Structural,magnetic and thermal properties of intermetallics Nd3Fe27−xCoxV2 (x = 0, 2 and 4) and their hydrides

Intermetallics of the Nd₃Fe_27?xCo_xV₂ (x = 0, 2 and 4) system with monoclinic type structure and their hydrides were prepared by arc melting and exposed to hydrogenation. The effects of Co substitution of Fe on the magnetic hyperfine fields of intermetallic compound Nd₃Fe₂₇V₂, as well as the effects of interstitial hydrogen additions on the symmetry and local magnetic properties in these systems were studied. The investigations were performed by powder X-ray diffraction, Mössbauer spectroscopy and simultaneous TGA-DSC. Both site populations and magnetic hyperfine field strength were analyzed in order to discriminate between the Co and H contributions to the local atomic environments in these systems. The values of magnetic hyperfine fields corresponding to inequivalent lattice sites were found to be enhanced after the increase in Co content and hydrogenation. Slight changes in the relative site occupancies were observed. Simultaneous TGA-DSC studies showed that the amount of hydrogen insertion decreased slightly with the increase of Co contents, while the enthalpy associated with hydrogen desorption decreased sharply under heat treatment. The Nd₃Fe_27?xCo_xV₂ (x = 0, 2 and 4) and their hydrides are not stable under treatment and two exothermic peaks are observed in the DSC curves, corresponding to the decomposition of the monoclinic phase with the precipitation of α-Fe phase.     

Substitutional effects in RFe3 intermetallics (R=Dy,Sm, Y)

Samples of RFe₃ (R=Dy, Sm, Y) were doped with Si, Al and Ti and studied by Mössbauer spectroscopy and X-ray diffraction. Most Mössbauer spectra were analyzed with three sextets, corresponding to the 6c, 3b and 18 h inequivalent lattice sites for iron. A quadrupole split doublet was also present in some spectra, which became dominant for the case of Al substitution. The collapse of the hyperfine magnetic structure is a magnetic effect due to substitutions: the X-ray diffraction patterns yielded a particle size of about 100 nm, thus precluding the occurrence of superparamagnetism in these systems.     

An investigation of the effect of structural order on magnetostriction and magnetic behavior of Fe–Ga alloy thin films

This paper reports results from a comprehensive study of Fe–Ga films fabricated over a wide range of growth conditions. Polycrystalline Fe_100?xGa_x films (14 ? x ? 32) were deposited (using three different combinations of growth parameters) on Si(1 0 0) using a co-sputtering and evaporation technique. The growth parameters (sputter power, Ga evaporation rate and chamber pressure) were used primarily to control the Fe:Ga ratio in the films. X-ray diffraction showed that all films had 〈1 1 0〉 crystallographic texture normal to the film plane. The lattice parameter increased with % Ga up to x = 22 and was independent of growth parameters. Conversion electron Mössbauer spectroscopy studies showed a predominance of the disordered A2 phase in all films. It appears that the use of vacuum deposition with appropriate parameters can effectively suppress the D0₃ ordered phase. Systematic studies of the effective magnetostriction constant as a function of composition support this conclusion. It was found that films of high effective saturation magnetostriction constant and low stress could be fabricated using low Ar pressure, irrespective of sputter power or evaporation rate, giving properties useful for application in microelectromechanical systems.     

Anwendung der Mößbauer-Spektroskopie auf die Untersuchung von Deckschichten in Naturumlaufkesseln

Application of Mößbauer spectroscopy to the study of surface layers in natural convection boilers Qualitative and quantitative analysis of oxide layers on tubes in steam generators have been performed by ⁵⁷Fe Mössbauer Spectroscopy in transmission and backscattering geometry. The tubes were exposed to the following conditions for ca. 127000 hours: 120–174 atm., 320–353 °C and 9.5–10.5 pH; the water contained < 10 and < 4 mg/l of P₂O₅ and SiO₂, respectively (The results of the chemical analysis of the tube materials were: 15 Mo 3 : 0.12% C, 0.15% Si, 0.5% Mn, 0.04% P, 0.04% S, ? 0.3% Cr, 0.25% Mo, rest Fe; St. 45.8. III: < 0.22% C, 0.1% Si, ? 0.45% Mn, 0.05% P, 0.05% S, ? 0.3% Cr, rest Fe). The Mössbauer investigations showed that the protective oxide layers contained mainly non-stoichiometric magnetite (Fe_3?xO₄; x ? 0.03), partly in microcristalline form (<500 Å). In addition to magnetite, hematite (α-Fe₂ O₃) was also detected in one of the samples. The composition of the top surface layers (～5000 Å) was studied by Conversion Electron Mössbauer Spectroscopy (CEMS). The high phosphate content in two of the four investigated samples can be probably attributed to apatite or hydroxylapatite.     

Role of Mo in the local configuration and structure stabilization of amorphous steels,a Synchrotron X-ray diffraction and Mössbauer study

Amorphous steels are promising materials with potential structural applications. The glass-forming ability (GFA) and mechanical properties of metallic glasses are intimately related to the local structure. In Fe-based alloys, Cr and Mo content seem to play a key role in stabilizing the amorphous atomic-level structure. Here we present a study on the effects of changing Mo content in Fe_72?xC₇Si_3.3B_5.5P_8.7Cr_2.3Al₂Mo_x amorphous steels. We study the local structure of these alloys by Synchrotron X-ray diffraction and Mössbauer spectroscopy. The results show how the amorphous phase evolves from a ferromagnetic Fe-rich structure to a structure with predominance of paramagnetic environments with the increase of Mo content. The changes in the distribution of magnetic environments cannot be attributed only to the Fe–Mo substitution but to a change of local configuration in the amorphous phase.     

Structural ordering and magnetic properties of arc-melted FeGa alloys

X-ray diffraction, Mössbauer spectrometry and magnetic properties have been performed on FeGa arc-melted binary alloys in order to study the compositional dependence of the structural ordering and magnetic properties of these alloys. The average magnetic hyperfine field at 300 K decreases with increasing Ga content as does the average magnetic moment per Fe atom, and a linear dependence between both quantities is found for Ga content up to 20 at%. The substitution of Fe by Ga atoms increases the lattice parameter and causes a change in the order of the crystal structure that has been modelled using a binomial distribution method. The effect of Ga atoms as near neighbours of Fe on the average magnetic hyperfine field is evaluated.     

Increase of TC in UFe2+x synthesized by ultrafast cooling

The uranium Laves phase UFe₂ was prepared by ultrafast cooling, which allows to incorporate excessive Fe of about 0.3 Fe/f.u. into the nanocrystalline Laves phase structure. ⁵⁷Fe Mössbauer spectroscopy reveals that such Fe atoms occupy the U sites. Stronger magnetic hyperfine field points to larger Fe moments of such antistructure Fe. Higher Fe excess leads to its segregation in the form of α-Fe. The Curie temperature of the UFe_2+x phase increases to the limit value 230–240 K.     

Evolution of structural changes in nanocrystalline alloys with temperature

Structural features of the NANOPERM-type alloys Fe_{91 ? x} Mo₈Cu₁B _x with x = 12, 15, and 17 have been investigated by Mössbauer spectroscopy. The room-temperature Mössbauer spectra of the as-quenched alloys are characteristic of disordered structural arrangement, but traces of bcc-Fe(Mo) as well as a FeMo₂B₂ phase have been revealed by X-ray diffraction in all the samples. These results have been confirmed by conversion-electron Mössbauer spectroscopy. The differences between the opposite sides of the ribbon-shaped samples have been shown to stem from structural distinctions. From the point of view of hyperfine interactions, the x = 12 sample exhibits paramagnetic behavior. With increasing x, a contribution from ferromagnetic regions appears gradually, thus leading to an increase in the magnetic ordering temperature in the as-quenched state. Partially crystallized samples have been prepared by controlled annealing of the original precursors for one hour at temperatures ranging from 330 to 650°C in a vacuum. The temperature of the onset of crystallization has been determined to be of 430, 450, and 470°C for x = 12, 15, and 17, respectively. During the first step of crystallization, bcc-Fe(Mo) nanosized grains are formed. Surface features of the samples investigated have also been characterized by using atomic force microscopy.     

Laser ablated Fe100−xPdx and Fe100−xMnx thin films (x=20 and 50)

Thin films (50 nm) of Fe_100−xPd_x and Fe_100−xMn_x (x=20 and 50) were deposited by laser ablation on a Si wafer and studied by X-ray diffraction (XRD) and conversion electron Mössbauer spectroscopy (CEMS). In all cases, the XRD patterns of the target surface before and after ablation show a congruent transfer of ablated material. For the Fe₈₀Pd₂₀ thin film, XRD spectra show the formation of FePd, iron, Pd oxide and traces of hematite in the ablated film. Complementary information obtained by CEMS indicates the presence of an upper 50-nm thick surface layer of hematite in the deposited film. Fe₅₀Pd₅₀ phase was observed by CEMS and a Pd oxide component was identified by XRD, along with traces of hematite. In the case of Fe₈₀Mn₂₀, the CEMS spectrum is consistent with the occurrence of the antiferromagnetic FeMn phase and the XRD pattern also shows the presence of Fe in the structure. For the system Fe₅₀Mn₅₀, we obtain by CEMS an antiferromagnetic phase of FeMn and by XRD we trace the presence of nanohematite and in-depth iron. For x=20 in both iron-rich systems investigated, we observe the segregation of iron with subsequent conversion to hematite. The study shows that the stoichiometry of the compounds plays a decisive role in determining their structural and magnetic properties.     

The Debye temperature of σ-phase Fe–Mo compounds as determined with Mössbauer spectroscopy

The Debye temperature, Θ_D, of σ-phase Fe_100−xMo_x compounds with 47 ≤ x ≤ 56.7 was determined from the temperature dependence of the centre shift of Mössbauer spectra recorded in the temperature range of 80–300 K. Its compositional dependence shows a weak increase with x the rate of which, in a linear approximation, is equal to 3.1 K/at%. The results are compared with the corresponding ones found previously for the σ-phase in Fe–Cr and Fe–V compounds showing that Θ_D-values are characteristic of a given alloy.     

First principles study of site substitution of ternary elements in NiAl

Site substitution of ternary elements in ordered compounds influences the electronic structure and hence the properties of compounds at the continuous level. The electronic structure and binding energy of a number of NiAl-X alloy systems (X=Ti, V, Cr, Mn, Fe, Co, Zr, Nb, Mo, Hf, Ta, W, Si, Ga, or Ge) were calculated using the discrete variational cluster method based on the local density approximation of the density functional theory. The site preference of the ternary additions to NiAl was investigated by employing the Bragg–Williams model to analyse the calculated binding energy. The results show that all the considered ternary elements possess stronger preference to the Al sublattice sites than a Ni atom does. A new method of identifying sublattice substitution of ternary additions in NiAl was proposed by comparison of the binding energies per atom of the ternary and the binary clusters involving the fourth nearest neighbours. The analysis suggests that Fe and Co atoms occupy the Ni sublattice sites, whereas Si, Ga and Ti atoms occupy the Al sublattice sites. The remaining elements may substitute for both sublattices: Mn is most likely to go for the Ni sublattice; V, Cr, Zr, Nb, Mo, Hf, Ta, W and Ge have a larger preference for the Al sublattice, but Cr and W do not show significant preference to any sublattice. The densities of states involving alloying additions of Co, Si and Cr were further investigated to clarify the site preference of the alloying additions.     

Hydrogenation effects in R2Fe14Si2 (R = Y,Nd, Dy and Er) offstoichiometric compounds

Samples of R₂Fe₁₄Si₂ (R = Y, Nd, Dy and Er) solid solutions were prepared by arc melting and exposed to hydrogenation. Characterization of hydrogenation effects was done using X-ray diffraction (XRD) and Mössbauer spectroscopy. Rietveld refining of the XRD spectra showed that both lattice parameters a and c increased after hydrogenation, with the exception of Er₂Fe₁₄Si₂, for which they decreased slightly. The values of the magnetic hyperfine fields corresponding to the four inequivalent lattice sites were found to increase after hydrogenation in all offstoichiometric compounds investigated. Changes in the relative site occupancies were observed. Our study is the first to evidence hydrogenation effects on the hyperfine magnetic fields in intermetallics.     

The formation of structure and phase composition and magnetic properties of Fe(Fe<Subscript>3</Subscript>C,Fe<Subscript>5</Subscript>SiC)-SiO<Subscript>2</Subscript> nanocomposites upon mechanical alloying

Structural and phase transformations that occur in Fe(Fe₃C, Fe₃SiC)-SiO₂ (amorphous quartz) systems during mechanical alloying in an Ar atmosphere and in air have been studied by X-ray diffraction, Mössbauer spectroscopy, IR spectroscopy, electron microscopy, and magnetometry. It has been shown that the mechanoactivation treatment leads to the formation of isolated particles 2–20 nm in size with a complex phase composition (Fe, FeSi alloy, oxides, silicates, and carbides), which depends on the milling atmosphere. It has been found out that the magnetic properties of such systems strongly depend on the oxygen and carbon compounds existing in the system, which cannot be detected by X-ray diffraction, but their presence is testified by the data of Mössbauer spectroscopy.     

A Mössbauer spectroscopy study of Fe based cemented carbides

Mössbauer spectroscopy has been used as a novel characterization technique to investigate Fe charge states, Fe complexes and hyperfine interaction parameters of different phases in WC-10Fe and WC-10(FeNi) materials sintered at three different temperatures (1350, 1430 and 1510 °C). The materials were characterized using standard cemented carbide quality control, and spectroscopic techniques to evaluate the structural changes and magnetic effects of the binder. The WC-10Fe grade had the highest Vickers hardness ranging from 1282 to 1320 HV₃₀, for the different sintering temperatures. X-ray diffraction data showed the presence of WC and the metal binder phase, α-Fe and γ-FeNi. Transmission Mӧssbauer spectroscopy spectra obtained for the milled powders revealed only the α-Fe phase with a hyperfine magnetic field, B_hf ~33 T. Conversion electron Mössbauer spectroscopy on the sintered compacts revealed the presence of multiple fields, suggesting the possibility of minor phases present in the binder which were not detected using X-ray diffraction. The corresponding spectra for the sintered WC-Fe grades exhibited two magnetic fields with hyperfine parameters of ~33 T and ~17 T, respectively. These fields were assigned to α-Fe with some W atoms in solution and a W-rich Fe phase, respectively. The Mӧssbauer spectrum for the FeNi binder sample at the lowest sintering temperature of 1340 °C showed a paramagnetic doublet with an isomer shift, δ = −0.08 mm/s and electric quadrupole splitting, ∆E_Q = 0.00 mm/s and a weak hyperfine magnetic field, B_hf = 15.7 T. The doublet has been assigned to γ-FeNi and the magnetic component to a W-rich γ-FeNi phase. For the higher sintering temperature, a distribution of magnetic fields (~33 T, 25 T and 9 T) was evident in the Mössbauer spectrum. These magnetic fields were tentatively assigned to multiple W-rich γ-FeNi phases.     

Mössbauer spectroscopy of interphase boundaries of Co/CoO bilayers

Co films and Co/CoO bilayers that were deposited by the method of magnetron sputtering on the MgO(100) and Al₂O₃(110) single-crystal substrates have been studied using electron microscopy and Mössbauer spectroscopy. The surface, bulk, and interphase Mössbauer spectra of the layers and bilayers under examination have been investigated. It is shown that the ⁵⁷Co(⁵⁷Fe) atoms located in the region of a Co/CoO interphase boundary can be in different magnetic states and have different valences and chemical surroundings.     

Kinetic paths for B2 order in nanocrystalline FeCo–Mo: a MÖssbauer spectroscopic study

The kinetic evolution of B2 order in nanocrystalline FeCo–Mo alloys was studied at different temperatures using Mössbauer spectroscopy. The kinetic paths in the space spanned by the two short-range order parameters α_Co/Fe and α_Mo/Fe were found to be independent of temperature within experimental errors. This behavior is to be compared with that observed in coarse-grained FeCo–Mo alloys, where the paths were not the same at different temperatures. This can be attributed to the presence of more grain boundary regions in the nanophase alloys, which provide short-circuited diffusion paths for atom movements.     

Influence of the order-disorder transition on the magnetic properties of Fe75Al25-xSix alloys

The influence of the different crystal structures and the variation of the lattice parameter on the evolution of the magnetism in the order-disorder transition produced by crushing and mechanical milling in the intermetallic Fe₇₅Al_25-xSi_x alloys (x = 7.5, 12.5, 17.5, 25) has been systematically studied by means of XRD measurements, Mössbauer spectroscopy and magnetic measurements. The results indicate that with the addition of Si to binary Fe₇₅Al₂₅ alloy the mechanical deformation needed to disorder the alloys increases. At the same time the variation of the lattice parameter due to the disorder is reduced as Si is added. The magnetic measurements indicate that there is a complex behaviour in ternary alloys with an opposite influence of Si and Al during the order-disorder transition. However, when the transition is fulfilled there is a linear relationship between structural and magnetic parameters.     

TbRhSn and DyRhSn – Detailed magnetic and 119Sn Mössbauer spectroscopic studies

The results of magnetic studies and Mössbauer spectroscopic investigations are reported for the stannides TbRhSn and DyRhSn crystallizing in the hexagonal ZrNiAl-type structure. The polycrystalline samples of these ternary intermetallics were synthesized by arc melting from metallic precursors. Detailed ¹¹⁹Sn Mössbauer spectroscopic studies are used to investigate the hyperfine interactions and their temperature evolutions at places occupied by the diamagnetic tin nuclei. Magnetic properties of DyRhSn and TbRhSn were studied by AC/DC magnetometry in a wide temperature range. The results show that both compounds are magnetically ordered at low temperatures. DyRhSn is a non-collinear antiferromagnet with the Néel temperature T_N = 7.5 K, whereas TbRhSn undergoes a transition from a paramagnetic to an antiferromagnetic state at T_N = 20.2 K. An additional transition at T_SR = 10.3 K is detected for TbRhSn which corresponds to some changes in the magnetic moments ordering. The role of the magnetostriction effect in the evolution of the hyperfine parameters and its influence on the observed TbRhSn Mössbauer spectra is discussed. Triangular-like antiferromagnetic arrangements with rare-earth magnetic moments lying in the hexagonal plane are proposed for both compounds at very low temperatures.     

Microstructure and magnetic properties of FeMoBCu alloys: Influence of B content

Fe_91–xMo₈Cu₁B_x (x = 12, 15, 17, 20) amorphous and nanocrystalline alloys were studied to examine the influence of B content on their microstructure and magnetic behaviour. Changes in the magnetic properties provoked by microstructural evolution upon thermal treatments of as-cast samples were also analyzed. Nanocrystallization kinetics can be described by an isokinetic approach except for the 20 at.% B content alloy. The Curie temperature of the amorphous as-cast samples increases with the alloy’s B content. Mössbauer results suggest the presence of Mo atoms in the nanocrystals. Crystalline volume fraction and mean grain size of the nanocrystals at the end of the nanocrystallization process are higher for the lowest B content alloy. The 20 at.% B content alloy develops a boride phase just after the early stages of the nanocrystallization process, which provokes a magnetic hardening in this alloy. The softest magnetic behaviour of the studied compositions corresponds to nanocrystallized 17 at.% B content alloy.