Local Fe Environment Research Articles

57FE Mössbauer spectroscopy studies of Tektites from Khon Kaen, Ne Thailand

Room temperature 57Fe Mossbauer effect spectroscopy has been used to investigate the local Fe environment in a set of indochinite tektites from Thailand. A MIMOS II spectrometer in backscattering geometry has been used in the study, so that no sample preparation at all was required. The spectra have been analysed in terms of discrete spectral components using Voigt functions. The results are similar to those obtained with analysis done with quadrupole splitting distributions. In all cases the Mossbauer spectra show a broadened asymmetric quadrupole split doublet. The Fe2+ sites have been distinguished in Fe with octahedral and tetrahedral coordination. The Fe3+/Fe2+ and Fe2+ tetrahedral/octahedral ratios have been determined from the relative areas of the various spectral components. Their values have been discussed in the light of actual literature.

Site determination and magnetism of Mn doping in protein encapsulated iron oxide nanoparticles

Soft x-ray absorption spectroscopy, soft x-ray magnetic circular dichroism, and alternating current magnetic susceptibility were performed on 6.7 nm iron oxide nanoparticles doped with (5%–33%) Mn grown inside the horse-spleen ferritin protein cages and compared to similarly protein encapsulated pure Fe-oxide and Mn-oxide nanoparticles to determine the site of the Mn dopant and to quantify the magnetic behavior with varying Mn concentration. The Mn dopant is shown to substitute preferentially as Mn+2 and prefers the octahedral site in the defected spinel structure. The Mn multiplet structure for the nanoparticles is simpler than for the bulk standards, suggesting that the nanoparticle lattices are relaxed from the distortions present in the bulk. Addition of Mn is found to alter the host Fe-oxide lattice from a defected ferrimagnetic spinel structure similar to γ-Fe2O3 to a nonferromagnetic spinel structure with a local Fe environment similar to Fe3O4.

γ -Fe 2 O 3 nanoparticle intrinsic magnetism dependence on iron-ion availability during synthesis

A single precursor technique was used to synthesize γ-Fe2O3 nanoparticles with diameters from 5 to 7 nm. The blocking temperature of the nanoparticles essentially increases with Fe precursor concentration. Transmission Mössbauer spectroscopy of the different γ-Fe2O3 nanoparticle systems reveal that the local Fe environments in the nanoparticle vary considerably with Fe precursor concentration. Not only does the nanoparticle size change, but the magnitude of the hyperfine field (a measure of the nanoparticle moment), the amount of chemical disorder in the nanoparticle, and the number of Fe moments in the octahedral and tetrahedral sites of the γ-Fe2O3 spinel structure vary with Fe concentration as well.

Analysis of the mechanically alloyed Fe 85–Nb 5–B 10 powder using a non-unique lattice parameter

X-ray diffraction patterns of mechanically alloyed Fe 85Nb 5B 10 powder were fitted using whole powder pattern decomposition procedure to resemble the presence of α-Fe type crystals with different Nb content. To do so, a distribution of lattice parameters was proposed based on the asymmetry of the diffraction maxima and the broadening of the distribution of the powder particles composition. Analogously, a set of discrete sextets was used to represent the different local Fe environments.

A Mössbauer effect study of structural ordering in rapidly quenched Fe–Ga alloys

Samples of Fe 100− x Ga x ( x=8.3, 17.9, 20.5 and 23.3) were prepared by rapid solidification from the melt using a single Cu roller. X-ray diffraction studies of all samples showed them to be single phase with the disordered BCC structure. No evidence of superlattice reflections from D0 3 ordering was observed for any of the samples. Room-temperature 57Fe Mössbauer effect spectra indicated that all samples were ferromagnetically ordered. Spectra were fit to distributions of hyperfine fields. The x=8.3 sample showed a hyperfine field distribution that was single peaked and indicated a reasonably random distribution of local Fe environments. The x=17.9 and 20.5 samples showed hyperfine field distributions that were bimodal and indicated two distinct local Fe environments. The x=23.3 sample showed three distinct field components. It is suggested that the x=8.3, 17.9 and 20.5 alloys are primarily a disordered BCC phase. The x=8.3 alloy shows a small amount of short-range Ga–Ga pairing, while this short-range pairing is significantly greater in the x=17.9 and 20.5 alloys. The three field components in the x=23.3 alloy correspond well to the two sites associated with the D0 3 phase and a third component corresponding to a remaining L1 2 phase suggesting the presence of at least short-range D0 3 clustering in this alloy.

Nitrogen-induced local magnetic and structural properties of sputtered FeAlN thin films

Conversion electron Mössbauer spectroscopy (CEMS) and x-ray diffraction were used to study a series of rf-reactively sputtered FeAlN thin films with the goal of gaining insight into the origins of their uniaxial magnetic anisotropy (UMA) and the effect of sputtering conditions on its thermal stability. At N/Fe=4.6%(±0.5%), N goes into interstitial vacancies without significant reductions in the hyperfine field (Bhf) of the Fe but causing lattice expansions, leading to an increased linewidth in the CEMS spectra of the film while the sample has an in-plane UMA. Reducing the target-to-substrate spacing used in the deposition process does not affect the nitrogen content of the films but yields an increased CEMS linewidth reflecting larger disorder in the local Fe environment. Heavy N doping causes the phase transform into γ′-Fe4N, ε-Fe3N, and ζ-Fe2N phases. A well defined UMA exists in mildly doped FeAlN films when doped N atoms mostly occupy the interstitial sites while FeAl and heavily doped FeAlN films are either isotropic or weakly anisotropic. UMA and its thermal stability are therefore concluded to have a correlation with the local environment of the Fe structure induced by N doping.

An investigation of Fe oxidation states and site distributions in a Tibetan tektite

Room temperature 57Fe Mossbauer effect measurements have been used to investigate the local Fe environment and the Fe3+/Fe2+ ratio in a Tibetan tektite. The spectrum has been analyzed in terms of two quadrupole splitting distributions; one corresponding to Fe2+ and one corresponding to Fe3+. The Fe2+ sites show a mean isomer shift of +1.011 mm/s relative to \alpha-Fe and a mean quadrupole splitting of 1.845 mm/s. The Fe2+ quadrupole splitting distribution showed a large peak for positive values, which has been identified with Fe2+ with local octahedral coordination, and a smaller peak for negative values, which has been identified with Fe2+ with local tetrahedral coordination. The Fe3+ sites showed an average isomer shift of +0.20 mm/s and a single peaked quadrupole splitting distribution with a mean near zero. The Fe3+/Fe2+ ratio, based on the relative areas of the spectra components was found to be 0.015. These results are discussed in terms of those obtained using fitting methods based on discrete sites and other distributions of parameters. Hyperfine parameters for the Tibetan tektite are compared with those obtained for tektites from other regions of the Australasian strewn field. All tektites studied show similar quadrupole distributions and similar mean hyperfine parameters. Fe3+/Fe2+ ratios ranged from 0.005 to 0.036. The relationship of the microstructure of the Tibetan tektite with that of tektites from various regions of the Australasian strewn field is discussed.

Mössbauer effect study of one-dimensional and two-dimensional quasicrystalline alloys in the AlPdFe system

Room temperature 57Fe Mössbauer effect measurements of the one-and two-dimensional quasicrystalline phases of Al 75Pd 15Fe 10 are reported. Both materials show Mössbauer spectra with asymmetric doublets which have been analyzed in terms of a distribution of quadrupole splittings, and in terms of a discrete expansion of quadrupole splittings. The one-dimensional material shows the following characteristics compared with the two-dimensional material; (1) a larger mean quadrupole splitting, (2) a more positive mean isomer shift and (3) a broader distribution of quadrupole splittings. These results indicate that the local Fe environments in the one-dimensional material are less isotropic than in the two-dimensional material and that this material is characterized by a higher concentration of open volume defects.

A Mössbauer study of bcc stainless steel films

Conversion electron Mössbauer spectroscopy is used to investigate the ferromagnetic state of bcc stainless steel films. First, a low temperature study allows the resolution of the local Fe environments. It is found that the main environments correspond to three Fe sites with different short-range order. Second, a phase transition takes place at temperatures higher than 750 K. Evidence is found that, during an isothermal kinetic at 913 K, the ferromagnetic part of the Mössbauer spectra splits up into two components, thus indicating the existence of two ferromagnetic iron sites. The index n and activation energy Q in the Johnson-Mehl-Avrami plot are found to be 0.53-0.68 and 366–462 kJ mol −1, respectively. The role of Cr atoms is also discussed.

High-symmetry transition-metal sites in Ti56Ni28−xFexSi16 quasicrystals

Rapidly solidified Ti-Ni-Fe-Si alloys are found to form a metastable phase with x-ray diffraction patterns which can be indexed to a structure with icosahedral symmetry. 57 Fe Mössbauer effect spectra show a negligible quadrupole splitting for the icosahedral phase. This observation, along with a comparison of the measured x-ray diffraction line intensities with calculated values, suggests a structure based on a decoration of a three-dimensional Penrose tiling, with transition-metal atoms in sites with local icosahedral symmetry. Magnetic susceptibility measurements show that a local moment of ∼0.2 μB forms on the Fe atom. These results are discussed in terms of possible local Fe environments.

Local atomic structure in amorphous Fe-P alloys

Amorphous alloys Fex P100−x have been fabricated by rf sputtering in the composition range 50≤x≤75. Only a-Fe75 P25 is ferromagnetic at 293 K, with its Mössbauer spectrum consisting of six broad lines. This simple spectrum differs markedly from that of c-Fe3 P. Similarly, thermogravimetric analysis yielded Tc=550 K for the amorphous phase, a value considerably lower than that of the crystalline phase (700 K). The Mössbauer spectra of the samples having x≤71 were all similar quadrupole-split doublets. Mean isomer shifts (IS) and P(H) and P(Q) distributions were determined using Window’s method. The isomer shift is nearly independent of concentration in the range studied here, but differs significantly from those of the crystalline compounds c-FeP, c-Fe2 P, and c-Fe3 P. The effective quadrupole interaction is also nearly constant for 50≤x≤71, indicating a similar local Fe environment. These results contrast sharply with c-Fe2 P and c-FeP, which have different crystal symmetries and whose quadrupole interactions differ substantially both from each other and from their amorphous counterparts.