Rafael Piccin

Magnetostatic interactions between two magnetic wires

The results of the magnetic dipolar field in a simple set of two amorphous ferromagnetic wires of composition Fe77.5Si12.5B15 placed side by side are presented. Owing to their peculiar domain structure, they could, in principle, be approximated by macroscopic magnetic dipoles, allowing the analysis of the magnetostatic field between these magnetic entities. Magnetization measurements as a function of the distance between the parallel wires were performed. Results can be explained considering the magnetostatic field created by one wire in the neighboring one. It is clearly shown that this field is responsible for changes of the reversal field of the wires, leading to the appearance of plateaux during the demagnetization process. Instead of pure dipolar model that does not fit experimental data, a multipolar model has been developed, showing a rather good agreement with the experimental results.

Influence of Rare-Earth Substitution for Iron in FeCrMoCB Bulk Metallic Glasses

The effects of rare earth addition on the glass forming ability of Fe50–xCr15Mo14C15B6Mx (x = 0, 2 and M = Y, Gd) bulks and ribbons are studied. The thermal and structural properties of the samples are measured by a combination of differential scanning calorimetry (DSC), x-ray diffraction and scanning electron microscopy. Chemical compositions are checked by energy dispersive spectroscopy analysis. The copper mold casting technique leads to a fully amorphous structure up to 2mm only for compositions containing Y or Gd. In the case of ribbons, a fully amorphous phase is observed for all the compositions. The roles of Y and Gd are discussed on the basis of melting behavior analyzed by high-temperature DSC. Such elements act as oxygen scavengers, avoiding heterogeneous nucleation.

Circular magnetic dichroism in Fe78−xNixSi8B14 (15⩽x⩽58) ferromagnetic amorphous ribbons

Abstract Fe 78− x Ni x Si 8 B 14 (15⩽ x ⩽58) amorphous ribbons were investigated by means of macroscopic magnetic characterization and X-ray magnetic circular dichroism (XMCD). The XMCD spectra were taken at the K and L edges of Fe and Ni, from which several indications of the local magnetic character of the alloy can be inferred. In both configurations, the spectra at Ni-edges display the same typical characteristic observed near Fe-edges, indicating that the amorphous alloy still behaves as a weak ferromagnet, with unfilled majority d-bands.

Quasicrystalline Phase Formation on Glassy ZrAINiCuPd Alloys by Linear Varying Joule Heating

Linear varying current Joule heating was employed to perform thermal treatments on bulk amorphous samples. Using this method it is possible to monitor the evolution of the materials structure through the on-line measurement of the samples electrical resistance. Therefore, it is possible to interrupt the treatment at any desired point. Using this method, we have succeeded to obtain icosaedral phases in some glassy alloys. The quasicristallization results in an increase of the samples resistance, which is visible in the Joule heating curves.

Crystallization Behavior of Fe50−x Cr15Mo14C15B6Mx (x = 0, 2 and M = Y, Gd) Bulk Metallic Glasses and Ribbons by in situ High Temperature X-Ray Diffraction

The effect of rare earth addition on thermal stability of Fe50−xCr15Mo14C15B6Mx(x = 0, 2 and M=Y, Gd) is studied. Thermal and structural properties are measured using differential scanning calorimetry and x-ray diffraction, respectively. The microstructure is observed by using a scanning electron microscope, and chemical composition is checked by energy dispersive spectroscopy analysis. The effect of high temperature on the isothermal crystallization of Fe50−xCr15Mo14C15B6Mx(x = 0, 2 and M=Y, Gd) bulk metallic glass and ribbons is investigated by high-temperature x-ray diffraction. It is found that the crystallization behavior of Fe50−xCr15Mo14C15B6Mx(x = 0, 2 and M=Y, Gd) bulk metallic glass strongly depends on the annealing temperature. The different crystallization behavior is believed to be due to the different structures that the metallic glass possesses at different temperatures.

Controlled Crystallization of Metallic Glasses Through Joule Heating

This work reviews the basic aspects of an alternative thermal treatment of metallic glasses by Joule heating, which explores the heat produced by an electrical current flowing through samples. The advantages of this annealing method are discussed, and a brief description of available models in a variety of materials is given. The experimental procedures are discussed in detail, with emphasis in a new variant of Joule heating, known as “linear varying current Joule heating”. This method allows one to precisely monitor the structural transformations that take place during annealing, by interrupting the thermal treatment at any desired point, leading to samples with controlled micro and/or nanostructures. Some examples of Joule heating curves in novel bulk amorphous samples are shown, to illustrate the excellent perspectives of this annealing technique.

Thermal and Magnetic Properties in (FeBSi)NbY Bulk Glassy Alloys

The thermal and magnetic properties of (Fe₇₅B₂₀Si₅)₉₃Nb₄Y₃ and (Fe₇₈B₁₄Si₈)₉₃Nb₄Y₃ metallic glasses, in cylinder and ribbon form, have been studied and compared. Bulk samples were prepared by water-cooled Cu-mold casting technique. Ribbons having the same compositions have been prepared by rapid quenching. Selected samples have been submitted to furnace annealing to study the crystallization behavior and the corresponding structure has been studied by X-ray diffraction technique. Room-temperature hysteresis loops and magnetisation behavior as a function of temperature have been measured by a Vibrating Sample Magnetometer (VSM). In this paper, the effect of small addition in the rapidly solidified ternary system Fe₇₈B₁₄Si₈ either on the glass formation or the magnetic properties will be shown.

Magnetic Power Losses in ((Fe1-xCox)75B20Si5)93Nb4Y3 (x = 0, 0.2, 0.4) Bulk Metallic Glasses

Magnetic power losses of ((Fe1-xCox)75B20Si5)93Nb4Y3 (x = 0, 0.2, 0.4) metallic glasses have been investigated. Bulk samples were prepared by water-cooled Cu-mold injection casting technique with shapes of cylinders (0.8 mm diameter and 30 mm length) and toroids (10 mm external diameter and 0.5 mm thickness). Ribbons prepared by the melt- spinning technique were also analyzed. Glassy structures were confirmed by the presence of a main halo in XRD and by crystallization signal in DSC. Power losses were studied with a digital wattmeter over a range of frequencies from 1 to 400 Hz at selected peak inductions. Ribbons show smaller losses than bulk samples, presenting 24.5 J/m 3 at 50 Hz and 0.65 T peak induction. It was observed that the Co addition reduces significantly the power losses. A separation theory was applied in order to explain the square root behavior of the measured power losses as a function of frequency and the results are in good agreement with the experimental data. The magnetic data were used to identify the presence of crystalline inclusions in the magnetic bulk metallic glasses. The effect of sample shape and composition on magnetic properties will be discussed.

Bulk Metallic Glasses

Rapid quenching techniques have been successfully applied since long time for the preparation of metallic glasses in ribbon form. Only in the recent years, the research activity addressed towards the synthesis of bulk metallic glasses (BMG), in form of ingots with a few millimetres in thickness. These materials can be obtained by casting techniques only for selected alloy compositions, characterised by a particularly high glass-forming tendency. Bulk amorphous alloys are characterised by a low modulus of elasticity and high yielding stress. The usual idea is that amorphous alloys undergo work softening and that deformation is concentrated in shear bands, which might be subjected to geometrical constraints, resulting in a substantial increase in hardness and wear resistance. The mechanical properties can be further improved by crystallisation. In fact, shear bands movement can be contrasted by incorporating a second phase in the material, which may be produced directly by controlled crystallisation. Soft magnetic properties have been obtained in Fe-based systems and they are strongly related to small variations in the microstructure, ranging from a fully amorphous phase to nanocrystalline phases with different crystal size. The high thermal stability of bulk metallic glasses makes possible the compression and shaping processes in the temperature range between glass transition and crystallisation. Aim of this paper is to present recent results on glass formation and properties of bulk metallic glasses with various compositions. Examples will be reported on Zr, Fe, Mg and Pd-based materials, focussing on mechanical and magnetic properties.

Influence of Co Addition on Magnetic Properties and Glass Formation of Fe-based Amorphous Alloys

In the present work, the influence of Co addition on the glass forming ability (GFA) and magnetic properties of the ((Fe 1− x Co x ) 75 B 20 Si 5 ) 93 Nb 4 Y 3 ( x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) alloy is evaluated. Ribbons and 1 mm diameter ingots were prepared by melt-spinning technique and Cu-mold injection casting technique, respectively. The presence of the amorphous phase was confirmed through X-ray diffraction and high temperature differential scanning calorimetry (HT-DSC). Rapid solidification leads to fully amorphous ribbons for all compositions. Conversely, ingots with Co content with x > 0.5 present boride crystalline phases. Magnetization measurements were performed by means of vibration sample magnetometer in a temperature range from room temperature to 850 °C in order to determine the Curie temperature ( T C ) of the amorphous phase and to follow the formation of magnetic phases at high temperatures. In addition, hysteresis loops measurements were carried out to study the magnetic response of the samples. Co substitution until x = 0.5 is seen to simultaneously induce in the as cast ingots a decrease of coercive field from 52 to around 10 A/m and an increase of the Curie temperature of the amorphous phase from 480 to 700 K. A maximum saturation magnetization value of 123 emu/g was observed in the sample having x = 0.2. The role of Co substitution on glass formation is discussed on the basis of crystallization and melting behaviors, analyzed by HT-DSC. Magnetic properties are correlated with the presence of crystalline phases and their microstructures.