[en] Mechanical alloying of high purity elemental powders in a planetary ball mill (Fritsch pulverisette 7) was used to prepare nanocrystalline Fe and equiatomic FeCo powders. Morphology changes, structural properties and local iron environment variations have been investigated by Scanning Electron Microscopy, X-ray diffraction and ⁵⁷Fe Moessbauer spectrometry. X-ray diffraction profile fitting, reveals the Co allotropic transformation from fcc to hcp form at the early stage of milling. The milling route implies diffusion of hcp-Co into α (Fe) leading to the formation, after 12 h, of the bcc-FeCo solid solution with a lattice parameter close to a = 0.2861 nm and a grain size of about 12 nm. The analysis of the evolution of the hyperfine magnetic field distribution, indicates that the disordered bcc-FeCo solid solution is formed after 12 h of milling, through the interdiffusion of Fe and Co elemental powders. The average hyperfine magnetic field of the bcc-FeCo solid solution (B = 35 T) suggests that the disordered Fe₅₀Co₅₀ system is formed

[en] We have studied the magnetization of melt spun amorphous Fe_74-xAl_xEr₆B₂₀ alloys with 0≤x≤15 under magnetic fields up to 8 T, and have analyzed the results at 4.2 K on the basis of the random magnetic anisotropy (RMA) model. The local random anisotropy constant is found to be (1.5±0.2)x10⁷ erg/cm³ and is aluminum content independent. The Moessbauer studies show that the average hyperfine field decreases linearly with the addition of Al

[en] We have studied the effect of water content on the magnetic and structural properties of goethite. For that purpose, four samples were prepared using two different hydrothermal methods, one of them is derived on the Fe(II) precursors and the other one from Fe(III) precursors. The samples were characterized by X-ray diffraction (XRD), TGA, BET, FTIR, Moessbauer spectrometry at RT, 77 and 4.2 K and ZFC and FC curves. The results suggest that the goethites from the Fe(II) precursors are less crystalline, have higher water contents and do not show magnetic ordered structure at RT in comparison to the goethites from the Fe(III) precursors. The goethites from the last systems exhibit good crystallinity, low water content and magnetic ordering at room temperature. Our results suggest that both structural and adsorbed water contents reduce the magnetic hyperfine field at 4.2 K. A linear correlation with regression coefficient of 0.91 between the saturation hyperfine field and both the structural hydroxyl content and the surface area could be derived

[en] Transmission Moessbauer spectra were performed on FeZrB nanocrystalline alloys with high crystalline fraction (∼60%)at temperatures comprised within the range 77-600 K. We report the temperature dependencies of the magnetic hyperfine data of the three contributions: the crystalline bcc-Fe grains, the residual amorphous phase, and the interface intermediate between the bulk of crystalline and amorphous phases. The evolution of the average value of the hyperfine field characteristic of the residual amorphous phase is discussed in terms of interactions between crystalline grains transmitting through the intergranular phase

[en] We discuss the concept of an interfacial zone resulting from the symmetry restriction at the periphery of crystalline grains in the nanocrystalline alloys. The different experimental features are essentially based on Moessbauer investigations performed in a wide temperature range. The results obtained are related to the atom probe field ion microscopy, EXAFS and FMR analysis. The experimental evidence for the existence of the interfacial zone (with a thickness of about 2-3 atomic layers) is presented. The structural, chemical and magnetic properties of the interfaces are discussed including their role in the overall magnetic interactions in nanocrystalline structure

[en] The effect of Co substitution on coercivity (H_C) and ⁵⁷Fe hyperfine parameters of mechanically alloyed SmFe_11-xCo_xTi (x≤2) were systematically studied. H_C exhibits a maximum at 1123 K resulting from the competition between surface defect effect and domain size. It is shown from the isomer shift (δ) assigned on the basis of Wigner-Seitz cell volumes that Co atoms occupy preferentially 8f sites of the ThMn₁₂-type structure. The mean hyperfine field (B_HF) increases with substitution

[en] Electric field gradient calculations using a point charge model were developed in corner-sharing octahedra random networks, simulating amorphous FeF₃ and resulting from the topological defect method. The η value is well estimated to 0.53-0.58 and the topology of the joint distribution P(V_zz, η) is similar to that deduced from previous dense random packing model. (orig.)

[en] High Field Moessbauer Spectroscopy has been carried out on an amorphous ferric fluoride: Fe₃, 0.4HF. Below the spin-freezing temperature, the external field cannot align the magnetic moments which are drawing a speromagnetic structure. At higher temperatures, the magnetic moments are colinearly aligned parallel to the external field direction. A progressive canting of the moments is observed in the intermediate temperature range resulting from a competition between molecular field distribution and applied field. (orig.)

[en] Nano structured ferrites have been prepared by milling starting from commercial ferrite powders. Subsequent compaction and sintering were applied to obtain cylindrical pieces. The magnetization as a function of the grain size follows the non-magnetic grain boundary model. The boundary thickness is estimated as about 1.5 nm, which is consistent with that calculated from the Moessbauer absorption areas. Due to the high lattice distortion, the small exchange length impedes the averaging of the anisotropy

[en] Spin texture in several MB 2826 (Fe₄₀Ni₃₈Mo₄B₁₈) ribbons has been investigated by Moessbauer spectroscopy. A tensile strain aligns the spins parallel to the ribbon length, this demonstrating a positive magnetostriction. The observed spin texture depends upon the width of the ribbons and varies from the centre to the edges. It is understood that the spin orientations originate from thermal stresses, which are close to tensile biaxial in the core of the ribbon (in-plane spins) and compressive biaxial at the surface (spins perpendicular to the plane). (orig./GSCH)