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.     

Competing anisotropy and random field effects in Mn1−xFexCO3

Previously published ⁵⁷Fe Mössbauer data for MnCO₃:Fe have been interpreted as arising from a competing anisotropy system. By considering the variation of the magnitude H_hf and direction θ_hf of the magnetic hyperfine field with temperature, the phase diagram for Mn_1?xFe_xCO₃ is deduced. The boundaries of the mixed phase are clear in the Mössbauer data. However, these boundaries are not sharp but are broadened by the effect of internal random magnetic fields.     

Preferential ordering of Mn and Fe Atoms in Y6(Fe1−xMnx)23

Neutron diffraction studies of the nomagnetic compositional range of the Y₆(Fe_1?xMn_x)₂₃ system reveal the presence of preferential ordering of Fe and Mn atoms on the four transition metal crystallographic sites. Throughout the entire compositional range of the ternary system, Mn atoms prefer to occupy the f₂ site and Fe atoms the f₁ site. Refinements of the data were carried out using the Rietveld profile method.     

Influence of hydrogen absorption on magnetic properties of Zr(Fe1−xVx)2 ternaries

Measurements of magnetization, susceptibility and Mössbauer effect were made on Zr(Fe_1?xV_x)₂ ternaries and their hydrides. Absorbed hydrogen leads to a large increase (20–30%) in volume without a change in the crystal structure. Ferromagnetism in the Fe-rich region is enhanced by hydrogen absorption, whereas hydrogenation leads to suppression of superconductivity in the V-rich range. The Fe moments in the Zr(Fe_1?xV_x)₂ hydrides are remarkably larger than those in the corresponding host compounds. The Fe moment in the β-ZrFe₂ hydride extrapolated reaches to 2.9μ_B, which exceeds the saturation value in bcc Fe. The hyperfine fields of ⁵⁷Fe in both Zr(Fe_0.8V_0.2)₂ and the hydride distribute widely, indicating that the Fe moments are very sensitive to the local environment arround the Fe atoms. Arguments are presented that it is possible to interprete the Fe moment increase by hydrogenation in terms of a decrease in occupancy of the 3d-band state due to electron transfers from Fe to hydrogen and/or vanadium.     

Local spin configurations of Fe atoms in the Rh1−x Fex (x=0.1, 0.2, and 0.3) system with competing exchange interactions

The local spin configurations of Fe atoms in the magnetically ordered alloys Rh_1?x Fe_x (x=0.1, 0.2, and 0.3) have been investigated by Mössbauer spectroscopy. The Mössbauer absorption spectra are measured in the range from 5 K to temperatures of the transition to the paramagnetic state. The measurements in magnetic fields with a strength up to 5 T are carried out at a temperature of 4.2 K. Analysis of the magnetic-hyperfinefield distribution functions demonstrates that Fe atoms form discrete sets of collinear spin configurations corresponding to different net moments of the nearest coordination sphere. The spin structure of the alloys is governed by a random distribution of Fe atoms over the lattice sites and the competition between the Fe-Rh ferromagnetic exchange interaction and the antiferromagnetic interaction of the neighboring Fe atoms. No spin frustration and spin “melting” effects characteristic of spin glasses are revealed in the Rh-Fe alloys.     

Low temperature conversion electron Mössbauer spectroscopy on FexGe1−x amorphous thin films

Conversion Electron Mössbauer Spectroscopy has been performed between room temperature and 4.2 K on a series of thin films 1000 Å thick of amorphous Fe_xGe_1?x alloys, 0.4 ? x ? 0.60. It shows that the onset of ferromagnetism for x > 0.4 is associated with the apparition of a small magnetic moment in Fe atoms, increasing rapidly with x. For a given composition, the distribution in the values of moments born by the Fe atoms is large, reflecting the statistical fluctuations of the nearest neighbour environment of the iron atoms.     

Relation between anisotropy and crystal structure in rare-earth (3d) transition compounds (R2M17)

The ⁵⁷Fe Mössbauer effect in Fe-rich compounds of the series Tm₂Fe_17?xCo_x was studied, using enriched iron. It was found that there exists a preference for occupancy by the Fe atoms for the f site. This fact proves useful to explain the changes in sign and magnitude of the anisotropy constant observed in other series of rare-earth transition compounds like Y₂Fe_17?xCo_x.     

Spin rotations in HoxEr1−xFe2 cubic laves compounds

The temperature dependence of the elastic moduli and the Mössbauer effect in Ho_x,Er_1?x, Fe₂ cubic Laves compounds (x, between 0.3 and 0.9) has been investigated in the temperature region where spin rotation occurs. The composition-dependence minima in the elastic moduli, and the Mössbauer effect measurements, were used to determine the boundaries between the various directions of easy magnetization in these compounds. The experimental spin orientation diagram was found to deviate from the predictions of a one-ion model based on the rare-earth ions alone. From the Mössbauer effect measurement it was deduced that in compounds for which a spin reorientation was observed, the spin rotates continuously with temperature between the major axes of the cubic symmetry. This was attributed to the contribution of higher-order magnetocrystalline anisotropy constants. The ΔE effect, measured in external magnetic fields up to 25 kOe, was found to be constant, and relatively small, in the holmium composition range of x = 0.45–1.00 in the Ho_xEr_1?xFe₂; compounds.     

Magnetic behaviour of Sm6Mn23−xFex compounds

Results of magnetic investigations on Sm₆Mn_23-xFe_x compounds performed over a wide temperature range are presented. It was found that replacing the Mn atoms in Sm₆Mn₂₃ and Fe atoms in Sm₆Fe₂₃ by Fe and Mn, respectively, causes a rapid reduction in both the Curie temperature and the magnetic moment. No magnetic order was found in the 4 < x < 8 range. The temperature dependence of the reciprocal susceptibility of the investigated compounds can be described using the Néel law.     

Magnetic phase transitions in the Fe1−xMnxSn2 alloys for 0≤x≤1.0: A Mössbauer and magnetization study

⁵⁷Fe Mössbauer study of the pseudo-Binary alloys Fe_1?xMn_xSn₂ for 0≤x≤1.0 reveal the antiferromagnetically ordered state at 79K for all the specimens of the series. The hyperfine field at Fe site decreases with increasing manganese concentration. Magnetic susceptibility measurements performed in the temperature range 80 K≤T≤300 K indicate that the Néel temperature decreases with increasing Mn concentration for the samples withx≤0.4 whereas it increases continuously for the specimens havingx>0.4 of the alloy series.     

Mössbauer studies of FexNbS2 (x = 0.25, 0.33 and 0.5)

Mössbauer studies of Fe_xNbS₂ (x = 0.25, 0.33 and 0.5) have been carried out for temperatures from 4.2K to about 715K. The iron exists in high spin divalent state for all the compositions. The temperature dependence of quadrupole splitting and center shift shows a reversible phase transition at about 600K and a possible disordering of Fe vacancies beyond this temperature for Fe_0.25NbS₂ and a reversible phase transition at 490K for Fe_0.5NbS₂. The hyperfine magnetic fields have been evaluated from the magnetically ordered spectra. The observed temperature dependent line intensities of the quadrupole doublet are attributable to the temperature dependence of the difference in the meansquare amplitude of vibrations parallel and perpendicular to the EFG axis.     

Magnetic properties of the ternary alloy system (FexNi1−x)11Se8 for 0.042<x<0.23

The ternary alloy system (Fe_xNi_1?x)₁₁Se₈ for 0.04257Fe Mössbauer experiments at 4.2 K. A preliminary analysis of the spectra reveals that all samples are magnetically ordered at 4.2 K and the magnetic hyperfine field increases with increasing iron concentration. Temperature dependent Mössbauer spectra measured for x=0.22 indicate that the transition temperature is about 108±2 K. The results are discussed in terms of the magnetic and structural properties of the system.     

Mössbauer spectroscopy on the double substituted lithium ferrite Li[Fe0.9(AlxGa1−x)0.1]5O8

Room temperature Mössbauer spectra of Ga and Al substituted lithium ferrite Li[Fe_0.9(Al_xGa_1?x)_0.1]₅O₈ with x = 0.00, 0.25, 0.50, 0.75 and 1.00 are reported. It is shown that the varying covalence of bonds caused by gallium and aluminium ions can explain the observed values of hyperfine fields and isomer-shifts.     

Hyperfine interactions and local magnetic ordering in Zr(Fe1-xCox)2 (0⩽x⩽0.2)

In the pseudobinary intermetallic compounds Zr(Fe_1-xCo_x)₂ (0?x?0.2) the hyperfine fields of all nuclei present are investigated by means of Mössbauer effect and NMR. While for the “nonmagnetic” site the Zr-hyperfine field depends on the configuration of the nearest Fe, Co neighbours, no such effect is observed for the hyperfine fields on the “magnetic” sites. A large pseudodipolar interaction is observed for the Fe and Co atoms, from which the coexistence of several directions of magnetization can be deduced. The easy direction seems to be determined by the respective Fe/Co configuration.     

Magnetic and electrical transport properties of amorphous Zr-Fe alloys

Amorphous Zr_1?xFe_x samples were prepared in the composition range 0.2 ? x ? 0.9 either by means of vapour deposition or melt spinning. The electrical resistivity was determined in the range 4.2–300 K. Negative temperature coefficients were observed in the whole concentration range. The extended Ziman theory (diffraction model) was found to be able to explain these results only if the effective valence of the Fe atoms involves not only s electrons but also d electrons. The magnetic properties and the ⁵⁷Fe Mössbauer effect of the Zr_1?xFe_x alloys were studied in the range 4.2–300 K. The Fe-rich alloys are ferromagnetic. The Fe moment vanishes in alloys of an Fe concentration lower than about 50 at%. In most alloys (x ? 0.8) the Curie temperature is below room temperature and continuously decreases with Zr concentration. By means of Mössbauer spectroscopy and magnetic measurements it is shown that compositional short-range order (CSRO) is present to a higher degree in melt-spun alloys than in vapour-deposited alloys. The effect of sign and magnitude of the heat of solution on CSRO and the magnetic properties is discussed.     

A Mössbauer study of the substituted hexagonal ferrites BaSrxCa1−xFe4O8 (0 ⩽ x ⩽ 1)

Ferrites of the formula BaCa_1?xSr_xFe₄O₈ (0 ? x ? 1) gave rise to magnetically split Mössbauer spectra indicative of antiferromagnetic ordering at room temperature. These spectra were analysed to yield the isomer-shift, quadrupole splitting, internal magnetic field and its orientation. The unit cell dimensions were obtained from X-ray powder patterns and were used in obtaining estimates of the electric field gradient (e.f.g.). The calculated and observed e.f.g. values indicate increasing symmetry as x → 1. A temperature dependence study of BaCaFe₄O₈ gave a Neel temperature T_N = 670 K and H_eff(O) = 485 kOe.     

The coordination of Co2+ions in Co substituted magnetites,Fe3−xCoxO4, with x⩽0.04

⁵⁷Fe Mössbauer spectra at room temperature, both with and without external magnetic field, indicate that Co²⁺ ions in Co_xFe_3?xO₄spinels (x?0.04) are situated on the octahedral B sites. The Mössbauer parameters are listed and the existence of unpaired Fe³⁺ ions is evidenced.     

A magnetic and Mössbauer study of magnetic ordering and vacancy clustering in Cu5FeS4

Magnetic susceptibilities and Mössbauer spectra recorded at temperatures between 4° and 300°K show that the low temperature form of Cu₅FeS₄, bornite, orders magnetically at 76±2°K. At a lower temperature 8°K a second magnetic phase transition occurs. The Mössbauer spectra suggest that there is significant partial disordering of cations and vacancies in tetrahedral holes of the face-centred cubic sulphur lattice. Thermoelectric power measurements indicate that bornite is a p-type semiconductor.For a three dimensional magnetic superexchange interaction between Fe(III) atoms a small super-transferred spin density would be required on intermediate Cu(I) atoms.     

Transferred hyperfine fields at 119Sn in Fe1−xMxSn [M≡Mn, Co and Ni]

The transferred hyperfine fields at ¹¹⁹Sn, using Mössbauer spectroscopy are reported for the hexagonal B-35 compounds with a general formula Fe_1?xM_xSn, where MMn, Co and Ni. In these compounds, Sn atoms occupy two crystallographically inequivalent sites. For FeSn the observed spectrum consists of a quadrupole doublet and a magnetic pattern corresponding to 2(d) and 2(a) sites respectively. The data have been analysed to resolve the controversy regarding hyperfine parameters. On replacing Fe by Mn atoms, additional lines appear in the higher velocity region of the Mössbauer spectrum and the intensity of the nuclear Zeeman pattern increases at the expense of quadrupole doublet. The resulting Mössbauer spectra have been analysed by taking only the nearest neighbour interactions into account. This analysis shows that on replacing each Fe atom by a Mn atom, the hyperfine field at 1(a) Sn site increases by about 40 kOe and a field of about 35 kOe is produced at the 2(d) Sn sites. Further, from the nuclear Zeeman pattern for 2(d) sites, the sign of quadropole splitting for these sites could also be determined and was found to be positive. However, the substitution of Co and Ni in place of Fe atoms results in a broad unresolved pattern suggesting that the hyperfine field at the 1(a) sites decreases and a finite field develops at the 2(d) site. The origin of transferred hyperfine fields at the two inequivalent Sn sites is discussed, the magnetic transition temperatures of these compounds have been estimated and the magnetic moments of M-atoms have been inferred.     

Magnetic order in palladium-based Heusler alloys Part I: Pd2MnIn1−xSnx and Pd2MnSn1−xSbx

Magnetization, susceptibility, X-ray and neutron diffraction measurements have been made on the two series of alloys Pd₂MnIn_1?xSn_x and Pd₂MnSn_1?xSb_x, for 0 ? x ? 1. All were single phase and were chemically ordered intermetallic compounds with the Heusler L2₁ structure in which the Mn atoms occupy an f.c.c. sub-lattice. At all compositions the alloys were magnetically ordered with a moment of ～4.3 μ_B located at the Mn sites. At the In-rich end the magnetic order is antiferromagnetic f.c.c. type 2. As Sn is increasingly substituted for In there is a change in magnetic order first to antiferromagnetic f.c.c. type 3A and then to ferromagnetism. All the alloys in the Sn/Sb series are ferromagnetic and in both series there is an increase in the ferromagnetic exchange interactions with increasing electron concentration.