57Fe Mössbauer effect studies on the magnetism of iron-rich bcc,fcc and hcp phases of Fe100−c Mn c

Iron-rich Fe_100?c Mn _c -alloys (0<c _Mn<35 at%) are found in three different phase structures depending on manganese concentration: bcc, fcc and hcp. Mössbauer effect studies on these phases reveal different magnetic properties: ferromagnetism, antiferromagnetism and paramagnetism. The evaluation of poorly resolved Mössbauer spectra of a sample with FeNi-Invar analogous composition (c _Mn=35%) and coexisting fcc and hcp phases was possible with the help of the Afanas'ev-Tsymbal sharpening method introduced recently [2,3].     

Mössbauer studies of the atomic distributions and hyperfine interactions in Mn-Fe alloys with β-Mn structure

Mn_20?x Fe_x alloys (x=1–6.4) with β-Mn structure are studied by Mössbauer spectroscopy. The electric field gradient tensor for the crystallographically nonequivalent sites 8(c) and 12(d) are calculated. Partial Mössbauer spectra are identified, and the ⁵⁷Fe hyperfine-interaction parameters determined. Iron atoms substituting for manganese in the Mn_20?x Fe_x system exhibit strong preference for occupying 8(c) sites. With increasing iron concentration, the longrange order parameter in the Fe atom arrangement increases, and the degree of order, decreases.     

From FeNi-Invar to FeNiMn reentrant spin-glasses

Thirty years of Mössbauer spectroscopy correspond roughly to twenty-five years of FeNi-invar research using this method. The first systematic study of hyperfine fields in FeNi alloys, in both bcc and fcc phases, appeared in 1963, the first spectra were published probably in 1964. A very important fact is that fcc iron is an antiferromagnet; however, this iron phase is unstable at low temperature. In 1963, the possibility of performing low-temperature Mössbauer spectroscopy experiments on fcc iron precipitates in copper was shown. We shall demonstrate that the low temperature⁵⁷Fe Mössbauer spectra reveal a spatially inhomogeneous magnetic structure in FeNi-invar alloys. This structure corresponds to magnetic invar anomalies. From the viewpoint of basic research in magnetism, it was opportune to enhance these anomalies. One possible way to do this was the alloying of manganese into the invar matrix. In this contribution, we limited our examples to the study of the classical Fe_0.65Ni_0.35 and the new (Fe_0.65Ni_0.35)_1−x .Mn _x alloys, and refer to the literature with respect to other transition metal systems with similar properties. We will show the goal of Mössbauer spectrometry investigating the spin structure in disordered alloy systems via the vector nature of the hyperfine fields. Additionally, the combination of Mössbauer spectrometry with global techniques, i.e. DC magnetization and AC susceptibility allowed us to study the changes in magnetic properties of our disordered 3d transition metal alloys from a nearly collinear ferromagnet (pure FeNi-invar) over the reentrant spin-glasses and “pure” concentrated spinglasses towards the antiferromagnetic behaviour by varying the manganese concentration in small steps.

     

Mössbauer studies of Fe x Mn1 − x S single crystals

Single crystals of iron manganese sulfides Fe _x Mn_{1 ? x} S (0.25 ≤ x ≤ 0.29) are experimentally investigated using Mössbauer spectroscopy and x-ray diffraction. The Mössbauer spectra measured at 300 K exhibit a single broadened line characteristic of paramagnets. The isomer shift of this line is equal to 0.92–0.94 mm/s, which is typical of Fe²⁺ ions in the octahedral position. The quadrupole splitting (0.18–0.21 mm/s) suggests a distortion of the coordination polyhedron of iron ions in the Fe _x Mn_{1 ? x} S compounds.     

Mössbauer studies of hyperfine fields in disordered Fe2CrAl

Heusler-like alloy Fe₂CrAl was prepared and studied. Structure determination was done by X-ray. The structure was found to conform to the B₂ type. Magnetic hyperfine fields in this sample were studied by the Mössbauer effect. The Mössbauer spectra were recorded over a range of temperature from 40 to 296 K. The Mössbauer spectra showed the co-existence of a paramagnetic part with a magnetic hyperfine portion at all recorded temperatures. Even with the distribution in the magnetic hyperfine field, the average hyperfine field follows the (T/T _c)^3/2 law. The paramagnetic part of the hyperfine field is explained in terms of the clustering of Cr atoms.     

Mössbauer effect on YFe3 compound

The thermal variation of the Mössbauer parameters of the YFe₃ compound between 78 K and T_c is studied. The results agrees well with the magnetic measurements. For the iron site 18 (h) the presence of an anisotropic interaction splits the Mössbauer spectrum into two six-lines patterns with the intensities in 2:1 ratio but above 400 K the intensities of the Mössbauer spectra of the sublattices are in 1 : 2: 6 ratio in agreement with crystallographic data.     

Mössbauer studies in amorphous Fe80−x−y Ni x Mn y B12Si8 alloys

Room temperature Mössbauer studies have been carried out on amorphous Fe_80?x?y Ni _x Mn _y B₁₂Si₈ (x=35, 40, 45, 50 and 0≤y≤1). Thex=40 and 45 samples, showed an increase in the average hyperfine field, wheny increased from 0.0 to 0.2 at% in the former and 0.0 to 0.5 at% in the latter. The hyperfine field decreased fory=1.0. This is similar to the results of the magnetic moment on these samples, published earlier.     

Magnetic properties of the pseudo-binary Fe2(Nb1-xMnx)

The magnetic behavior of the pseudo-binary system Fe₂(Nb_1-xMn_x) is investigated by means of the experimental techniques of X-ray diffraction (XRD), Mössbauer Spectroscopy (MS) and magnetization studies. The XRD results indicate that, up to x=0.3, all samples are single phase with hcp structure. This corresponds to the solubility limit of manganese in this phase. Above x=0.3, all prepared samples present the coexistence of three phases, two with hcp structure and one fcc. The magnetization measurements at low temperatures indicate that the transition temperature increases with the addition of Mn atoms in the Fe₂Nb host (T_N=10 K) up to 58 K for x=0.1. The Mössbauer spectra were fitted with a quadrupole splitting distribution, which indicates that the average quadrupolar splitting increases slightly with the increase of the manganese concentration.     

The nature of spin glass state in γ-Fe70Ni30-cMnc alloys

The transition paramagnet—spin glass is investigated in γ-Fe₇₀Ni_30-cMn_c alloys using a.c.-susceptibility, Mössbauer effect and small angle neutron scattering measurements. Different behaviours are proposed depending on the Mn concentration.     

Polaronic and charge ordering effects in Pr 0.5 Sr 0.5 Mn 1− x Fe x O 3

In this work we report the results of X-ray diffraction and Mössbauer spectroscopy for the systems Pr_0.5Sr_0.5Mn_1???x Fe _x O₃ (with x?=?0.05, 0.10, 0.15, 0.20, 0.25, 0.30). XRD patterns indicated that all samples were single phase with slightly distorted orthorhombic symmetry. Room temperature Mössbauer spectra are all quadrupole split, indicating paramagnetic relaxation of the Fe moment for all values of Fe concentrations. The spectra are fitted with two doublet components associated with Fe^3?+? ions in octahedral sites with different distortions. Mössbauer spectra recorded at liquid nitrogen temperature for this system also indicate paramagnetic relaxation of the Fe moments down to liquid nitrogen temperature (LNT). In these spectra a third quadrupole component with quadrupole splitting close to zero develops. This component is associated with the delocalization of the charge carriers and the consequent disappearance of lattice distortions produced by the polaronic effect at room temperature. The component with the high quadrupole splitting (0.81 to 1.07 mm/s) results from Jahn–Teller distortion as a consequence of charge ordering transition at low temperature.     

An57Fe Mössbauer effect study of the highT c superconductor GdBa2Cu3O7−y

Split source⁵⁷Fe Mössbauer effect spectroscopy has been performed between 4 K and 295 K on the superconducting perovskite GdBa₂Cu₃O_7?y. No evidence is seen for magnetic splitting at low temperatures as reported in some split absorder⁵⁷Fe Mössbauer experiments on this material. There is evidence for phonon mode softening, as observed for¹¹⁹Sn Mössbauer spectra of some other highT _c superconductors.     

Mössbauereffekt- und Magnetisierungsmessungen an EuxYb1-xSn3

Mössbauer effect and magnetization measurements were performed in the intermetallic system Eu_xYb_1-xSn₃ for 0≦x≦100 at%. The magnetic behavior of the system depends only on the Eu concentration. For allx the system has the same crystal structure (fcc) and is ordered antiferromagnetically. The dependence of the paramagnetic Curie temperature, the ordering temperature, and the hyperfine fields at the Sn and Eu sites on the Eu concentration are discussed in the frame of the RKKY theory.     

Mössbauer study of Tl containing highT c superconductors

Mössbauer spectroscopy, X-ray diffractometry, ESR and electrical resistivity measurements, were used to study highT _c (above 100 K) superconducting materials in order to get information about the effect of the preparation circumstances as well as about the anomalous behaviour in⁵⁷Fe and¹¹⁹Sn doped TlBaCaCu(⁵⁷Fe)O_4.5+δ and TlBaCuCu(¹¹⁹Sn)O_4.5+δ superconductors. We have found that the Mössbauer parameters strongly depend on the preparation circumstances. In many cases the presence of Mössbauer lines of nonsuperconducting oxide phases indicated that the iron and tin could not entirely built in into the lattice of superconductor. Four valence state tin and four valence state iron sites were identified in the superconducting phases. We have found anomalous temperature dependent changes in the⁵⁷Fe Mössbauer spectra of TlBaCaCu(⁵⁷Fe)O_4.5+δ samples around theT _c between 105–135 K. In this temperature range the total area of the spectrum temporary increases.     

Mössbauer insight to metallurgy, materials science and engineering

A brief overview of the contributions which Mössbauer effect spectroscopy has made to areas of materials science is presented. A survey of the literature reveals the decreasing trends of established areas, with emergence in the past decade or so of new areas such as nanostructured materials and materials produced by mechanochemical treatment and the continuing importance of rare-earth magnetic materials. Examples of applications of ⁵⁷Fe and ¹¹⁹Sn Mössbauer spectroscopy, both transmission and backscattering, are discussed. The complementary nature of Mössbauer spectroscopy and neutron diffraction in delineation of the magnetic behaviour and structures of materials is demonstrated by the La_1?x Y _x Mn₂Si₂ series of rare-earth intermetallic compounds.     

Investigations of crystal and magnetic properties of the Mn5−xFexGe3 intermetallic compounds

The Mn_5?xFe_xGe₃ intermetallic compounds are investigated with X-ray, neutron diffraction, magnetometric and Mössbauer effect methods. It is found that crystal structure of x = 1 compound is of D8₈ type while the structure of x = 3, 4 and 5 compounds is of B8₂ type. All are ferromagnets with collinearly ordered atomic spins. The lattice constants are derived from X-ray diffraction patterns, while magnetometric measurements yield the Curie temperatures and Weiss constants as well as the values of magnetic moments per molecule in ferromagnetic and paramagnetics states. The distributions of Fe and Mn atoms among two non-equivalent crystal sites are determined with the neutron diffraction method and are confirmed by the Mössbauer effect measurements. The parameters of hyperfine interactions are derived from Mössbauer absorption spectra and are attributed to iron atoms in two non-equivalent crystal sites.     

Evidence of Clustering in Heusler like Ferromagnetic Alloys

Two Heusler like alloys Fe₂CrAl, and Mn₂CoSn had been prepared and studied using the Mössbauer effect. Both these alloys were ferromagnetic in nature. Their Mössbauer spectra showed the co-existence of a paramagnetic portion along with magnetic hyperfine part upto temperatures well below their Curie temperatures. Low temperature Mössbauer spectra for both these alloys showed a steady variation in the intensity of the paramagnetic portion. X-ray diffraction studies made on these samples ruled out the possibilities of a separate phase. Explanation of the observed phenomena is given by clustering around the magnetic ions present in these samples.     

Hydrogen absorption effects in the Zr(Fe0.5Cr0.5)2 compound

Zr(Fe_0.5Cr_0.5)₂ Laves phase samples have been made to absorb different hydrogen amounts up to 3.3 H/f.u. XRD showed all ZrFeCrH _y samples to have the C14 structure and, for 0 < y < 2.6, to be composed of a H-poor α-phase and a H-rich β-phase having different c/a ratios. Both phases could be distinguished in the Mössbauer spectra. The β-phase fraction was determined both from X-ray and Mössbauer analysis, with excellent agreement. β-phase lattice constants and hyperfine parameters vs. y exhibited a small jump at y ≈ 1.75, suggesting a phase transition. Mössbauer spectra of ZrFeCrH_3.3 in the 78–350 K range revealed that, for Fe at the 6h site, quadrupole splitting and linewidth increased for decreasing temperature. These effects were attributed to H atomic jump diffusion, and an activation energy of ≈70 meV was estimated.     

Phenomenological simulation and density functional theory prediction of 57 Fe Mössbauer parameters: application to magnetically coupled diiron proteins

The use of phenomenological spin Hamiltonians and of spin density functional theory for the analysis and interpretation of Mössbauer spectra of antiferromagnetic or ferromagnetic diiron centers is briefly discussed. The spectroscopic parameters of the hydroxylase component of methane monooxygenase (MMOH), an enzyme that catalyzes the conversion of methane to methanol, have been studied. In its reduced diferrous state (MMOH _Red ) the enzyme displays ⁵⁷Fe Mössbauer and EPR parameters characteristic of two ferromagnetically coupled high spin ferrous ions. However, Mössbauer spectra recorded for MMOH _Red from two different bacteria, Methylococcus capsulatus (Bath) and Methylosinus trichosporium OB3b, display slightly different electric quadrupole splittings (ΔE _Q ) in apparent contradiction to their essentially identical active site crystallographic structures and biochemical functions. Herein, the Mössbauer spectral parameters of MMOH _Red have been predicted and studied via spin density functional theory. The somewhat different ΔE _Q recorded for the two bacteria have been traced to the relative position of an essentially unbound water molecule within their diiron active sites. It is shown that the presence or absence of the unbound water molecule mainly affects the electric field gradient at only one iron ion of the binuclear active sites.     

Magnetic properties of amorphous rare-earth—Iron alloys

The magnetic properties of various amorphous alloys of the type R_1-xFe_x (R = Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu) have been determined in the concentration range 0.3 ? x ? 0.5. With the exception of the Gd and Lu alloys pronounced thermomagnetic history effects were observed in the temperature dependence of the magnetization. These effects are due to a strong temperature dependence of the coercive force (H_c) which is found to obey an exponential law of the form H_c ∞ exp(-αT). ⁵⁷Fe Mössbauer spectra were obtained on the alloys of a composition close to 40 at% Fe. From the combined results of the Mössbauer spectroscopy and magnetic measurements it is derived that the Fe moment decreases if one passes through the rare-earth series. It is postulated that this decrease is due to small differences in the compositional short-range order in the amorphous alloys caused by the heat of mixing becoming more negative in the same sense.