Marcel Miglierini

Mössbauer spectrometry of Fe(Cu)MB-type nanocrystalline alloys: I. The fitting model for the Mössbauer spectra

A fitting model based on the use of two independent blocks resulting from distributions of a hyperfine field and of one sextet of lorentzian lines is discussed for Mossbauer spectra recorded for Fe(Cu)MB nanocrystalline alloys. One distributed subspectrum is ascribed to the amorphous residual matrix, while the other independent block, from the hyperfine-field distribution, is attributed to Fe atoms located in the so-called interface zone. This region comprises atoms of nanocrystalline-grain surfaces and also atoms originating from the amorphous precursor, in close contact with the nanocrystalline grains. A sextet of lorentzian lines is attributed to the crystalline grains that have emerged from the amorphous alloy, which are unambiguously identified as phase. The distribution with low hyperfine fields can be eventually analysed in terms of two components accounting for the coexistence of electric and magnetic hyperfine interactions. In such an analysis, distributions of both quadrupolar splittings and hyperfine magnetic fields are employed. Examples of the present fitting model are provided for Mossbauer spectra of FeCuMB ( M= Zr, Ti, and NbCr) nanocrystalline alloys in the first stage of crystallization. The spectra have been recorded under various experimental conditions comprising low (77 K) and high (373 K) temperatures as well as an external magnetic field. More detailed discussion about the consequences of this novel fitting procedure with respect to the topography of hyperfine interactions within Fe-based nanocrystalline alloys is reported in part II, the following paper.

Mössbauer spectrometry of Fe(Cu)MB-type nanocrystalline alloys: II. The topography of hyperfine interactions in Fe(Cu)ZrB alloys

Changes in the microstructure, crystallization behaviour and magnetic state of alloys upon annealing for one hour at are examined by Mossbauer spectrometry, x-ray diffraction and differential thermal analysis measurements. The presence of crystallites is reflected in the Mossbauer spectra taken in the temperature range 77 K to 473 K as sharp lines which are superimposed on broad features of the residual amorphous phase. The principal crystalline phase is identified as bcc-Fe. An early stage of crystallization is found even after only a anneal. The total fraction of crystalline phase increases as the temperature is increased. Hyperfine fields of Fe atoms positioned in grain boundaries of the crystallites are affected by the ordered structure of the crystallites from one side, and by the disordered amorphous arrangement from the other. The behaviour of this amorphous/crystalline interface zone is studied by considering two independent hyperfine-field distributions. One is attributed to the interface zone and the other represents the amorphous remainder of the original precursor. The three-dimensional distributions of hyperfine fields obtained from this fitting procedure, which was introduced in part I, the previous paper, enable us - after their decomposition into several gaussian components - to sketch a model of the topography of the hyperfine interactions within nanocrystalline samples.

Temperature behaviour of iron nanograins in Nanoperm-type alloys

The evolution of hyperfine interactions with temperature is studied for Fe80M7Cu1B12 (M = Mo, Nb and Ti) nanocrystalline alloys with the help of 57Fe Mossbauer spectrometry. The nanocrystalline structure features an amorphous residual matrix surrounding the crystalline nanograins of bcc-Fe. In addition, interfacial regions comprising atoms located on the surface of nanocrystals are considered. Special attention is paid to the temperature behaviour of hyperfine magnetic fields of the nanograins. The temperature dependence of hyperfine magnetic fields pointed out significant differences between bulk and nanosized bcc-Fe, suggesting a decrease in the corresponding magnetic ordering temperature. The higher the crystalline content is, the lower the difference between the hyperfine fields of bulk bcc-Fe and nanocrystalline Fe grains. This tendency is observed for M = Nb and Ti whereas it is completely opposite for M = Mo. The present results are explained in terms of mechanical stresses induced during the transformation from the amorphous to the nanocrystalline state, thus excluding a significant number of impurities diffused into the nanocrystalline grains.

Formation of Fe(0)-Nanoparticles via Reduction of Fe(II) Compounds by Amino Acids and Their Subsequent Oxidation to Iron Oxides

Iron nanoparticles were prepared by the reduction of central Fe(II) ion in the coordination compounds with amino acid ligands. The anion of the amino acid used as a ligand acted as the reducing agent. Conditions for the reduction were very mild; the temperature did not exceed 52°C, and the optimum pH was between 9.5 and 9.7. The metal iron precipitated as a mirror on the flask or as a colloid in water. Identification of the product was carried out by measuring UV/VIS spectra of the iron nanoparticles in water. The iron nanoparticles were oxidized by oxygen yielding a mixture of iron oxides. Oxidation of Fe(0) to Fe(II) took several seconds under air. The size and properties of iron oxide nanoparticles were studied by UV/VIS, TEM investigation, RTG diffractometry, Mossbauer spectroscopy, magnetometry, thermogravimetry, and GC/MS.

AFM and Mössbauer spectrometry investigation of the nanocrystallization process in Fe–Mo–Cu–B rapidly quenched alloy

In this paper, the effect of heat treatment on the development of nanocrystallites in rapidly quenched Fe79Mo8Cu1B12 alloy is investigated. The surface morphology is examined using non-contact mode atomic force microscopy (AFM). The results are compared with those obtained by transmission Mossbauer spectroscopy (TMS), conversion electron Mossbauer spectrometry (CEMS) and x-ray diffraction (XRD). It was found that the sample is not fully amorphous even in the as-quenched state. Minor amounts of bcc-Fe grains were detected on the wheel side of the ribbon-shaped samples while no indications of the crystalline phase were observed in the bulk. The crystallization onset is observed after annealing at 410 ◦ C, when bcc-Fe nanograins are quite well developed. More intense crystallization is evidenced after annealing at higher temperatures, when the content of the crystalline phase increases progressively. The second crystallization, not discussed in the present paper, is characterized by the occurrence of additional crystalline phases, and appears after annealing at 650 ◦ C. We suggest a crystallization model assuming no drastic change in the size of the primarily formed bcc-Fe nanograins with temperature as proved by XRD. The increase in annealing temperature induces the formation of a higher number of crystalline particles, which form large irregular agglomerates (80- 130 nm in height), in accordance with the AFM data. (Some figures in this article are in colour only in the electronic version)

Iron-based nanocrystalline alloys investigated by 57Fe Mössbauer spectrometry

Soft magnetic nanocrystalline alloys have attracted great fundamental interest in recent years due to their two-phase structural and magnetic behaviour. We review first the reliability of the fitting procedures of spectra obtained by 57Fe Mössbauer spectrometry which is a very efficient tool to investigate such materials. Then, we report the common features which characterise the temperature dependence of Mössbauer spectra; the hyperfine field temperature dependence of both the crystalline grains and the intergranular phase is discussed for different crystalline fractions in order to model the magnetic behaviour of the nanocrystalline alloys.

CEMS studies of structural modifications of metallic glasses by ion bombardment

Fe76Mo8Cu1B15 and Fe74Nb3Cu1Si16B6 amorphous metallic alloys were exposed to ion bombardment with nitrogen ions and protons to ensure different degree of radiation damage. The radiation damage profiles were calculated in the “full cascade” mode. Conversion electron Mössbauer spectrometry was employed to scan structural modifications in the surface regions of the irradiated alloys. In Fe76Mo8Cu1B15, the irradiation with 130 keV N+ has caused a significant increase of the hyperfine magnetic fields and isomer shift due to changes in topological and chemical short-range order (SRO), respectively. No appreciable effects were revealed after bombardment with 80 keV H+ ions. Fe74Nb3Cu1Si16B6 amorphous metallic alloy was irradiated by 110 keV N+ and 37 keV H+ and only changes in chemical SRO were revealed after bombardment with nitrogen ions. The observed alternations of the structure depend primarily on the total number of displacements of the resonant atoms which are closely related to the fluence as well as type and energy of the incident ions.

High temperature Mössbauer effect study of Fe90Zr7B3 nanocrystalline alloy

Fe90Zr7B3 NANOPERM alloy is investigated in as-quenched and nanocrystalline forms by means of high temperature (up to 1040 K) Mossbauer spectroscopy. These studies are aimed at revealing the relationship of microstructure to magnetic properties for57Fe phases and their temperature dependences in NANOPERM-type ternary alloy at temperatures exceeding the onset of the second crystallization. For this purpose the nanocrystalline sample was prepared by annealing an amorphous precursor at 893 K for 1 h providing 54% of bcc α-Fe nanocrystalline grains. At this stage the first crystallization is almost completed. Because of the progress of the crystallization process during the acquisition of Mossbauer spectra beyond the temperature of the first crystallization, the results obtained are discussed for three temperature intervals: below the first crystallization (782 K), between the first and the second crystallization, and above the second crystallization temperature (931 K). Conclusions related to the evolution of the crystalline fraction, interfacial regions and the amorphous residual phase are derived by comparing spectral parameters obtained from the in situ high temperature Mossbauer effect measurements with those from room temperature Mossbauer spectra acquired immediately after each high temperature experiment. The latter revealed structural modifications imposed during Mossbauer spectroscopy at high temperatures, whereas the in situ experiments identify thermally induced dynamic processes.

Laser-induced structural modifications of FeMoCuB metallic glasses before and after transformation into a nanocrystalline state

The effects of laser treatments on the structural and magnetic properties of metallic ribbons have been studied using the melt-spun Fe76Mo8Cu1B15 alloy in as-quenched and nanocrystalline states. 57Fe Mossbauer effect techniques, comprising transmission geometry measurements (TM) and detection of conversion electrons (CEMS), have been employed in addition to magnetization measurements, differential scanning calorimetry and x-ray diffraction. The Curie temperature of the as-quenched alloy was about 70 °C. The distributions of hyperfine magnetic fields as well as quadrupole splitting obtained from TM and CEM spectra have revealed the possibility of observing laser-induced structural modifications even at room temperature when the system is only weakly magnetic. Consequently, both types of hyperfine interactions have been detected and they are nearly in equilibrium (having the same strength or occurring to the same extent). After treatments with a pulsed XeCl excimer laser (with a homogeneous beam of 5×5 mm2, 308 nm, 55 ns, 1 Hz), the significance of magnetic dipole interactions rises as a function of the number of laser pulses (up to 64) and the laser beam fluence (up to 3 J cm-2). No traces of laser-induced crystallization have been found. In the nanocrystalline Fe76Mo8Cu1B15 alloy, surface crystallization was already completely removed after the first pulse of 1 J cm-2.

The optimalization of the Ba-hexagonal ferrite phase formation

Abstract The Ba-hexagonal ferrite phase has been prepared by the citric precursor method. The decomposition of the precursor started at 360°C. The optimal Fe:Ba ratio was 10.8. The crystalline Ba-ferrite starts forming at a temperature of 600°C. Mono-phase Ba-ferrite is obtained at 950–1100°C.