Hyperfine Field Distribution Research Articles

Analysis of broadened Mössbauer spectra using simple mathematical functions

Simulated and experimental broadened Mossbauer spectra are analyzed using several distribution functions. The resolution Hesse and Rubartsch data are reproduced in order to analyze the origin of the oscillations appearing in the recovered distribution function. The lined triangular distribution is used and some of its properties are described. The no implicit nth-nomial distribution function P(x) = (aCos (πx) + bSin (πx)) n is introduced, complementing the Window and Hesse and Rubartasch no implicit distribution functions. This new no implicit distribution function gives similar results of those of Window’s method. In addition, the Window method has also been modified by inserting a smoothing factor λC. For 0 1, the opposite is obtained. The Levenberg-Marquardt algorithm is used to solve the involved Fredholm integral equation rather than the typical second order regularized algorithm. From the extracted hyperfine field distribution functions of the Mossbauer spectra of the amorphous and crystallized Fe70Cr2Si5B16 magnetic alloy the short range atomic order for the amorphous state of this alloy can be inferred.

Magnetic properties and spin dynamics in Ba(Fe1−xMnx)2As2 compounds studied by 57Fe Mössbauer spectroscopy

Ba(Fe1−xMnx)2As2 compounds with x = 0.016 and 0.064 have been studied by 57Fe Mössbauer spectroscopy in the temperature range from 30 to 300 K. The unusual magnetic splitting spectra at lower temperatures have been analyzed using the distribution of hyperfine field. It is found that the influence of Mn dopant spreads beyond the nearest Fe magnetic moments, and the Fe 3d electrons behave more localized compared with those in the electron-doped compounds. This reduces the hyperfine interactions between iron nucleus and the sounding electrons. The shape of the spin density wave is near-rectangular at 6.4%-Mn doping, indicating quite different interband interactions compared with electron-doped compounds. A distinct broadening of the spectral linewidth around the spin density wave transition temperature has been observed and the spin correlation time is deduced according to the linewidth. The correlation time is further related to the spin–lattice relaxation rate by a simple model and the magnetic fluctuations can be explored effectively. It is found that the magnetic fluctuations of iron spins in Mn-doped compounds can be described well using a phenomenological two-component model and the resulting Curie–Weiss temperature is far from the quantum critical point at the present doping levels.

Iron nano-clusters in ytterbium films: a 57Fe Mössbauer spectroscopic study

We have performed a Mossbauer study on iron clusters that are formed in ytterbium films prepared by vapor co-deposition onto kapton substrates kept at room temperature. The film thicknesses were chosen in a range between 2.5 and 2.8 µm. XRD of the films reveals a mixture of fcc- and hcp-like ytterbium. Iron concentrations were between 0.3 and 5 at %. All samples reveal hyperfine spectra attributed to only two types of iron clusters with well defined hyperfine parameters. The clusters are supposed to be formed at boundaries of hcp- and fcc-like grains. In addition there is found a small contribution from monomeric iron. Spectra taken at 4.2 K reveal a complex distribution of magnetic hyperfine fields. The dynamic origin of the spectral shape is supported by susceptibility data revealing spin-freezing at temperatures below ca. 10 K. This proves that the iron clusters have sizes on the order of nm.

XRD and Mössbauer spectroscopy study of Ho doped BiFeO3

The structural and magnetic properties of Ho substituted BiFeO3 (BHFO) have been investigated using 57Fe Mossbauer spectroscopy and X-Ray diffraction (XRD) as a function of temperature. The Mossbauer spectrum obtained at room temperature for the as-synthesized BHFO sample exhibits broadened features due to the hyperfine field distributions related to the local variation of the neighbourhood of Fe and the magnetic hyperfine splitting patterns are indicative of magnetic ordering, mostly probably screwed or slightly antiferromagnetic. The spectrum was fitted with two superimposed asymmetric sextets, with similar hyperfine magnetic fields of Bhf1 = 48.0(1) T and Bhf2 = 49.0(1) T, corresponding to rhombohedral BFO. The hyperfine fields of the magnetic components decreased systematically with increasing temperature to a ‘field distribution’ just below the Neel temperature, TN ~ 600 K. At temperatures above 600 K, the spectral line associated with the Bi25FeO40 impurity phase dominates the spectra. This phase is confirmed by XRD measurements. From the temperature dependence of the site populations of the spectral components an average Debye temperature of θD = 240(80) K has been estimated.

Mössbauer study of EUROFER and VVER steel reactor materials

57Fe Mossbauer spectroscopy and X-ray diffractometry were used to study EUROFER or VVER ferritic reactor steels mechanically alloyed with TaC or NbC. Significant changes were found in the Mossbauer spectra and in the corresponding hyperfine field distributions between the ball milled pure steel and that alloyed with TaC or NbC. Spectral differences were also found in the case of use of same carbides with different origin, too. The observed spectral changes as an effect of ball milling of the reactor material steels with carbides can be associated with change in short range order of the constituents of steel.

Structure and Magnetic Properties of Aerosol Nanoparticles of Fe and Its Alloys

Structure and magnetic properties of aerosol nanoparticles of Fe and its alloys (FeMn, FeNi, FeNiMn, FePt, FeCr, FeCo, and FeCu) have been reviewed. It has been shown that, compared to a bulk material, the particles have a number of specific features being of much fundamental and applied interest. The effect of both a quenched high-temperature Fe modification and its oxides on the structure and magnetism of nanoparticles has been considered in detail. Particular attention has been paid to the recently observed fine structure in the hyperfine field distribution at iron nuclei in Mössbauer spectra for pure iron and its alloys both as a bulk and aerosol nanoparticles. This phenomenon makes it possible to reveal very weak magnetic interactions in the system under study. The plausible origin of these magnetic interactions has been also discussed.

Influence of cation disorder on the magnetic properties of ball-milled ilmenite (FeTiO3)

We have investigated the evolution of crystal structure, cation disorder and magnetic properties of ilmenite (FeTiO3) after increasing time of high-energy ball-milling in an inert atmosphere. Refinement of X-ray diffraction data show that the hexagonal crystal structure of ilmenite is maintained after high-energy ball-milling of up to 128 h, but neutron diffraction studies reveal significant cation redistribution of Fe2+ and Ti4+ ions in the ball-milled samples. Mössbauer spectroscopy studies show that the magnetic hyperfine field of Fe2+, which is around 5 T before ball-milling, increases, and after milling times longer than 4 h a broad distribution of hyperfine fields with values up to around 40 T for Fe2+ is seen. This can be explained by the cation disorder induced by the ball-milling which affects the orbital contribution to the magnetic hyperfine field. In contrast to some ball-milled spinel ferrites, the Néel temperature of ilmenite is not significantly affected by the cation disorder.

Low-temperature Mössbauer study of heterosite, (Fe, Mn)PO4

The heterosite phase occurring in a pegmatitic rock sample was characterized by X-ray diffraction, by energy-dispersive X-ray spectroscopy and by Mössbauer spectroscopy. The orthorhombic unit-cell parameters, expressed in Å, were found as a=9.733 (1), b=5.837 (1) and c=4.776 (1). The composition was determined to be (Fe0.54Mn0.43Mg0.04)PO4. Mössbauer spectra recorded at temperatures T of 65K and higher consist of two broadened quadrupole doublets. Their isomer shifts δ are both diagnostic for the ferric state. The dominant doublet (∼60% of total area) exhibits an average quadrupole splitting ΔEQ,av of 1.62mm/s at room temperature, while the weaker broader doublet has ΔEQ,av=0.68mm/s. For temperatures T≤60K the spectra are composed of a broad sextet and a central quadrupole doublet. The doublet persists down to the lowest applied temperature of 17K. It is concluded that this doublet is due to an Fe-bearing phase other than heterosite and which gives rise to the inner doublet appearing in the spectra recorded at T≥65K. The broad sextets, attributable to the heterosite phase, were fitted with model-independent hyperfine-field distributions. However, it was consistently experienced that using the common Lorentzian-shaped elementary sextets composing the distribution, could not adequately reproduce the observed line shapes. Instead, the calculations had to be based on the diagonalization of the complete hyperfine-interaction Hamiltonian. This is due to the unusually strong quadrupole interaction. The as-such calculated hyperfine parameters of the heterosite phase at 17K may be summarized as follows: maximum-probability hyperfine field Bhf,m=473kOe, isomer shift δFe=0.54mm/s, average quadrupole coupling constant ½e2qQ=1.50mm/s, asymmetry parameter of the EFG η=0.80, and polar angles of the hyperfine field with respect to the EFGs principal axes frame Ω=40° and Ψ=90°. The temperature variation of the hyperfine field was interpreted in terms of the Bean–Rodbell (BR) model. The BR parameter, ηBR, was found to be 0.90, indicating a first-order magnetic transition at TN=59.7K. The temperature variation of the isomer shift is explained by the second-order Doppler shift δSOD. Using the Debye model for the lattice vibrational spectrum for calculating δSOD, the characteristic Mössbauer temperature ΘM was found to be 400K, which is unusually low for a ferric compound.

Incommensurate spin density wave versus local magnetic inhomogeneities in Ba(Fe1−xNix)2As2: a 57Fe Mössbauer spectral study

We report 57Fe Mössbauer spectral results in pure and doped Ba(Fe1−xNix)2As2 with x = 0.01 and 0.03. We show that all these materials present a first-order magnetic transition towards a magnetically ordered state. In the doped compounds, a broad distribution of Fe hyperfine fields is present in the magnetic phase. We successfully fit the Mössbauer data in Ba(Fe1−xNix)2As2 in the framework of two different models: (i) an incommensurate spin density wave (IC-SDW); (ii) a dopant-induced perturbation of the Fe polarization, recently proposed to interpret 75As NMR data in Ba(Fe1−xNix)2As2, which is valid only in the very dilute limit x = 0.01. Moreover, we show here that these NMR data can also be successfully analysed in terms of the ‘incommensurate model’ for all doping contents by using the parameters obtained from the Mössbauer spectral analysis. Therefore it is not possible to rule out the presence of an IC-SDW on the basis of the 75As NMR data.

Structural and magnetic characteristics of Co1−xNix/2Srx/2Fe2O4 nanoparticles

Co1−xNix/2Srx/2Fe2O4 (x=0–0.5 in steps of 0.1) ferrite nanoparticles have been synthesized at room temperature, without calcination, using a reverse micelle process. The site preference was determined by Mössbauer spectroscopy at 300K. The hyperfine parameters were obtained, for the whole series of solid solutions. For the X≤0.20 samples, the spectra were fitted with two discrete sextets and for the X>0.20 samples, a magnetic hyperfine field distribution and a doublet were also imposed in the fit procedure. Hysteresis loops were measured using a superconducting quantum interference device magnetometer at 2K and 300K. The results indicate that the relative decrease in saturation magnetization of nanoparticles compared to the submicron particles could be attributed to a surface spin termination and disorder. Magnetic dynamics of the nanoparticles was studied by the measurement of ac magnetic susceptibility versus temperature at different frequencies and it is found that the results are well described by the Vogel–Fulcher model.

Magnetic and Structural Characterization of Mechanically Alloyed Fe50co50 Samples

Samples of nominal composition Fe50Co50 were produced by mechanical alloying byusing a planetary ball mill and different milling times. The samples were studied via X-raydiffraction, Mössbauer spectroscopy, and X-ray photoelectron spectroscopy to characterize thephase distribution resulting from the milling process. The Mössbauer data indicated that Co startsdiffusing into Fe after 8 h of milling. Between t = 8h and t = 24 h the sample has a heterogeneouscomposition, presenting a bimodal hyperfine field distribution with maxima centred at 34.3 T and35.8 - 36.4 T, compatible with the presence of different Fe environments (richer in Co and richerin Fe, respectively). After 48 h of milling, the sample presents a more homogeneous compositionshowing an almost symmetric hyperfine magnetic field distribution centred at H=34.9 T,indicating that a disordered equiatomic FeCo solid solution has already been formed. The X-rayphotoelectron spectroscopy data indicate that the native oxide layer formed on the freshly milledsamples contains Co2+, Fe2+ and Fe3+ oxides. After complete removal of this native oxide layer byAr ion bombardment, X-ray photoelectron spectroscopy analysis yields the composition of thenominal equiatomic Fe50Co50 alloy.

Probing the SmRhSn magnetic state by AC/DC magnetic measurements and 119Sn Mössbauer spectroscopy

Bulk magnetic AC and DC susceptibility studies have been performed on a polycrystalline SmRhSn sample. The measurements revealed ferromagnetic ordering with a Curie temperature of T C = 14.9(2) K. However, additional magnetic transitions have been found. The thermal variation of the hyperfine parameters has been studied in detail by 119Sn Mössbauer spectroscopy. The distribution of magnetic hyperfine fields observed at the tin sites points to a rather complicated magnetic structure of probably non-collinear character but with the magnetic samarium moments arranged in directions close to the ab-basal plane.

On the conditions eliciting a detailed structure in the hyperfine field distribution at 57Fe nuclei

A technique has been developed to extract a fine structure in the hyperfine field distribution (HFD) at Mössbauer resonating nuclei such as 57Fe. It has been shown that the NORMOS program package widely spread for fitting Mössbauer spectra may be used for this purpose under some restrictions for variable parameters. In a case of iron and its some alloys both as a bulk and aerosol nanoparticles, applying the technique has permitted to bring out a discrete structure in HFD at 57Fe nuclei caused with a set of weak magnetic interactions.

Mössbauer investigations of crystalline and quasicrystalline Al3(Mn,Fe) compounds

57Fe Mössbauer and magnetic measurements are reported on Taylor phase compounds T-Al3(Mn,Fe), which can be seen as complex metallic alloys. The orthorhombic unit cell contains 156 atoms. These investigations are further extended to Al71Mn19Fe10 crystallizing as a decagonal quasicrystal. Common to both structures are layers, which are oriented perpendicular to the b axis in the crystalline or to the periodic axis in the quasicrystalline compound, allowing the formation of characteristic building blocks. At low temperatures for all samples spin glass behavior is observed with freezing temperatures Tf increasing with Fe content. Above Tf the 57Fe Mössbauer spectra were analyzed by a superposition of two subspectra with intensity ratio around 75:25, which can be allocated to Fe substituted on Mn sites surrounded either by Al and Mn or solely by Al with Mn only at the edge of the nearest neighbor shell. For both subspectra quadrupole splitting and center shift do not change significantly with Fe content and with structure. Below Tf broad hyperfine field distributions with non-vanishing contributions at zero hyperfine field are present. Magnetic and electrostatic hyperfine interactions are governed by length scales which are determined by the very similar interatomic configurations forming the T-Al3Mn phase as well as the decagonal compound. The presence of long range atomic order is of less importance.

Crystal and magnetic structures in Perovskite-related [formula omitted] ( x=0.2, 0.33)

Using sol–gel synthesis, single phase perovskite-related compounds in the family La 1 − x Ca x FeO 3 − δ have been formed for x=0.2 and x=0.33, but not for x=0.5. The x=0.2 and x=0.33 compounds are isostructural with LaFeO 3 ( Pnma). The magnetic structure of La 0.8 Ca 0.2 FeO 3 − δ has been studied through Mössbauer spectroscopy and neutron powder diffraction. La 0.8 Ca 0.2 FeO 3 − δ is a G-type antiferromagnet with a magnetic moment magnitude of 3.0 ± 0.2 μ B at room temperature. The reduction in the magnitude of the antiferromagnetic moment compared to that published for LaFeO 3 is explained by the measurement being taken at room temperature rather than 4 K and by the presence of Fe 4 + ions which have weaker exchange interactions than Fe 3 + , causing a strong reduction in T N . Room temperature Mössbauer shows a broad magnetic hyperfine field distribution on the Fe sites in both La 0.8 Ca 0.2 FeO 3 − δ and La 0.67 Ca 0.33 FeO 3 − δ . On cooling, disproportionation of Fe 4 + into Fe 3 + and Fe 5+ is apparent, and the resulting Fe 5+ sextet measured at low temperature gives a reliable measure of the Fe 4 + fraction. This in turn shows that creation of high-oxidation-state Fe is the dominant charge balance mechanism on doping Ca 2 + into the Ln 3 + site indicating a disordered distribution of Fe 3 + and Fe 4 + . The lack of broadening of the Fe 5+ sextet suggests that there may be ordering in the distribution of Fe 5+.

Specific features of the hyperfine field at iron nuclei in aerosol nanoparticles of FeCo alloy

178 We previously detected a fine structure in the distri� bution of hyperfine fields at iron nuclei in nanoparti� cles of the Fe 1– xCox alloy [1]. We also established [2] that the average hyperfine field H i at iron nuclei in such alloys, with increasing cobalt concentration, ini� tially rapidly increases from H i = 33 T (pure iron) to the maximal value Hi = 36.6 T at 30 at % Co and then slowly decreases to H i ≈ 33 T again at x ≈ 80 at % Co and remains constant until a limiting cobalt concen� tration of x = 95 at %. Such behavior was observed for both monolithic alloys and aerosol nanoparticles pro� duced from them, as well as for samples prepared by ball milling. This indicates the absence of surface con� tribution to the hyperfine field of small particles. The still unclear cause of the observed concentration dependence of the hyperfine field in monolithic alloy and aerosol particles should probably be found at the atomic level.

Mössbauer study of local environment effects in the ordered Fe70Al30 Invar alloy: Temperature dependence of isomer shift in the spin-glass phase

The systematic studies of the magnetic hyperfine field distribution for 57Fe in the spin-glass (SG) phase of the ordered Fe70Al30 Invar alloy have been performed using Mössbauer spectroscopy technique in the temperature range from 5 to 80K. Particular emphasis has been placed on the low-field component of the distribution, which is considered as corresponding to the Fe sites in the SG magnetic configurations. The main result is the observation of the pronounced temperature dependence of isomer shift for several atomic SG configurations. The temperature behavior of the local electron density is strongly correlated to the Invar properties of the Fe70Al30 alloy. We argue that the observed temperature dependence of the isomer shift due to a local volume effect. The temperature range, for which the pronounced decrease in atomic volume is observed, coincides with the range of the existence of the Invar effect. The influence of the competition between opposite in sign exchange interactions on the Invar properties is discussed.

Mössbauer and Magnetization Measurements of Al<sub>3</sub>Mn(Pd,Fe) Compounds

Magnetic and 57Fe Mössbauer measurements are reported on Taylor-phase compounds T-Al3Mn(Pd,Fe), which are complex metallic alloys containing 156 atoms in the unit cell. These investigations are extended to Al71Mn19Fe10 which crystallize as decagonal quasicrystal. Common for all samples spin glass behavior is present at low temperatures with freezing temperatures Tf increasing (decreasing) with Fe (Pd) content. Below Tf rejuvenation and memory effects are observed in aging investigations. Above Tf the 57Fe Mössbauer spectra were analysed by a superposition of two subspectra with intensity ratio around 75:25, which might be allocated to Fe substituted on Mn sites surrounded either by Al and Mn or completely by Al in the nearest neighborhood. Below Tf broad hyperfine field distributions are present.

Maximum Entropy Method in Mössbauer Spectroscopy - a Problem of Magnetic Texture

A reconstruction of the three dimensional distribution of the hyperfine magnetic field, isomer shift and texture parameter z from the Mossbauer spectra by the maximum entropy method is presented. The method was tested on the simulated spectrum consisting of two Gaussian hyperfine field distributions with dierent values of the texture parameters. It is shown that proper prior has to be chosen in order to arrive at the physically meaningful results.

Ultrafine metallic Fe nanoparticles: synthesis, structure and magnetism.

The results of the investigation of the structural and magnetic (static and dynamic) properties of an assembly of metallic Fe nanoparticles synthesized by an organometallic chemical method are described. These nanoparticles are embedded in a polymer, monodisperse, with a diameter below 2 nm, which corresponds to a number of around 200 atoms. The X-ray absorption near-edge structure and Mössbauer spectrum are characteristic of metallic Fe. The structural studies by wide angle X-ray scattering indicate an original polytetrahedral atomic arrangement similar to that of β-Mn, characterized by a short-range order. The average magnetic moment per Fe atom is raised to 2.59 µB (for comparison, bulk value of metallic Fe: 2.2 µB). Even if the spontaneous magnetization decreases rapidly as compared to bulk materials, it remains enhanced even up to room temperature. The gyromagnetic ratio measured by ferromagnetic resonance is of the same order as that of bulk Fe, which allows us to conclude that the orbital and spin contributions increase at the same rate. A large magnetic anisotropy for metallic Fe has been measured up to (3.7 ± 1.0)·105 J/m3. Precise analysis of the low temperature Mössbauer spectra, show a broad distribution of large hyperfine fields. The largest hyperfine fields display the largest isomer shifts. This indicates a progressive increase of the magnetic moment inside the particle from the core to the outer shell. The components corresponding to the large hyperfine fields with large isomer shifts are indeed characteristic of surface atoms.