Efectos de tamaño en de hierro ultrafino. Nuevas estructuras: 2D - 3D

This article is devoted to the analysis of the size of iron nanoparticles impact on the structure, to comparison of the results obtained for the nanopowders in the various authors’ researches. The article considers factors that may impact on the form and parameters of the Mössbauer spectra of iron nanopowders obtained by the inert gas condensation technique (Gen-Miller’s method). Possible causes of the new state of the iron are proved with the effective magnetic field at the 57 Fe nucleus (H=365 kOe). But the results related to size effects differ from the researches of other authors. It was revealed that nanoparticles with a mean (X-ray data) particle size of 50 nm have also Angstrem patterns, which can meet the new structure. Presence of small amounts of superparamagnetic oxide could be a catalyst, impetus for the formation of the new structure, and also, at the exchange interactions, could modify the charge of the electron density at the Fe nuclei. Reviewed and other factors can result in appearing of such a high value of the effective magnetic field at the iron nuclei.Este artículo se dedica al análisis del impacto del tamaño de nanopartículas de hierro en su estructura y a la comparación de los resultados obtenidos para los nanopolvos en estudios llevado a cabo por autores diferentes. Aquí también se consideran los factores que pueden afectar la forma y los parámetros de los espectros Mössbauer del nanopolvo de hierro producido por medio de condensar el gas inerte (método Gen-Miller). Se demuestran las causas posibles del estado nuevo del hierro a través de un campo magnético efectivo para el nucleo de  57 Fe (H=365 kOe). Pero los resultados relacionados a efectos del tamaño son diferentes de lo obtenido por otros autores. Se revela que los nanopartículas del tamaño medio de 50 nm (datos de examinaciones con rayos X) también tienen modelos Angstrem que pueden corresponden a la estructura nueva. La ocurrencia del óxido superparamagnético en cantidades pequeñas puede catalizar la formación de la estructura nueva y también, en caso de interacciones de intercambio, modificar la carga de la densidad del electrón en el núcleo de Fe. Los factores examinados y otros factores pueden resultar en la generación del campo magnético de un alta intensidad alrededor del núcleo de hierro

Structure and magnetic properties of mechanically alloyed Fe3-xAlCrx powders

Mixtures of elemental powders of Fe3-xAlCrx composition, where x ∈ (0.8 -1.2) were mechanically alloyed in the AGO-2U planetary ball mill. Obtained powders consist of particles with median size of 0.5-1.6 µm; increase in the Cr content results in the decrease in the size of particles. According to the X-ray data, all of the as-milled alloys possess a single-phase disordered b.c.c. structure. Increase in Cr content results in the decrease in the grain size of alloys, whereas the dependence of the lattice parameter on the chemical composition is not conclusive. Mössbauer spectra for most of the alloys shows that iron atoms in this alloys, in spite of the fact that only one phase was detected using the X-ray diffraction, exist both in ferromagnetic and non-ferromagnetic conditions

Crystallochemical aspects of solid state reactions in mechanically alloyed Al-Cu-Fe quasicrystalline powders

A number of elemental mixtures having initial compositions close to Al65Cu23Fe12 were mechanically alloyed at different energy intensities using a planetary mill. Laboratory X-ray diffraction analysis, differential scanning calorimetry and Mössbauer spectroscopy were used for characterization of the phase and structural state of mechanically alloyed powders after different periods of milling and annealing. Several exothermic effects were found, and these were ascribed to specific solid-state reactions. Quantitative phase analysis was applied in order to identify the mechanism of solid-state reactions taking place in the vicinity of the quasicrystalline phase domain in the Al-Cu-Fe system as a result of mechanical and thermal excitation and homogenization. Metastable intermetallics were identified which possess certain structural and topological elements identical to those found in quasicrystals. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd

Fe-Mn mechanically alloyed powders characterised by local probes

Abstract High-energy ball milling of Fe\u2013Mn elemental powder mixtures has been carried out for Mn atomic concentrations in the range 10\u201390%. X-ray diffraction (XRD), M\uf6ssbauer spectroscopy (MS) and perturbed angular correlations (PAC) have been used to investigate the crystalline structure in the milled samples. It is found that ball milling gives rise to concentration ranges of existence of terminal solid solutions more extended than in the equilibrium phase diagram. Particular attention has been paid to the environment of 57Fe (MS) and 111In (PAC) probe atoms: 57Fe atoms are a constituent of the system, while 111In atoms have been implanted at 400 keV into pills made from the milled powders. The PAC spectra of milled Fe show the ferromagnetic \u3b1-phase with a high fraction of a single vacancy next to the probe. Accordingly, the M\uf6ssbauer spectrum is a sextet with the characteristic splitting of \u3b1-Fe and a broadening which indicates a certain degree of atomic structure disorder. With an amount of Mn up to 15% PAC probes with one and two Mn next neighbours are identified by their smaller magnetic hyperfine field, corresponding to two distinct magnetic components in the M\uf6ssbauer spectra. At 20 and 30% of Mn the PAC magnetic signal of the a-phase disappears in favour of a distorted cubic apparently non-magnetic signal. From 40 to 70% of Mn a broad distribution of hyperfine magnetic fields is observed by PAC. M\uf6ssbauer spectra in the 20\u201370% of Mn range can be fitted with an unresolved low field magnetic sextet. Finally, the PAC spectra at 80 and 90% are quite similar to the one which is obtained after milling pure Mn, while the corresponding M\uf6ssbauer data can be roughly approximated to those of the \u3b1-Mn(Fe) solid solution

THE MÖSSBAUER STUDY OF ULTRASOUND STIMULATED BY RADIOFREQUENCY FIELD IN FERRODIELECTRICS

La production d'ultrasons dans des ferrodiélectriques de structure à domaines multiples est observée avec un pompage à radiofréquence. On propose un mécanisme de production du changement d'aimantation dans les parois de Bloch à 180°, qui exciterait les vibrations ultrasonores de la plaque ferromagnétique. On étudie expérimentalement les spectres Mössbauer des YIG polycristallins soumis à un champ de radiofréquence. L'analyse spectrale a montré que le comportement sous excitation par un champ à radiofréquence confirme l'hypothèse de la production des ultrasons par les parois de Bloch à 180°.Ultrasonic generation in ferrodielectrics with multidomain structure is considered at rf field pumping. The mechanism inducing the magnetization change in 180° Bloch walls and leading to the longitudinal ultrasonic vibrations of the ferromagnetic plate is assumed. Mössbauer spectra of the polycrystalline YIG are investigated experimentally, under applied rf field. The spectra analysis showed that the rf field excitation behavior confirms the hypothesis of the ultrasound generated by 180° Bloch walls

Crystallochemical aspects of solid state reactions in mechanically alloyed Al-Cu-Fe quasicrystalline powders

A number of elemental mixtures have initial compositions close to Al₆₅Cu₂₃Fe₁₂ were mechanically alloyed at different energy intensities using a planetary mill. Laboratory X-ray diffraction analysis, differential scanning calorimetry and Mössbauer spectroscopy were used for characterization of the phase and structural state of mechanically alloyed powders after different periods of milling and annealing. Several exothermic effects were found, and these were ascribed to specific solid-state reactions. Quantitative phase analysis was applied in order to identify the mechanism of solid-state reactions taking place in the vicinity of the quasicrystalline phase domain in the Al-Cu-Fe system as a result of mechanical and thermal excitation and homogenization. Metastable intermetallics were identified which possess certain structural and topological elements identical to those found in quasicrystals