the average isomer shift and the magnetite content was found, allowing the estimation of the amounts of magnetite and maghemite in an unknown sample without the need of performing chemical analysis.

Experimental and theoretical study of the σ-phase Fe–Re alloys

X-ray diffraction (XRD) and Mössbauer spectroscopy techniques combined with theoretical calculations based on the Korringa-Kohn-Rostoker (KKR) electronic structure calculation method were used to investigate σ-phase Fe100−xRex alloys (x = 43, 45, 47, 49 and 53). Structural data such as site occupancies and lattice constants were derived from the XRD patters, while the average isomer shift and distribution curves of the quadrupole splitting were obtained from the Mössbauer spectra. Fe-site charge-densities and the quadrupole splittings were computed with the KKR method for each lattice site. The calculated quantities combined with the experimentally determined site occupancies were successfully used to decompose the measured Mössbauer spectra into five components corresponding to the five sublattices.

Hyperfine interactions and structural features of Fe–44Co–6Mo (wt.%) nanostructured powders

Nanocrystalline Fe–44Co–6Mo (wt.%) powders have been prepared by high-energy ball milling from elemental Fe, Co and Mo pure powders in a P7 planetary ball mill. The obtained powders were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Mössbauer spectrometry techniques. The influence of milling process and Mo substitution for Co in equiatomic FeCo have been examined in order to study structural evolution and formation mechanism of nanostructured Fe(CoMo) solid solution. XRD results show the formation of a BCC Fe(CoMo) solid solution (a = 0.2874 nm) where unmixed nanocrystalline Mo with a BCC structure is embedded. Disordered Fe(CoMo) solid solution is characterized by a broad hyperfine magnetic field distribution with two regions centered at B1 = 35.0 T and B2 = 30.7 T, respectively, attributed to disordered Fe(Co) solid solution and CoMo enriched environments. Prolonged milling and Mo addition cause the decrease of average hyperfine magnetic field while the average isomer shift remains nearly constant.

Magnetic and structural characterization of Mo-Hitperm alloys with different Fe/Co ratio

The influence of the Co content on the microstructure and magnetic behaviour of a series of amorphous and nanocrystalline (FeCo) 79Mo 8Cu 1B 12 alloys is reported. Changes in the magnetic properties provoked by the microstructural evolution upon different thermal treatments of as-cast samples are analyzed as well. Kinetics of nanocrystallization process can be described by an isokinetic approach. As the Co content in the alloy increases, the Curie temperature of the amorphous as-cast samples increases while the crystallization onset temperature decreases. The crystalline volume fraction as well as the mean grain size of the nanocrystals at the end of the nanocrystallization process are slightly higher for the lowest Co content alloy but smaller than in similar Hitperm Mo-free alloys. The average magnetic field and the average isomer shift of the as-cast samples show a linear increase with two slopes with the Co content of the alloy. The same tendency is found for the saturation magnetization. Combined TEM, XRD, and MS data indicate the presence of Mo atoms in the nanocrystals. At the end of the nanocrystallization process the softest magnetic behaviour corresponds to the lowest Co content alloy. The second crystallization process provokes a magnetic hardening of these alloys.

Hyperfine interactions and crystallization kinetics of Co-substituted NANOPERM-Type alloys

Evolution of structural changes after (as well as during) heat treatment and their consequence upon hyperfine interactions is discussed for (Fe1−xCox)79Mo8Cu1B12 and (Fe1-xCox)76Mo8Cu1B15 (0 ≤ x ≤ 0.75) alloys which eventually exhibit nanocrystalline arrangement. Along with Mössbauer spectrometry, results obtained from AFM, and MFM are reported. The kinetics of crystallization is characterized by in-situ diffraction of synchrotron radiation. Substitution of Fe by Co has a remarkable effect on the average hyperfine field and average isomer shift values in the as-quenched alloys. This indicates structural modifications which lead to increase of Curie temperature with higher Co contents. In nanocrystalline state, presence of bcc FeCo crystalline phase is identified. The progress of crystallization is shown to depend upon the annealing conditions as well as on the Co contents. Differences in the onset of crystallization on both sides of the ribbon-shaped samples are revealed. They correlate with the degree of quenched-in surface crystallization.

Magnetic phase transition in MnFeP0.5As0.4Si0.1

We have carried out a detailed investigation of the magnetic phase transition in MnFeP0.5As0.4Si0.1. Temperature hysteresis has been observed in the variable temperature magnetization curves (Bappl = 0.01 T) with TCW ~ 302 K on warming and TCC ~ 292 K on cooling. The first order nature of this transition in MnFeP0.5As0.4Si0.1 is confirmed by the negative slope obtained from isotherms of M2 versus B/M around the critical temperature. Linear thermal expansion measurements reveal a large volume change, ΔV/V~8.7×10−3 at the magnetic phase transition and that this magnetovolume effect is suppressed to ΔV/V ~ 5.5×10−3in an applied field of Bappl = 1.0 T. Analyses of 57Fe Mössbauer spectra (4.5 – 300 K) using a random distribution model and taking nearest-neighbour environments into account, indicate that the paramagnetic and ferromagnetic phases coexist over a temperature range of ~ 45 K around the Curie temperature. The Debye temperature for MnFeP0.5As0.4Si0.1 has been evaluated as θD = 350 ± 20 K from the temperature dependence of the average isomer shift.

Magnetic phase transitions in Pr1−xLuxMn2Ge2 compounds

The effects of replacing Pr by Lu on the magnetic behaviour and structures ofPr1−xLuxMn2Ge2 (x = 0.2,x = 0.4)have been investigated using x-ray diffraction, Mössbauer spectroscopy, magnetization and neutrondiffraction measurements. The substitution of Lu for Pr leads to a decrease in the lattice constantsa,c and the unitcell volume V at room temperature with this contraction of the unit cell resulting in modifications of thePr1−xLuxMn2Ge2 magnetic structures. Four and five magnetic phase transitions—linked primarilywith temperature driven changes in the intralayer Mn–Mn separationdistances—have been detected within the temperature range 4.5–550 K forPr0.8Lu0.2Mn2Ge2 and Pr0.6Lu0.4Mn2Ge2, respectively, with re-entrant ferromagnetism being detected aroundTCPr∼31 K forPr0.6Lu0.4Mn2Ge2. It was foundthat TCinter andTCPr increase with increasingapplied field while TNinter decreases for Pr0.6Lu0.4Mn2Ge2, indicating that the canted antiferromagnetic AFmc regioncontracts with increasing field. The Debye temperatures forPr1−xLuxMn2Ge2 withx = 0.2 and 0.4 wereevaluated as θD = 320 ± 40 K and θD = 400 ± 20 K respectively from the temperature dependence of the average isomer shift. The magneticstructures of both compounds have been determined by means of neutron diffractionmeasurements over the temperature range 3–300 K with formation of the Fmi magnetic state belowTc/c = 192 Kfor Pr0.8Lu0.2Mn2Ge2 and the occurrence of re-entrant ferromagnetism belowTCPr = 31 Kfor Pr0.6Lu0.4Mn2Ge2 being confirmed.

Mössbauer studies of GdFe2 − x Hf x alloys

GdFe2 − x Hf x alloys, where x = 0, 0.10, 0.15, 0.20, and 0.30, are produced by arc-melting of pure elements. The samples are investigated by x-ray diffraction and Fe57 Mossbauer spectroscopy at 78 K and 300 K. We find that the alloy system GdFe2 − x Hf x have the single phase cubic Cu2Mg type structure in the whole concentration range. Mossbauer spectroscopic results show that all the samples studied are magnetically ordered at 78 K, and at room temperature. The room temperature spectra are fitted with two magnetic components where the direction of magnetization is along the [111] while the spectra at 78 K are fitted with four magnetic subspectra indicating a complex direction of magnetization for all samples under investigation. The average magnetic hyperfine field and the average isomer shift are found to decrease almost linearly with increasing the Hf concentration at 78 K and 300 K due to the replacement of Fe by nonmagnetic Hf.

Hexanuclear Iron(III) Salicylaldoximato Complexes Presenting the [Fe6(μ3-O)2(μ2-OR)2]12+Core: Syntheses, Crystal Structures, and Spectroscopic and Magnetic Characterization

The use of salicylaldehyde oxime (H2salox) in iron(III) carboxylate chemistry has yielded two new hexanuclear compounds [Fe6(mu3-O)2(O2CPh)10(salox)2(L)2].xMeCN.yH2O [L = MeCONH2, x = 6, y = 0 (1); L = H2O, x = 2, y = 3 (2)]. Compound 1 crystallizes in the triclinic space group P with (at 25 degrees C) a = 13.210(8) A, b = 13.87(1) A, c = 17.04(1) A, alpha = 105.79(2) degrees , beta = 96.72(2) degrees , gamma = 116.69(2) degrees , V = 2578.17(2) A(3), and Z = 1. Compound 2 crystallizes in the monoclinic space group C2/c with (at 25 degrees C) a = 21.81(1) A, b = 17.93(1) A, c = 27.72(1) A, beta = 111.70(2) degrees , V = 10070(10) A(3), and Z = 4. Complexes 1 and 2 contain the [Fe6(mu3-O)2(mu2-OR)2]12+ core and can be considered as two [Fe3(mu3-O)] triangular subunits linked by two mu2-oximato O atoms of the salox2- ligands, which show the less common mu3:eta1:eta2:eta1 coordination mode. The benzoato ligands are coordinated through the usual syn,syn-mu2:eta1:eta1 mode. The terminal MeCONH2 ligand in 1 is the hydrolysis product of the acetonitrile solvent in the presence of the metal ions. Mössbauer spectra from powdered samples of 2 give rise to two well-resolved doublets with an average isomer shift consistent with that of high-spin Fe(III) ions. The two doublets, at an approximate 1:2 ratio, are characterized by different quadrupole splittings and are assigned to the nonequivalent Fe(III) ions of the cluster. Magnetic measurements of 2 in the 2-300 K temperature range reveal antiferromagnetic interactions between the Fe(III) ions, stabilizing an S = 0 ground state. NMR relaxation data have been used to investigate the energy separation between the low-lying states, and the results are in agreement with the susceptibility data.

Structure, magnetic properties and atomic immigration of CoFe2O4 nanoparticles

CoFe2O4 nanoparticles have been prepared by sol-gel method. M?ssbauer spectra at temperature varying from 80 to 873 K showed that the atomic site preference of nanoparticles is different from that of the bulk material. The results also showed that the Tc of nanosized CoFe2O4 is much lower than that of the bulk counterpart. This decrease of Tc in the nanoparticles may be attributed to the redistribution of Fe and Co cations at the A and B sites. The M?ssbauer spectra also revealed that the hyperfine field varied with the temperature and the hyperfine interaction follows T3/2+T5/2 law approximately. The average isomer shift decreases almost linearly with increasing temperature.

An analysis of the hyperfine parameters of theRFe11Ti and RFe11TiH compounds, where R is a rare-earth element

The Mössbauer spectra of CeFe11Ti and CeFe11TiH obtained between 4.2 and 295 K are analysed in terms of the model used to analyse the spectra of theRFe11Ti and RFe11TiH compounds, where R is Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er and Lu. The hyperfine parametersobtained with a consistent model that considers both the easy magnetization direction andthe titanium preferential site occupancy are discussed as a function of rare-earth atomicnumber, temperature and hydrogen content. The average hyperfine fields in theRFe11Ti and RFe11TiH compounds are described with a two-sublattice model in which theiron sublattice contributions coincide with the fields observed inLuFe11Ti and LuFe11TiH, respectively. In both series, the rare-earth sublattice contributes a transferredfield which occurs as a result of indirect exchange between the rare-earth 4f andiron 3d electrons and depends on the nature of the rare-earth element. Theincrease in average hyperfine field and isomer shift upon hydrogenation of theRFe11Ti compounds results from the unit-cell expansion upon hydride formation. The observedaverage quadrupole shift is closely related to the magnetic anisotropy exhibited by eachcompound.

Mössbauer effect studies of RFe 11Ti and RFe 11TiH ( R=Y, Gd, Er)

The Mössbauer spectra of ferromagnetic RFe 11Ti and RFe 11TiH x ( R = Y , Gd, Er, x = 0 , 1) have been investigated in the temperature range 20–300 K and analysed in terms of a new model which allows a better fit of the spectra using less constraints than previously reported analyses. In addition to the dependence of magnetic hyperfine field and isomer shift on the number of Ti nearest-neighbours (NN) of the Fe atoms, this new model also considers the effect of the average interatomic distances between Fe atoms on the 8i, 8f and 8j crystallographic sites. Magnetic hyperfine fields were found to decrease with the average interatomic distances for Fe atoms on the same sites and with the same number of iron NN. This decrease was however lower than that due to the presence of one additional titanium NN. Furthermore the present model distinguishes the 8f from the 8j sites on the basis of the relative areas of the corresponding sextets. It leads to higher average magnetic hyperfine fields and isomer shifts for the 8f site as compared to the 8j sites which may be understood on the basis of electronic charge transfer between the rare-earth and Fe atoms. The dependence of hyperfine parameters on the number of titanium NN and on hydrogenation is similar to those previously reported.

Magnetic properties of Y2Fe17−x Gax and Sm2Fe17−xGax compounds

The magnetic properties of Y2Fe17−x Ga x and Sm2Fe17−xGa x for 3 ≤ x ≤ 7 have been investigated using the 57Fe Mossbauer spectroscopy at room temperature. These compounds have the rhombohedral Th2Zn17 structure. X-ray diffraction analyses of aligned powders show that the easy direction of magnetization is parallel to the c-axis in Y2Fe10Ga7 and Sm2Fe14Ga3 and is perpendicular to the c-axis in Y2Fe14Ga3, Y2Fe12Ga5, Sm2Fe12Ga5 and Sm2Fe10Ga7. Mossbauer studies indicate that all the samples studied are ferromagnetically ordered. The 57Fe hyperfine field decreases with increasing Ga content. This decrease results from the decreased magnetic exchange interactions resulting from Ga substitution. The average isomer shift, δ, for Y2Fe17−xGa x and Sm2Fe17−xGa x at room temperature is positive and the magnitude of δ increases with increasing Ga content.