XAS and XMCD studies of amorphous FeCo-based ribbons

This paper reports x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) studies at Fe L_3,2 and Co L_3,2-edges to investigate the electronic structure of (Fe_100 ? xCo_x)₇₈Si₉Nb₃B₉Cu₁ (0 < x < 95) alloys. The influence of controlled Co addition on electronic structural and magnetic properties of (Fe_100 ? xCo_x)₇₈Si₉Nb₃B₉Cu₁ (x = 0, 20, 40, and 60) alloys has been investigated and it has been observed that Co exists as Co²⁺/Co³⁺, while Fe exists as a mixture of Fe⁰ (metallic) and Fe²⁺. The XMCD studies confirm these results and reveal that Co-ions are responsible for the room temperature ferromagnetism (RTFM) in the system, while at Fe L_3,2-edge it shows a diamagnetic behavior.     

Bulk glassy (Fe0.474Co0.474Nb0.052)100 − x(B0.8Si0.2)x alloys prepared using commercial raw materials

The alloy compositions have been optimized by modifying the B and Si contents in (Fe_0.474Co_0.474Nb_0.052)_100 ? x(B_0.8Si_0.2)_x alloy system with commercial materials. The thermal stability of the supercooled liquid improves with the increased B and Si contents from x = 22 to 28. The composition of the alloy with x = 26 is close to a eutectic point. By copper mold casting, (Fe_0.474Co_0.474Nb_0.052)_100 ? x(B_0.8Si_0.2)_x bulk glassy alloys with diameters up to 5 mm were synthesized for the composition range of x from 22 to 28. In addition to high glass-forming ability (GFA), the (Fe_0.474Co_0.474Nb_0.052)_100 ? x(B_0.8Si_0.2)_x glassy alloys exhibit good soft magnetic properties as well, i.e., rather high saturation magnetization of 0.84–1.07 T, low coercive field of 1.8–3.2 A/m, high initial permeability of 10100–24100 at 1 kHz under a field of 1 A/m and Curie temperature of 620–730 K.     

In-situ crystallization of amorphous Fe73−xNbxAl4Si3B20 alloys through synchrotron radiation

In the present work Fe_73−xNb_xAl₄Si₃B₂₀ (x = 5, 10) alloys have been processed by melt-spinning with the aim of studying the crystallization sequence through annealing in suitable temperatures. Melt-spun ribbons were characterized by differential scanning calorimetry (DSC), X-ray diffractometry (XRD) through Cu-Kα (λ = 1.54 Å) and synchrotron radiation (λ = 1.77 Å) and transmission electron microscopy (TEM). Soft magnetic properties were measured through the hysteresis loop tracer. In-situ XRD through synchrotron radiation was very accurate in phase identification. Fe_73−xNb_xAl₄Si₃B₂₀ (x = 5, 10) alloys showed the possibility of forming ferromagnetic amorphous alloys composed of commercial Fe-based master alloys with fine nanocrystalline structure and good soft magnetic properties.     

Magnetic and magnetoimpedance properties of Mn-doped FINEMET

In this study, the effect of substituting Mn for Fe on the magnetic and magnetoimpedance (MI) properties of amorphous and nanocrystalline Fe_73.5?xMn_xCu₁Nb₃Si_13.5B₉ (x = 1, 3, 5, 7) alloys has been investigated. A higher Mn content causes a decrease in the saturation magnetization and Curie temperature. For as-received Fe_73.5?xMn_xCu₁Nb₃Si_13.5B₉ (x = 0, 1, 3, 5, 7) ribbons, the measured values of the MI are 11%, 12%, 16%, 12% and 15% for x = 0, 1, 3, 5, 7, respectively. The magnitude of the MI ratio increases in annealed samples, indicating that they are magnetically softened by nanocrystallisation. The change in MI is found to be ～70% for the x = 3 sample annealed at 550 °C. It is also found that replacing a small amount of Fe by Mn improves the magnetic softness of Fe_73.5?xMn_xCu₁Nb₃Si_13.5B₉ alloys.     

Plastic deformation behavior of amorphous Fe78Si9B13 alloy at elevated temperature

The plastic deformation behavior of amorphous Fe₇₈Si₉B₁₃ alloy was investigated under uniaxial tension and gas pressure bulging. The deformed specimens were studied by means of XRD and SEM. It is shown that amorphous Fe₇₈Si₉B₁₃ ribbon has good deformation ability in the temperature range from 450 °C to 500 °C. The elongation of 36.3% and the relative bulging height (RBH) of 0.45 were obtained at 500 °C. In the uniaxial tension test, the amount of the crystalline α-Fe phase increases with deformation temperature below the crystallization temperature, which also suggests that the tensile stress promotes the crystallization of amorphous Fe₇₈Si₉B₁₃ ribbon. The suitable gas pressure bulging conditions for the specimens are 500 °C, 3.4 MPa and 30 min.     

Corrosion behavior of Fe-based ferromagnetic (Fe,Ni)–B–Si–Nb bulk glassy alloys in aqueous electrolytes

Fe-based ferromagnetic [(Fe_1?xNi_x)_0.75B_0.2Si_0.05]₉₆Nb₄ (x = 0, 0.2 and 0.4) bulk glassy alloys (BGAs) with a diameter of 2 mm were prepared by copper mold casting. The corrosion behavior of glassy alloy rods obtained was investigated in 0.5 M NaCl, 0.5 M NaOH and 0.5 M H₂SO₄ aqueous electrolytes, respectively, using weight loss and electrochemical polarization measurements. It was found that the corrosion rates significantly decrease with an increase in Ni content in all examined solutions. The Ni-containing BGAs are spontaneously passivated with wide passive regions and low passive current densities in NaCl and NaOH solutions, but exhibit the active–passive–transpassive behavior in H₂SO₄ solution. The partial substitution of Ni for Fe results in a considerable improvement on the corrosion resistance of [(Fe_1?xNi_x)_0.75B_0.2Si_0.05]₉₆Nb₄ BGAs, because of the structural and chemical homogeneousness of the amorphous phase and the effect of Ni on promoting the formation of a passive film. Besides their high glass-forming ability (GFA), excellent soft-magnetic properties and good mechanical performance, which have been reported before, these FeNi-based BGAs also feature rather high corrosion resistance.     

Torsion annealing influence on the impedance behaviour in amorphous FeSiB and CoSiB wires

An experimental study on the influence of a dc magnetic field on the real and imaginary parts of axial diagonal (ζ_zz) and off-diagonal (ζ_ϕz) components of the surface magnetoimpedance (MI) tensor in amorphous Fe_77.5Si_7.5B₁₅ and Co_72.5Si_12.5B₁₅ wires has been performed. The impedance characteristics have been analysed in the as-cast wires and after being torsion annealed. The ac drive current was 5 mArms flowing along the wires in the frequency range from 0.45 to 0.8 MHz. The MI behaviours can be ascribed to the different domain structures of positive and negative magnetostriction wires, before and after the intrinsic anisotropy distribution has been also modified by the annealing treatment.     

Thermal behavior of substituted FeCo-based metallic glasses

Three compositions of metallic glasses, Fe₅₈Co₂₄Nb₃Ta₁Mo₁B₁₃, Fe_61.5Co_20.5Nb₃Ta₁Mo₁B₁₃ and Fe₆₆Co₁₈Si₁B₁₅ were prepared by rapid quenching from the melt and subsequently annealed for 1 hour at 450, 650 and 750 °C. The samples were analyzed by X-ray diffraction (XRD) and Mössbauer spectroscopy. All Mössbauer spectra were fitted with a six-line pattern corresponding to the crystalline α-(FeCo) phase and a hyperfine magnetic field distribution representing the amorphous component. The Mössbauer spectra of annealed Fe₆₆Co₁₈Si₁B₁₅ revealed the presence of a secondary crystalline phase, namely (FeCo)₃(BSi). Moreover, the last Mössbauer spectrum in the set was fitted with an additional sextet, corresponding to the appearance of hematite in the system. It is inferred that the addition of Nb, Ta and Mo considerably delays the onset of bulk crystallization and iron oxidation in these systems. The XRD patterns are in qualitative agreement with the Mössbauer results and are consistent with a surface layer of hematite nanoparticles in the system. The activation energy of 4.34 eV for oxidation was estimated from the Mössbauer results.     

Influence of annealing on the high frequency magnetotransport properties of melt-spun Fe31Co31Nb8B30 alloys

The influence of furnace annealing and Joule heating on microstructure and high frequency magnetotransport properties of Fe₃₁Co₃₁Nb₈B₃₀ alloy has been studied using differential scanning calorimetery, hysteresis and Mössbauer spectroscopy measurements. Annealing reduces disorder in the specimens via structural relaxation improving soft magnetic properties. In as-cast specimens the spins preferentially remain within ribbon plane whereas annealing above 400 °C they become random. Magnetoimpedance shows a progressive deterioration of the magnetoimpedance response upon furnace annealing as well as Joule heating of the samples.     

Influence of nickel content on the electrochemical behavior of Finemet type amorphous and nanocrystalline alloys

The aim of this work has been the systematic study of the influence of partial substitution of Fe by Ni in the Ni_xFe_73.5−xSi_13.5B₉Nb₃Cu₁ alloy within the range 0 ⩽ x ⩽ 10% atom (x = 0, 2, 4, 6, 8, 10) on electrochemical behavior, corrosion rate and the structural changes in amorphous and nanocrystalline alloys. The amorphous nature of the alloys was confirmed by X-ray diffraction and the chemical compositions were determined by ICP. The glass transition and kinetic crystallization of amorphous alloys were studied by DSC. The technique of XPS was used for evaluating the chemical states of elements present in native oxide films. The electrochemical behavior of amorphous and nanocrystalline alloys have been investigated in 0.5 M KOH using cyclic voltammetry. The experimental results show that the formation of different nanocrystalline phases is not excessively transformed by the addition of small amount of nickel and the electrochemical behavior is improved as nickel content increased.     

Nd5Fe64B23Mo4Y4 bulk nanocomposite permanent magnets produced by crystallizing amorphous precursors

The glass forming ability and magnetic properties of Nd₅Fe_68 ? xB₂₃Mo₄Y_x (x = 0, 2, 4, 6) alloys prepared by copper mold casting technique have been studied. Amorphous rods with a diameter of 2 mm were obtained in the Nd₅Fe₆₄B₂₃Mo₄Y₄ alloy. After annealing for 10 min at 1013 K, the Nd₅Fe₆₄B₂₃Mo₄Y₄ alloy showed optimal hard magnetic properties with the coercivity of 764.2 kA/m, remanence of 0.6 T and maximum energy product of 57.3 kJ/m³, respectively. The enhanced magnetic properties can be ascribed to the strong exchange coupling among the magnetically soft α-Fe (25–30 nm), Fe₃B (30–35 nm) and hard Nd₂Fe₁₄B (40–50 nm) grains present in the magnet microstructure. Large size bulk nanocomposite magnets with sound magnetic properties make the Nd–Fe–B–Mo–Y alloy system a promising candidate for industrial applications.     

Angular dependence of low-field microwave absorption in Co-rich amorphous alloys

Microwave power absorption measurements at 9.4 GHz (X-band) were carried out on as-cast amorphous ribbons of nominal composition Co₆₆Fe₄B₁₂Si₁₃Nb₄Cu, prepared by melt spinning. The angular dependence of low-field absorption (LFA) was investigated from θ = 0° to 180° in two orientations. In both cases the ribbon axis was orientated parallel to the DC magnetic field. In orientation 1, the ribbon plane was parallel to the AC microwave field. In orientation 2, it was perpendicular to the AC field. LFA spectra showed an antisymmetric shape (a minimum and a maximum), whose separation increased as a function of the angle between the DC field and the ribbon axis. A similar angular behavior was observed for the resonance field of the ferromagnetic resonance (FMR). For the orientation 1, the resonance field increased as a function of the angle, which can be explained in terms of the contribution of the shape anisotropy to the total anisotropy field. A similar behavior was observed in LFA, which suggests that the shape anisotropy field can be observed in the angular dependence of LFA. In the orientation 2, the observed changes occur within the plane associated with the induced anisotropy due to the rapid solidification process. The LFA signal is capable therefore to detect different contributions to the total anisotropy.     

Influence of Cr additions on corrosion resistance of Fe- and Co-based metallic glasses and nanocrystals in H2SO4

The influence of Cr concentration on the corrosion resistance of Fe_73.5Si_13.5B₉Nb₃Cu₁ and Co_73.5Si_13.5B₉Nb₃Cu₁ metallic glasses was studied by DC electrochemical methods in different concentrations of H₂SO₄ (1, 3 and 5 M) at room temperature. Gravimetric tests were performed to establish the reliability of the data obtained electrochemically. Every test was determined with the same material in different states: amorphous, nanocrystalline and crystalline. In all cases metallic glasses and nanocrystals showed better resistance to corrosion in the tested conditions with increasing chromium concentration. However, in test conditions, the Fe-based materials maintained a high corrosion rate. The corrosion resistance of Co-based materials was slightly greater than that of Fe-based materials.     

Influence of composition in the crystallization process of Fe75−xNb10B15+x metallic glasses

The devitrification process of Fe_75−xNb₁₀B_15+x (x = 0, 5, 10) metallic glasses produced by melt-spinning has been analyzed by calorimetric, microstructural and magnetic measurements. The experimental results show large differences in the behavior of these alloys as a function of composition. The alloy x = 0 undergoes a primary crystallization process separated in two stages while only one is observed for alloys with x = 5 and x = 10. This difference, observed by DSC, is correlated with a microstructural change in the phases that precipitate. For alloys x = 5 and 10, Fe₂₃B₆ and bcc-Fe are identified after the first calorimetric peak. In the sample x = 0, bcc-Fe and an unidentified phase precipitate in the first peak but massive crystallization of bcc-Fe is observed after the second stage. Finally the thermal dependence of magnetization has been measured and the Curie temperatures for the metallic glasses are determined. The change of these quantities with heat treatment and composition is discussed.     

Crystallization kinetics and magnetic properties of Fe63.5Co10Si13.5B9Cu1Nb3 nanocrystalline powder cores

Amorphous ribbons of the alloy Fe_63.5Co₁₀Si_13.5B₉Cu₁Nb₃ were prepared by the standard single copper wheel melt spinning technique in air and their crystallization kinetics was analyzed by non-isothermal differential scanning calorimetry (DSC) measurements. The crystallization activation energies (E_x, E_a1 and E_a2) of amorphous ribbons calculated from Kissinger model were 448, 385 and 396 kJ/mol for the first and the second crystallizations, respectively. The Avrami exponent n was calculated from the Johnson–Mehl–Avrami (JMA) equation and was used to identify the crystallization mechanism for the amorphous ribbons. The ribbons were milled into different sized flakes, which were molded subsequently to cores using 3 wt.% epoxy as a binder. The effective permeability of the cores showed high frequency stability and increased with the size of the flakes. For the cores made from small sized flakes (? 75 μm), the quality factor was increased at high frequencies, which was attributed to the reduction in the eddy current loss.     

Magnetoimpedance effect in surface pinned nanostructured Fe-based alloys

Amorphous Fe-based ribbons of the composition Fe_73.5Si_13.5B₉Nb₃Cu₁ have been nanocrystallized using conventional heat treatment at 550 °C for 60 min. Nanostructured ribbons have been irradiated using pulsed Nd:YAG laser (1064 nm). To investigate the pinning effect, different energies have been used to illuminate the ribbons. Pinning behavior has been studied using magnetoimpedance data. It has been shown that an increase in laser beam energy makes the pinning centers more effective.     

Magnetic properties and low frequency magnetoimpedance of novel amorphous CoFeTaBCr ribbons

In this report, we study the effect of Cr addition on the dc an ac magnetic properties of novel melt spun amorphous Co_43−x/2Fe_20−x/2Cr_xB_31.5Ta_5.5 (x = 0, 1, 2, 4) alloy series. The saturation magnetization decreases with increasing Cr content as well as the ac relative initial permeability, which showed a high relaxation frequency of 3 MHz. In addition, the magnetoimpedance effect was detected in this alloy series, with a maximum variation ratio of 3% at x = 0.     

Viscosity of glass forming alloys based on Fe–Si–B system

The temperature dependences of kinematic viscosity of the amorphizing Fe₈₀B₁₄Si₆, Fe₇₇B₁₅Si₈, Fe₇₈Ni₁B₁₂Si₉, Fe_78.5Ni₁Mo_0.5B₁₄Si₆ and Fe_73.6Nb_2.4Cu₁B_7.2Si_15.8 melts in the temperature interval ranging from 1200 to 1600 °C under heating and cooling conditions have been studied. When heating the anomalies in the shape of sharp viscosity decrease have been revealed in the polytherms. On further cooling its hysteresis has been observed as well.     

Structural and magnetic properties of nanocrystalline particles in an amorphous Fe73.5Nb3CuSi13.5B9 matrix

We report structural and magnetic properties of fine particles embedded in an amorphous magnetic matrix. As-quenched amorphous Fe_73.5Nb₃CuSi_13.5B₉ ribbons (FINEMET) were submitted to the thermal treatments of several times (1 ? t ? 240 min) at 570 °C using a conventional furnace. The analyses of the X-ray diffraction patterns at room temperature reveal that our samples consist of single phase Fe₃Si nanocrystals embedded in a residual amorphous phase. Magnetic measurements show that the saturation moment at T = 450 °C increases as a function of annealing time. This behavior is attributed to an increase of the fraction of nanocrystallites in the residual amorphous phase.     

Effect of germanium on the microstructure and the magnetic properties of Fe–B–Si amorphous alloys

In this work, we present a systematic study on the crystallization kinetics and the magnetic properties of melt-spun Fe₈₀B₁₀Si_10 ? xGe_x (x = 0.0 ? 10.0) amorphous alloys. The activation energy for crystallization, determined by differential scanning calorimetry, displayed a strong dependence on the Ge content, reflecting a deleterious effect on the alloys' thermal stability and their glass forming ability with increasing Ge concentration. On the other hand, the alloys exhibited excellent soft magnetic properties, i.e., high saturation magnetization values (around 1.60 T), alongside Curie temperatures of up to 600 K. Complementary, for increasing Ge substitution, the ferromagnetic resonance spectra showed a microstructural evolution comprising at least two different magnetic phases corresponding to a majority amorphous matrix and to Fe(Si, Ge) nanocrystallites for x ≥ 7.5.