The influence of the local surrounding on the hyperfine interactions in Zr(Fe1−x Ni x )2 compounds

Mössbauer spectroscopy and the TDPAC method have been used to study Zr(Fe_1–x Ni _x )₂ compounds forx0.30. The hyperfine magnetic field at the Fe sites and the quadrupole splitting as functions of nickel concentration were analysed by use of⁵⁷Fe Mössbauer spectroscopy. Values of the internal magnetic field on¹⁸¹Ta nuclei have been found by means of the TDPAC method.     

Mössbauer study of Zn x Ni5/3−x Fe1Sb1/3O4 spinel ferrite

The antimony substituted nickel ferrite Zn_xNi_5/3–xFe₁Sb_1/3O₄ with different values ofx are prepared, checked by x-ray and studied with⁵⁷Fe Mössbauer spectroscopy over a wide temperature range. Characteristic spectra of paramagnetic, magnetic and electronic relaxation types for the different compositions have been observed. The interpretation of the spectra allows the cation distribution of the compounds to be deduced. The Mössbauer effect parameters at different temperatures are calculated and their dependence on the substitution of non-magnetic Zn⁺² for the magnetic nickel ions are discussed. The temperature dependence of the hyperfine magnetic fields and the respective Néel temperature points are obtained.     

Magnetic properties of R2Fe17−xGax (R=Y and Gd) compounds

The magnetic properties of Y₂Fe_17−xGa_x for 3≤x≤7 and Gd₂Fe_17−xGa_x for 5≤x≤7 have been investigated using ⁵⁷Fe Mössbauer spectroscopy. These compounds have the rhombohedral Th₂Zn₁₇ structure. X-ray diffraction analysis of aligned powders shows that the easy direction of magnetization is parallel to the c-axis in Y₂Fe₁₀Ga₇ and Gd₂Fe₁₀Ga₇ and is perpendicular to the c-axis in Y₂Fe₁₄Ga₃, Y₂Fe₁₂Ga₅, Gd₂Fe₁₂Ga₅ and Gd₂Fe₁₁Ga₆. Mössbauer studies indicate that those samples are ordered ferromagnetically. The ⁵⁷Fe hyperfine field decreases with increasing Ga content. This decrease results from the decreased magnetic exchange interactions resulting from Ga substitution. The average isomer shift, δ, for R₂Fe_17−xGa_x (R=Y and Gd) at room temperature is positive and the magnitude of δ increases with increasing Ga content.     

Magnetic properties of RE0.7Ca0.3Mn0.95Fe0.05O3 (RE=Sm and Gd) manganites at low temperature

The magnetic properties of RE_0.7Ca_0.3Mn_0.95Fe_0.05O₃ perovskite with rare-earth cations (RE=Sm and Gd) were investigated by means of X-ray diffraction, Mössbauer spectroscopy, and low temperature (4.2-266 K) magnetization measurements. Structural characterization of these compounds shows that they both have orthorhombic (Pbnm) structure. The Mössbauer spectra show clear evidence of local structural distortion of the Mn(Fe)O₆ octahedron, which is based on the non-zero nuclear quadrupole interactions for high-spin Fe³⁺ ions. It was found that the local structural distortion increases significantly when Sm³⁺ is replaced by Gd³⁺. This distortion is attributed to the Jahn-Teller coupling strength as estimated from the Mössbauer effect results. The magnetic results indicate that the Curie temperature decreases as a result of replacing Sm by Gd. This is due to the decrease of the average A-site cationic radius 〈r_A〉. The rapid increase of magnetization at low temperature indicates the magnetic ordering of rare earth ions at the A-site.     

TDPAC andemission Mössbauer studies of Fe3O4(99Ru)

Hyperfine interactions of⁹⁹Ru(»⁹⁹Rh) nuclei in Fe₃O₄ were studied by means of TDPAC andemission Mössbauer spectroscopy. The isomer shift obtained from an emission Mössbauer spectrum andthe temperature dependence of the hyperfine magnetic field obtained from TDPAC measurements indicate that⁹⁹Ru ions arising from⁹⁹Rh nuclei dilutely doped in Fe₃O₄ exist as a mixed state of Ru²⁺ andRu³⁺, which is not common in ruthenium oxides. Fe₃O₄(⁹⁹Ru) is the first example where a Ru ion in a low valence state exhibits its own magnetization in oxides.     

57Fe Mössbauer spectroscopy on disordered crystalline media with Ca-gallogermanate type structure. II. Below the magnetic ordering temperature

Magnetic phase transitions in disordered crystalline iron germanates of the type Sr₂LnFe₃Ge₃O₁₄ (Ln=La; Nd) were observed at low temperatures by means of⁵⁷Fe Mössbauer spectroscopy. The hyperfine interaction of Fe³⁺ magnetic moments ordered in one octahedral and two tetrahedral magnetic sublattices is discussed. A local environment computation on the octahedral sites confirms the assumption that the wide distribution found for the 4.2 K octahedral hyperfine fields is due to the random occupation by Fe³⁺ and Ge⁴⁺ ions of the nearest cation environment of the octahedral iron. The effect of substituting Ln rare earth ions in the Thompson cubes on the magnetic behaviour of these compounds is pointed out.     

Mössbauer and magnetic studies of orthorhombic FeSiO3

Magnetic properties of orthoferrosilite FeSiO₃ have been examined using susceptibility, magnetization measurements and Mössbauer spectroscopy. From magnetic and Mössbauer measurements, one obtains close values of the magnetic ordering temperature, T_N=39±1 K and T_N=41±1 K, respectively. The magnetic order is characterized by strong ferromagnetic coupling of Fe²⁺ moments within the ribbons and a weak antiferromagnetic coupling of the moments between adjacent ribbons. The 4.2 K Mössbauer spectra can be fitted with two different hyperfine magnetic fields H_hf=68 kOe and H_hf=314 kOe which can be assigned to Fe²⁺ in the octahedrally coordinated M1 and M2 sites, respectively, of the FeSiO₃ structure.     

Mössbauer,X-ray diffraction and AC susceptibility studies on nanoparticles of zinc substituted magnesium ferrite

Nanoparticles of zinc substituted Mg-ferrite with compositions Mg_(1-x)Zn _x Fe₂O₄ (x = 0.15, 0.30 and 0.50) having particle sizes in the range 6.4 nm to 21.4 nm prepared by the co-precipitation method were characterized by ⁵⁷Fe Mössbauer spectroscopy, X-ray diffratometry and AC magnetic susceptibility measurements. Mössbauer measurements at room temperature and down to 20 K clearly indicate presence of superparamagnetic particles in all the samples. AC magnetic susceptibility data show lowering of blocking temperature with decrease of particle size. Superparamagnetic relaxation was observed for larger particle size in samples with higher Zn content, which is attributed to the weakening of A-B exchange interaction in ferrite lattice due to replacement of Fe^{3 +} in tetrahedral site by Zn^{2 +} ions.Received: 16 April 2004, Published online: 23 July 2004PACS: 75.50.Tt Fine-particle systems; nanocrystalline materials - 76.80. + y Mössbauer effect; other gamma-ray spectroscopy - 75.30.Cr Saturation moments and magnetic susceptibilitiesS. Das: Present address: Department of Physics, Jadavpur University, Kolkata - 700032, India     

Characterization of a soil ilmenite developed from basalt

A natural ilmenite was extracted from the silt fraction of a soil developed from basalt of Minas Gerais State, Brazil. The sample was characterized by chemical analysis, X-ray diffraction, magnetic measurements and Mössbauer spectroscopy. The Mössbauer spectrum at 293 K shows two doublets: a major subspectrum of Fe²⁺ (relative area 74%) and a minor one of Fe³⁺ (26%). An incipient magnetic structure appears at 85 K, with a broadline sextet. These results may be explained by supposing that the separate is a solid solutionxFeTiO₃ · (1–x)Fe₂O₃, withx=0.85, which is magnetically ordered at low temperature. The lattice parameters for this hexagonal structure area=0.5082±0.0001 nm andc=1.398±0.001 nm.On leave of absence from the Departamento de Química, Universidade Federal de ViÇosa, 36570 000 ViÇosa (MG), Brazil.     

Mössbauer effect study of the pseudo-binary system (Ti1−x Nb x )Fe2

Mössbauer spectroscopy and X-ray diffraction measurements have been done on (Ti_1–x Nb _x )Fe₂ compounds in order to investigate the effect of Nb on the magnetic properties of TiFe₂. The experimental results show that Nb enters the lattice by filling Ti sites, thereby forming a continuous phase over the whole range of Nb concentrations. The Mössbauer spectra at 80 K fitted with a magnetic hyperfine field distribution show a continuous decrease of the average magnetic hyperfine field with increasing Nb concentration, as well as several different magnetic configurations forx0.3.     

Enhancement of the recoilless fraction in R2Fe10Co4Si2 (R=Gd, Er, Y) offstoichiometric compounds after hydrogenation

Samples of R₂Fe₁₀Co₄Si₂ (R=Gd, Er and Y) solid solutions were prepared by arc melting and exposed to hydrogenation. Characterization of hydrogenation effects was done using Mössbauer spectroscopy. 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. Slight changes in the relative site occupancies were observed. Transmission Mössbauer spectra were also recorded for absorbers consisting of the intermetallic and the stainless steel etalon. The recoilless fraction was derived from these spectra using the method developed by us. Our study is the first to evidence that the recoilless fraction is enhanced after hydrogenation.     

Phase analysis of hard magnetic Nd4Fe77B19 ribbon

Amorphous Nd₄Fe₇₇B₁₉ ribbon was prepared by conventional single-roller technique. Room temperature transmission⁵⁷Fe Mössbauer spectra were taken in the as-prepared state and after annealing 650°C/30 min in N₂-atmosphere and compared with that of the crystalline master alloy. The broad hyperfine induction distribution in the amorphous state was fitted by two symmetric Gaussians reflecting its asymmetry. In the annealed state, the spectrum decomposes into a series of sharp sextets corresponding to -Fe (6%), Nd₂Fe₁₄B (38%) present also in the master alloy, tetragonal Fe₃B (39%) and probably an inter-grain remainder (17%). No traces of the Nd₂Fe₁₇ admixed in the master alloy were found in the crystalline state. The results of the Mössbauer spectroscopy are compared with those of the X-ray and magnetic methods.     

Mössbauer spectroscopy on RE2Fe14BHx (RE-Y,Ce, Dy,Er) alloys

RE₂Fe₁₄B alloys and some of their hydrides have been studied through⁵⁷Fe and¹⁶¹Dy Mössbauer spectroscopy (MS). For both the paramagnetic and ordered phases a consistent and physically sound analysis of the⁵⁷Fe spectra is presented. Fe hyperfine fields and the local magnetic moments are obtained and compared with magnetic and structural data. A spin reorientation is observed for Er₂Fe₁₄B. A constant collinear structure down to 4.2 K was determined for the other samples. The¹⁶¹Dy spectra are interpreted for a pure ¦J_Z=15/2 > ground multiplet in Dy₂Fe₁₄B and its hydride. The B₂ crystal field parameters were estimated empirically for the Dy-alloy. Their values can account for the anisotropy in the compounds.     

Structure-property relationships in the R-type hexaferrites: Cation distributions and magnetic susceptibilities of MX2Fe4O11

The strong, and heretofore unexplained, dependence of the magnetic properties of the R-type hexaferrites MX₂Fe₄O₁₁ on heat treatment has been investigated by means of⁵⁷Fe and¹¹⁹Sn Mössbauer spectroscopy at 297 K.⁵⁷Fe Mössbauer spectra of samples of BaTi₂Fe₄O₁₁, BaSn₂Fe₄O₁₁, and SrSn₂Fe₄O₁₁, annealed at several temperatures consist of three partially resolved quadrupole doublet patterns. The pattern with the very large quadrupole splitting, i.e. 1.3 mm s^–1 E _Q 1.7 mm s^–1, is readily assigned to the trigonal bipyramidal site, and exhibits no dependence of its parameters, including the relative intensity, on heat treatment. The remaining two patterns are assigned to Fe³⁺ ions on octahedral sites, and exhibit relative intensities that are strongly dependent on heat treatment. The¹¹⁹Sn spectrum of BaSn₂Fe₄O₁₁ is a broad, apparently single line that, with the exception of verifying the presence of tin as Sn⁴⁺, provides little insight into the details of the structure. The distributions of the cations over all the lattice sites have been deduced from the Mössbauer spectra and compared with the magnetic susceptibility data. It is concluded that the cation distributions of the Sn-containing phase are not strongly dependent on heat treatment, and the magnetic properties exhibit a similar insensitivity to heat treatment. The Ti analogs exhibit a strong dependence of cation distribution on heat treatment, which is reflected in widely varying magnetic ordering temperatures, a variation that is confirmed by the incipient magnetic hyperfine splitting despite the strong magnetic frustration in the R-type hexaferrites.     

Magnetic coupling in Gd(Fe6−x Cr x )Al6: A57Fe Mössbauer study

⁵⁷Fe Mössbauer measurements have been made on the ternary ThMn₁₂-type intermetallic compounds Gd(Fe_6–x Cr _x )Al₆ withx=0, 0.5, 1.0, 1.5 and 2.0, at temperatures of 4.2 and 77 K. The principal effect of the Cr substitution is to reduce the⁵⁷Fe magnetic hyperfine field at 4.2 K in this series. The analysis of the⁵⁷Fe Mössbauer spectra is consistent with a ferromagnetic coupling between the Gd and Cr magnetic moments. These results are in agreement with previous studies by Felner et al. on GdCr₆Al₆, in which a ferromagnetic ordering withT _C=170 K was observed.On leave from Applied Acoustics Institute, Shaanxi Teachers University, Xian, PR China.     

Magnetic characterization of the non centrosymmetric Ba3NbFe3Si2O14 langasite

The potential relevance of the Ba₃NbFe₃Si₂O₁₄ langasite in the field of multiferroism was investigated. Interesting properties were expected, in particular in view of the frustrating stacking of its magnetic Fe³⁺ cations into triangular planes of triangle units. We hereafter report results of specific heat and magnetic measurements, Mössbauer spectroscopy and neutron diffraction. A peculiar magnetic structure is found out at low temperature. The Fe³⁺ magnetic moments adopt a triangular 120° configuration within each triangle, which is in-phase propagated in each triangular plane and is helically modulated from plane to plane.     

High pressure Mössbauer spectroscopy using a diamond anvil cell

Recent applications of high pressure Mössbauer spectroscopy using a diamond anvil cell are presented. High pressure Mössbauer studies of two perovskite-related iron oxides SrFeO_2.97 and CaFeO₃, magnetite Fe₃O₄, and wüstite Fe_1– O have been carried out at 300 K at pressures of up to 74 GPa. A preliminary result by the resonant forward scattering of synchrotron radiation for high pressure Mössbauer spectroscopy using a diamond anvil cell is also presented.     

57Fe high pressure Mössbauer and X-ray studies of Gd2Fe17C x interstitial compounds

The unit cell volume dependence of the Curie temperature of Gd₂Fe₁₇ is studied by means of high-pressure Mössbauer spectroscopy. From the pressure dependence of the average isomer shift of Ge₂Fe₁₇ at room temperature, the contribution of the pure magnetovolume effect to the change of the average isomer shift is derived. By combining the Mössbauer measurements of Gd₂Fe₁₇C _x with different C concentrations at room temperature, the change ofIS in Gd₂Fe₁₇C _x is dominated by the magnetovolume effect. The contribution of the chemical bonding effect is about 24% of that of the magnetovolume effect, but with negative sign.     

Structure and magnetic properties of FeMnAl alloys investigated by Mössbauer spectroscopy and X-ray diffraction

The system (Fe_0.88Mn_{0.12 1–x} Al _x has been investigated in a concentration range from 5 to 14 at.% Al. We applied Mössbauer spectroscopy in the temperature range from 4 up to 900 K and X-ray diffractometry at room temperature. The as-cast samples show a bcc phase for all concentrations and exhibit broadened six-line Mössbauer spectra typical for disordered alloys. The Mössbauer spectra during a high temperature treatment show dramatic changes. These are due to ordering processes appearing at temperatures above 700 K. As an example of the observed changes, we present results obtained for the alloy withx= 14 at.% Al.