Microstructure, magnetic and transport properties of magnetoresistive Cu80Fe x Ni20?x (x?=?5, 10 and 15 at.%) ribbons

The generation of copper nanoparticles in an arc furnace by the evaporation/condensation method is systematically investigated. The evaporation/condensation process is advantageous because it allows direct synthesis using pure metals as starting materials avoiding reactions of expensive and potentially poisonous precursors. In the presented system, a transferred direct current arc provides the energy for evaporation of the metal target. In order to prevent an oxidation of the particles in the process, the synthesis is conducted in an atmosphere of inert gases (purity grade 5.0). The arc stability and its effect on particle synthesis are investigated. The experiments reveal excellent long-term arc stability for at least 8?h continuous operation delivering aerosols with high reproducibility (±10?% of average particle size). The influences of the arc current and length, the flow rates of the applied gases and the injection of hydrogen in the plasma zone on the particle size distributions and the agglomerate structure are studied. The produced copper nanoparticles are characterized by scanning mobility particle sizing and scanning electron microscopy. The average particle size could be well controlled in a size range 4?C50?nm by selecting appropriate operating parameters.     

Structural and magnetic properties of rapidly solidified (Ce0.2Fe0.8)1-x Al x ribbons determined by X-ray diffraction and Mössbauer spectrometry

(Ce_0.2Fe_0.8)_1-x Al _x (0 x 0.9) ribbons have been prepared by planar flow casting under an He atmosphere with a linear velocity of 29 m s^-1. Analyses of the ribbons by X-ray diffraction and⁵⁷Fe Mössbauer spectrometry in the temperature range 4–300 K show that all the ribbons are crystalline. With increasingx, the observed phases are Ce(Fe, Al)₂, Ce₂(Fe, Al)₁₇, CeFe_4+y,Al_8-y and the single fcc aluminium phase. For the different phases, the line intensities of the Mössbauer spectra agree with previous results on the preferential substitution sites for aluminium. A coherent hyperfine parameters set was deduced from fitting spectra in the temperature range 4–300 K.     

Structure and stability of Fe−Mg amorphous alloys prepared by coevaporation

The structure, stability and magnetic properties at 77K of Fe₆₀Mg₄₀ amorphous thin films prepared by coevaporation are investigated using mainly Mössbauer effect working in scattering geometry and transmission electron microscopy. The temperature range of the amorphous to crystal transition is determined; moreover a superparamagnetic-like behaviour of the films is pointed out at 77K.

     

X-ray and Mössbauer study of rapidly solidified Ce20Fe80 ribbons; effect of the quenching rate

Ce₂₀Fe₈₀ ribbons have been produced by planar flow casting under an He atmosphere at linear wheel velocities between 19 and 29 m s^–1. Analysis of ribbons by X-ray diffraction and⁵⁷Fe Mössbauer spectrometry in the temperature range 77–300 K shows that the ribbons are crystallized. For higher velocities, the ribbon is constituted of the two equilibrium phases CeFe₂ and Ce₂Fe₁₇, but, for lower velocities, there appears a third iron metallic phase, which can be explained by the quenching rate of the melt. A coherent hyperfine parameter set was deduced from fitting Mössbauer spectra in the whole temperature range.     

CEMS Investigations of Swift Heavy Ion Irradiation Effects in Tb/Fe Multilayers

Hyperfine Interactions - In nanometric Tb/Fe multilayers, the effect of the electronic energy loss of fast heavy ions on the interfaces has been selectively investigated by 57Fe conversion electron...     

Study of Tb/Fe multilayers by temperature-dependent CEMS

A comparative study is accomplished in the domain where iron layers are amorphous. The dependence of the magnetic structure of Tb/Fe multilayered films on temperature have been investigated by Mössbauer spectrometry. When the iron layer is thinner than 2.3 nm, the average hyperfine field at the iron site remains nearly constant at 4.2 K, while it decreases strongly for iron thickness higher than 1.5 nm at room temperature. This decrease of H is due to the decrease of the Curie temperature, which can be explained from the structure of iron layers.     

Damage processes in Fe 3 O 4 magnetic insulator irradiated by swift heavy ions. Experimental results and modelisation

The behavior of the magnetic properties of magnetite Fe₃O₄ irradiated by swift heavy ions is investigated by magnetization measurements. Although there is no induced structural phase transformation, both coercive field and saturation magnetization are sensitive to ion irradiation and exhibit different behaviors depending on the ion fluence range. In the low fluence regime, the coercive field increases, which is evidence for a strong pinning of magnetic domain boundaries by the induced defects. The magnetization shows a decrease in the saturation value and tends to reorient perpendicularly to the ion track axis. At high fluence, the initial magnetic properties of the sample are nearly restored. The changes in the magnitude and the direction of magnetization are interpreted by magnetostrictive effects related to the stress induced by irradiation. A phenomenological model is applied to reproduce the fluence evolution of the saturation magnetization, assuming relaxation of the stress induced around the core of defects of the tracks by overlapping effects at high fluence. The results are compared to those obtained in the case of yttrium iron garnet Y₃Fe₅O₁₂. Received 18 April 2001 and Received in final form 24 July 2001     

Transmission electron microscopy and X-ray diffraction study of microstructural evolution in magnetoresistive Cu–Fe–Ni ribbons

The evolution of the microstructure of a granular Cu₈₀Fe₁₀Ni₁₀ (at%) melt-spun ribbon is studied by transmission electron microscopy (TEM), energy-filtered transmission electron microscopy (EFTEM) and X-ray diffraction. This system is interesting as large giant magnetoresistance (GMR) values have been measured for this composition. We have shown the presence of two face-centred cubic phases, an (Fe,Ni)-rich phase and a Cu-rich phase. The lattice parameters of these two phases are close and no diffraction or elastic contrast is involved in displaying the two phases in TEM bright-field mode. With EFTEM imaging, we have shown the presence of a fine-scale (Fe,Ni)-rich precipitation inside the Cu-rich fcc matrix. The precipitates are 2–4 nm in the as-spun state and 4–6 nm after annealing for 2 h at 400°C. The lattice parameter of the Cu-rich phase in the as-spun sample is 0.3608 nm and 0.3610 nm for the (Fe,Ni)-rich phase. After a 24-h annealing treatment at 600°C, the mean diameter of the particle is 20 nm and the lattice parameter of the (Fe,Ni)-rich phase has decreased to 0.3600 nm, while that of the Cu-rich phase has increased to 0.3613 nm, which is consistent with a segregation of Fe and Ni in the precipitates. The composition and volume fraction of the two phases measured for this annealed sample are in good agreement with the Thermocalc® predictions.     

Magnetic and electronic studies in the granular (Ni0.84Fe0.16)54(alumina)46 sputtered thin films

We have studied the magnetization in the granular (Ni_0.84Fe_0.16)₅₄(alumina)₄₆ alloy. The thermomagnetization curve is found to obey the Bloch law. Spin wave stiffness constant D and the exchange constant A were calculated from the experimental results. The magnetic experimental measurements have been interpreted in the framework of random magnetic anisotropy (RMA) model. The results have shown that it is possible to extend the application of RMA to the granular alloy. From an analysis of the approach to saturation magnetization some fundamental parameters have been extracted. In addition, self-consistent ab initio calculations, based on Korringa–Kohn–Rostocker (KKR), are performed to investigate magnetic and electronic properties of the granular alloy. Spin polarization within the framework of the coherent potential approximation (CPA) is considered.     

Ion-implantation induced phase transitions: Order-disorder and amorphous-crystal transformations (CEMS studies)

The implantation induced disordering of intermetallic compounds is studied using the CEMS technique. The results mainly concern a magnetic transition in Fe−Al 40%at. and the crystallization process in⁵⁷Fe implanted Ni−Ti 50%at.