Metallic Nucleus Research Articles

Dependence of Magnetic Properties of As-Prepared Nanocrystalline Ni2MnGa Glass-Coated Microwires on the Geometrical Aspect Ratio.

We have prepared NiMnGa glass-coated microwires with different geometrical aspect ratios, ρ = dmetal/Dtotal (dmetal-diameter of metallic nucleus, and Dtotal-total diameter). The structure and magnetic properties are investigated in a wide range of temperatures and magnetic fields. The XRD analysis illustrates stable microstructure in the range of ρ from 0.25 to 0.60. The estimations of average grain size and crystalline phase content evidence a remarkable variation as the ρ-ratio sweeps from 0.25 to 0.60. Thus, the microwires with the lowest aspect ratio, i.e., ρ = 0.25, show the smallest average grain size and the highest crystalline phase content. This change in the microstructural properties correlates with dramatic changes in the magnetic properties. Hence, the sample with the lowest ρ-ratio exhibits an extremely high value of the coercivity, Hc, compared to the value for the sample with the largest ρ-ratio (2989 Oe and 10 Oe, respectively, i.e., almost 300 times higher). In addition, a similar trend is observed for the spontaneous exchange bias phenomena, with an exchange bias field, Hex, of 120 Oe for the sample with ρ = 0.25 compared to a Hex = 12.5 Oe for the sample with ρ = 0.60. However, the thermomagnetic curves (field-cooled-FC and field-heating-FH) show similar magnetic behavior for all the samples. Meanwhile, FC and FH curves measured at low magnetic fields show negative values for ρ = 0.25, whereas positive values are found for the other samples. The obtained results illustrate the substantial effect of the internal stresses on microstructure and magnetic properties, which leads to magnetic hardening of samples with low aspect ratio.

Tuning of magnetic bistability and domain wall dynamics in magnetic microwires

A unique combination of unusual magnetic properties, such as magnetic bistability associated with ultrafast domain wall propagation or ultrasoft magnetic properties, together with excellent mechanical and corrosion properties can be obtained in amorphous microwires. Such ferromagnetic microwires coated with insulating and flexible glass-coating with diameters ranging from 0.1 to 100 can be prepared using the Taylor-Ulitovsky method. Magnetic properties of glass-coated microwires are affected by chemical compositions of the metallic nucleus and can be substantially modified by post-processing. We provide an overview of the routes allowing tuning of hysteresis loops and domain wall dynamics in amorphous microwires and new experimental results on the dependence of hysteresis loops on external stimuli, such as applied stress and temperature.

Preparation and magnetic properties of MnFePSi-based glass-coated microwires

We prepared a Mn48Fe22P15Si15 glass-coated microwires (GCMWS) with metallic nucleus diameter, d = 11.2 μm and total diameter, D = 28.3 μm (geometrical aspect ratio d/D = 0.4) for the first time by using the Taylor–Ulitovsky Technique. This low-cost, single-step fabrication approach enabled the preparation of kilometers-long GCMWS from a few grams of low-cost components (Mn, Fe, P, and Si) for a variety of applications. The analysis of the magnetic measurements revealed a well-defined magnetic anisotropy in the whole temperature range. Moreover, relatively hard magnetic properties were observed for the temperature range of 5–400 K, where the average of coercivity, Hc ≈ 465 Oe. Notable magnetic field, H, and temperature dependencies of the magnetic properties were observed. Substantially irreversible magnetic behavior with a blocking temperatures Tb = 97 K at H = 1 kOe and Tb = 50 K at H = 5 kOe upon field cooling was observed. The modification in the magnetic properties of MnFePSi-glass-coated microwires is ascribed to the presence of various magnetic phases resulting from internal stresses induced by the glass coating. Moreover, the elevated Curie temperature (Tc > 400 K) observed in the investigated sample, makes this material as an appealing choice for several industrial applications.

Enhancing the Squareness and Bi-Phase Magnetic Switching of Co2FeSi Microwires for Sensing Application.

In the current study we have obtained Co2FeSi glass-coated microwires with different geometrical aspect ratios, ρ = d/Dtot (diameter of metallic nucleus, d and total diameter, Dtot). The structure and magnetic properties are investigated at a wide range of temperatures. XRD analysis illustrates a notable change in the microstructure by increasing the aspect ratio of Co2FeSi-glass-coated microwires. The amorphous structure is detected for the sample with the lowest aspect ratio (ρ = 0.23), whereas a growth of crystalline structure is observed in the other samples (aspect ratio ρ = 0.30 and 0.43). This change in the microstructure properties correlates with dramatic changing in magnetic properties. For the sample with the lowest ρ-ratio, non-perfect square loops are obtained with low normalized remanent magnetization. A notable enhancement in the squareness and coercivity are obtained by increasing ρ-ratio. Changing the internal stresses strongly affects the microstructure, resulting in a complex magnetic reversal process. The thermomagnetic curves show large irreversibility for the Co2FeSi with low ρ-ratio. Meanwhile, if we increase the ρ-ratio, the sample shows perfect ferromagnetic behavior without irreversibility. The current result illustrates the ability to control the microstructure and magnetic properties of Co2FeSi glass-coated microwires by changing only their geometric properties without performing any additional heat treatment. The modification of geometric parameters of Co2FeSi glass-coated microwires allows to obtain microwires that exhibit an unusual magnetization behavior that offers opportunities to understand the phenomena of various types of magnetic domain structures, which is essentially helpful for designing sensing devices based on thermal magnetization switching.

Magnetostriction of amorphous Co-based and Fe-Ni-based magnetic microwires: Effect of stresses and annealing

The magnetostriction coefficient and its stress dependence are evaluated in Co- and Fe-Ni- based glass-coated microwires with different character of hysteresis loops. The hysteresis loops as well as the magnetostriction coefficients of the microwires depend mostly on the chemical composition of the metallic nucleus. We found that the stress dependence of the magnetostricition coefficient, λs, is linear for both families of studied microwires, however a decrease with applied stress is observed for Co-rich microwires with negative magnetostriction coefficient. In contrast, a linear increase of the magnetostriction coefficient with applied stress is observed for Fe-Ni rich microwires with positive magnetostriction coefficient. Additionally the λs - value is affected by annealing. We evaluated the dependence of λs on annealing time, tann, for different annealing temperatures. In Co-rich microwires a change of the λs sign is observed. For both kinds of microwires (either with positive or with negative λs) λs(tann) at Tann = 400 °C present similar features: an increase with tann up to tann ≈ 10 min followed by a decrease for tann> 10 min. The λs(tann) dependencies are affected by the annealing temperature. Observed results are discussed in terms of influence of internal stresses and short range ordering induced by annealing.

Preparation and Magneto-Structural Investigation of High-Ordered (L21 Structure) Co2MnGe Microwires

We used the Taylor–Ulitovsky technique to prepare nanocrystalline Co2MnGe Heusler alloy glass-coated microwires with a metallic nucleus diameter of 18 ± 0.1 µm and a total diameter of 27.2 ± 0.1 µm. Magnetic and structural studies were carried out to determine the fundamental magneto-structural characteristics of Co2MnGe glass-coated microwires. XRD revealed a well-defined nanocrystalline structure with an average grain size of about 63 nm, lattice parameter a = 5.62 and a unique mixture of L21 and B2 phases. The hysteresis loops measured at different temperatures indicated a well-known ferromagnetic behavior for the reduced remanent, where a monotonic increasing in the reduced remanent and saturation magnetization occurs. The coercivity shows anomalous behavior compared to the Co2Mn-based glass-coated microwires. The magnetization curves for field cooling and field heating (FC–FH) demonstrate a considerable dependence on the applied magnetic field, ranging from 50 Oe to 20 kOe. Internal stresses, originated by the production process, resulted in various magnetic phases, which were responsible for the notable difference of FC and FH curves on magnetization dependence versus temperature. Furthermore, the ferromagnetic behavior and expected high Curie temperature, together with high degree of the L21 order, make it a promising candidate for many applications.

Temperature influence on magnetic properties and magnetoimpedance effect of Fe-rich glass-coated microwires

Giant magnetoimpedance, GMI, effect and magnetic properties upon temperature influence of as-prepared and stress-annealed amorphous Fe75B9Si12C4 glass-coated microwires produced by the Taylor-Ulitovsky technique are analyzed. Remarkable change in the hysteresis loops and GMI effect is observed for both samples upon heating. Tuning of the stress-annealing conditions allows one to vary the temperature dependence. Furthermore, it is observed almost complete reversibility of the changes induced by the temperature. Observed dependences are explained by the heating effect on the internal stresses relaxation, by the modification of the thermal expansion coefficients of the metallic nucleus and the glass coating, and by the Hopkinson effect.

Preparation and Magneto-Structural Investigation of Nanocrystalline CoMn-Based Heusler Alloy Glass-Coated Microwires

In this work, we have successfully fabricated nanocrystalline Co2MnSi Heusler alloy glass-coated microwires with a metallic nucleus diameter (dnuclei) 10.2 ± 0.1 μm and total diameter 22.2 ± 0.1 μm by the Taylor–Ulitovsky technique for the first time. Magnetic and structural investigations have been performed to clarify the basic magneto-structural properties of the Co2MnSi glass-coated microwires. XRD showed a well-defined crystalline structure with a lattice parameter a = 5.62 Å. The room temperature magnetic behavior showed a strong in-plane magnetocrystalline anisotropy parallel to the microwire axis. The M-H loops showed unique thermal stability with temperature where the coercivity (Hc) and normalized magnetic remanence exhibited roughly stable tendency with temperature. Moreover, quite soft magnetic behavior has been observed with values of coercivity of the order of Hc = 7 ± 2 Oe. Zero field cooling and field cooling (ZFC-FC) magnetization curves displayed notable irreversible magnetic dependence, where a blocking temperature (TB = 150 K) has been observed. The internal stresses generated during the fabrication process induced a different magnetic phase and is responsible for the irreversibility behavior. Moreover, high Curie temperature has been reported (Tc ≈ 985 K) with unique magnetic behavior at a wide range of temperature and magnetic fields, making it a promising candidate in magnetic sensing and spintronic applications.

Fabrication and Magneto-Structural Properties of Co2-Based Heusler Alloy Glass-Coated Microwires with High Curie Temperature

In this work, we were able to produce Co2FeSi Heusler alloy glass-covered microwires with a metallic nucleus diameter of about 4.4 µm and total sample diameter of about 17.6 μm by the Taylor–Ulitovsky Technique. This low cost and single step fabrication process allowed the preparation of up to kilometers long glass-coated microwires starting from a few grams of high purity inexpensive elements (Co, Fe and Si), for a wide range of applications. From the X-ray diffraction, XRD, analysis of the metallic nucleus, it was shown that the structure consists of a mixture of crystalline and amorphous phases. The single and wide crystalline peak was attributed to a L21 crystalline structure (5.640 Å), with a possible B2 disorder. In addition, nanocrystalline structure with an average grain size, Dg = 17.8 nm, and crystalline phase content of about 52% was obtained. The magnetic measurements indicated a well-defined magnetic anisotropy for all ranges of temperature. Moreover, soft magnetic behavior was observed for the temperature measuring range of 5–1000 K. Strong dependence of the magnetic properties on the applied magnetic field and temperature was observed. Zero field cooling and field cooling magnetization curves showed large irreversibility magnetic behavior with a blocking temperature (TB = 205 K). The in-plane magnetization remanence and coercivity showed quite different behavior with temperature, due to the existence of different magnetic phases induced from the internal stress created by the glass-coated layer. Moreover, a high Curie temperature was reported (Tc ≈ 1059 K), which predisposes this material to being a suitable candidate for high temperature spintronic applications.

Vol4 num4-16 Magnetic properties and applications of glass-coated ferromagnetic microwires

The impact of post-processing on soft magnetic properties and the giant magnetoimpedance (GMI) effect of Fe- and Co-based glass-coated microwires is evaluated. A remarkable improvement of magnetic softness and GMI effect is observed in Fe-rich glass-coated microwires subjected to stress annealing. Frequency dependence of GMI ratio of stress-annealed Fe-rich microwires has been discussed considering frequency dependence of the skin penetration depth, δ, as well as magnetic anisotropy distribution within the metallic nucleus. Annealed and stress-annealed Co-rich microwires present rectangular hysteresis loop and single and fast domain wall propagation. However, Co-based stress-annealed microwires present high magnetoimpedance ratio. Observed stress-induced anisotropy and related changes of magnetic properties are discussed considering internal stresses relaxation and “back-stresses”.

Post-Annealing Influence on Magnetic Properties of Rapidly Quenched Ni–Mn–Ga Glass-Coated Microwires

In this article, we showed how the post-annealing process affects the magnetic properties of rapidly quenched Ni-Mn-Ga glass-coated microwires. Microwires with Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">63</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">25</sub> chemical composition, total diameter D = 54.6 μm and metallic nucleus diameter d = 26.7 μm, were prepared by the Taylor-Ulitovsky technique. The annealing temperature was fixed at 873 K, and the time was 1 and 10 h. The microwires were slowly cooled to room temperature after annealing. Based on thermomagnetic curves and hysteresis loops, we suggest crystallographic texture in the wires that considerably affect magnetic properties. The wires' internal structure indicates nanocrystalline state with radial micrometers and submicrometer grains distribution in the wire annealed during 10 h. A ferromagnetic-to-paramagnetic phase transition occurs around room temperature. No signs of a martensitic transformation were found. The difference in magnetization saturation, remanence, and Curie temperature values for 1- and 10-h annealed microwires indicates the influence of internal stresses and that created by the presence of a glass coating.

The study of adhesion between the glass shell and the metallic nucleus of a Fe-based amorphous microwire

A method for measuring adhesion between the glass shell and the metallic nucleus of an amorphous microwire was developed. It was revealed that the stress of separation of the glass shell from the metallic nucleus (pull-off stress) can vary from hundreds of MPa to units of GPa. It was found that the adhesion value between the glass shell and the metallic nucleus is 270 MPa. It was found that the annealing of microwires below crystallization temperature can lead to a more than 2-fold increase in the adhesion. The effect of adhesion on the stress state and coercivity of microwires under tension was analyzed.

Optimization of Magnetic Properties of Magnetic Microwires by Post-Processing

The influence of post-processing conditions on the magnetic properties of amorphous and nanocrystalline microwires has been thoroughly analyzed, paying attention to the influence of magnetoelastic, induced and magnetocrystalline anisotropies on the hysteresis loops of Fe-, Ni-, and Co-rich microwires. We showed that magnetic properties of glass-coated microwires can be tuned by the selection of appropriate chemical composition and geometry in as-prepared state or further considerably modified by appropriate post-processing, which consists of either annealing or glass-coated removal. Furthermore, stress-annealing or Joule heating can further effectively modify the magnetic properties of amorphous magnetic microwires owing to induced magnetic anisotropy. Devitrification of microwires can be useful for either magnetic softening or magnetic hardening of the microwires. Depending on the chemical composition of the metallic nucleus and on structural features (grain size, precipitating phases), nanocrystalline microwires can exhibit either soft magnetic properties or semi-hard magnetic properties. We demonstrated that the microwires with coercivities from 1 A/m to 40 kA/m can be prepared.

The effect of stress distribution in the bulk of a microwire on the magnetization processes

Abstract The effect of tensile stresses on the magnetic properties of amorphous microwires with positive magnetostriction has been studied. It has been determined that the experimental dependence of coercivity on the tensile stresses is reversible and consists of linear and square root parts. The evolution of an average stress level in the “core” and surface domain layer has been theoretically estimated depending on the external tensile stress. The value of internal stress level may vary from 150 to 600 MPa in “core” for microwires with different ratio of amorphous metallic nucleus diameter to thickness of glass shell. Crucial differences of tension of glass-coated and uncoated microwires have been investigated. The analysis of these differences with relation to effect under tension has been performed.

Stress-induced magnetic anisotropy enabling engineering of magnetic softness of Fe-rich amorphous microwires

Stress-annealing allows considerably modify magnetic properties of Fe-rich glass-coated microwires. Varying the stress-annealing conditions (temperature, time and stress applied during the annealing) we can observe either remarkable improvement of magnetic softness and giant magnetoimpedance (GMI) effect. Observed changes have been attributed to the transverse magnetic anisotropy. An improvement of the circumferential permeability in the surface layer of metallic nucleus is evidenced from observed magnetic softening and remarkable GMI effect rising. We assumed that stress-annealing leads to an increase in the outer domain sheath with transverse magnetic anisotropy rise in expense of inner axially magnetized core typically existing in as-prepared Fe-rich microwires. Consequently, stress annealing enabled us to design the magnetic anisotropy distribution beneficial for optimization of the GMI effect and magnetic softness of Fe-rich magnetic microwires.

Synchrotron, X-Ray, and Electron Microscopic Studies of Catalyst Systems Based on Multiwalled Carbon Nanotubes Modified by Copper Nanoparticles

The results of the studies of a composite based on multiwalled carbon nanotubes (MWCNTs), onto the outer surface of which nanosized copper particles are deposited by the pyrolysis of copper formate, via electron microscopy, X-ray diffractometry, and ultrasoft X-ray spectroscopy, are discussed. It is found that, in an inert argon atmosphere, copper nanoparticles are deposited onto the surface of MWCNTs in the form of different-size nanoparticles consisting of a metallic copper nucleus and a cuprous oxide Cu2O shell. However, after being taken out into the atmosphere, a layer of copper oxide CuO is formed on the surface of the copper nanoparticles. It is shown that the good adhesion of the copper nanoparticles on the surface of the MWCNTs is provided by the formation of a chemical bond between the carbon atoms of the outer graphene layer of the nanotube and the oxygen atoms of cuprous oxide Cu2O that initially covers the metallic nucleus of the nanoparticles.

Синхротронные, рентгеновские и электронно-микроскопические исследования каталитических систем на основе многостенных углеродных нанотрубок, модифицированных наночастицами меди

Abstract The results of the studies of a composite based on multiwalled carbon nanotubes (MWCNTs), onto the outer surface of which nanosized copper particles are deposited by the pyrolysis of copper formate, via electron microscopy, X-ray diffractometry, and ultrasoft X-ray spectroscopy, are discussed. It is found that, in an inert argon atmosphere, copper nanoparticles are deposited onto the surface of MWCNTs in the form of different-size nanoparticles consisting of a metallic copper nucleus and a cuprous oxide Cu_2O shell. However, after being taken out into the atmosphere, a layer of copper oxide CuO is formed on the surface of the copper nanoparticles. It is shown that the good adhesion of the copper nanoparticles on the surface of the MWCNTs is provided by the formation of a chemical bond between the carbon atoms of the outer graphene layer of the nanotube and the oxygen atoms of cuprous oxide Cu_2O that initially covers the metallic nucleus of the nanoparticles.

Engineering of Magnetic Softness and Domain Wall Dynamics of Fe-rich Amorphous Microwires by Stress- induced Magnetic Anisotropy

We observed a remarkable improvement of domain wall (DW) mobility, DW velocity, giant magnetoimpedance (GMI) effect and magnetic softening at appropriate stress-annealing conditions. Beneficial effect of stress-annealing on GMI effect and DW dynamics is associated with the induced transverse magnetic anisotropy. An improvement of the circumferential permeability in the nearly surface area of metallic nucleus is evidenced from observed magnetic softening and remarkable GMI effect rising. We assumed that the outer domain shell with transverse magnetic anisotropy associated to stress-annealing induced transverse magnetic anisotropy affects the travelling DW in a similar way as application of transversal bias magnetic field allowing enhancement the DW velocity. Observed decreasing of the half-width of the EMF peak in stress-annealed microwires can be associated to the decreasing of the characteristic DW width. Consequently, stress annealing enabled us to design the magnetic anisotropy distribution beneficial for optimization of either GMI effect or DW dynamics.

Interferência de artefatos na detecção de fraturas radiculares verticais em dentes bovinos com núcleo metálico fundido em imagens tomográficas de feixe cônico

Objetivos: Avaliar a interferência de artefatos (beamhardening) na detecção de fratura radicular vertical por meio da tomografia computadorizada de feixe cônico, bem como a sensibilidade, a especificidade e a acurácia. Materiais e métodos: Foram utilizadas 15 raízes de incisivos inferiores bovinos que, após a realização do tratamento endodôntico e desobturação de 10 mm, foram reabilitadas com núcleo metálico fundido, cuja cimentação foi efetuada com cimento de fosfato de zinco. Os exames de tomografia computadorizada de feixe cônico foram realizados antes (T0) e após (T1) a indução das fraturas, as quais foram avaliadas por dois examinadores, a fim de identificar a presença e a localização das fraturas radiculares, bem como a localização do beamhardening e sua interferência na detecção das fraturas radiculares verticais, utilizando o filtro Angio_Sharpen High 5X5. Resultados: O beamhardening foi observado em 100% das raízes e foi visualizado com mais frequência no terço radicular médio. Em ambos os momentos de avaliação (T0 e T1), os locais de beamhardening eram proporcionalmente os mesmos que os locais das fraturas, gerando diagnósticos positivos de fratura mesmo quando elas não existiam. Conclusões: A tomografia computadorizada de feixe cônico apresentou 78% de sensibilidade, 63% de especificidade e 68% de acurácia, demonstrando ser apenas um método auxiliar no diagnóstico de fraturas radiculares em dentes reabilitados com núcleo metálico fundido.

Nucleation and growth mechanisms of palladium, nanoflower-shaped, and its performance as electrocatalyst in the reduction of Cr(VI)

Palladium electrocrystallization on Ti film is studied through theoretical models of nucleation and growth. At the applied potential + 0.1 V, palladium electrodeposition followed a mechanism through these steps: adsorption, 2D nucleation with ad-atoms incorporation-controlled growth, and 3D nucleation with diffusion-controlled growth. At − 0.3 V, the metallic nucleus and its growth were accompanied by proton reduction, followed by hydrogen adsorption on palladium nucleation sites; thus nucleation turned more progressive. When Pd° was electrodeposited on glassy carbon (GC) electrode at + 0.05 V and + 0.1 V, this led to Volmer–Weber mechanism, with a 3D instantaneous nucleation and diffusion-controlled growth. The structural and morphological characterizations of both substrates determined a Pd° fcc lattice and nanoflowers of Pd° between 100 and 125 nm. Pd/Ti systems are applied in electroanalytic determinations, performing the nanoflowers as highly sensitive electrocatalysts in the chromium(VI) determination in acidic media. Pd/Ti showed a reduction peak at + 0.58 V Ag/AgC in deoxygenated 0.1 mol L−1 H2SO4. Unlike Ti film, the palladium nanostructures contributed to reduction of Cr(VI). Dependence of current peak versus Cr(VI) concentration with Pd/Ti electrocatalysts was linear from 1.9 10−5–10−4 mol L−1, even when the electrode was tested in a real water sample. For comparison, a glassy carbon (GC) electrode and Pd/GC electrode synthesized at + 0.05 V were assayed with Cr(VI). Unlike Ti film, GC contributed to reduction of Cr(VI). Scan rate studies confirmed that Cr(VI) reduction with Pd/Ti is a diffusion-controlled process. Differential pulse voltammetry (DPV) allowed Pd/Ti electrode to obtain a detection limit (LOD) of 5.8 ppb Cr(VI) and a quantitation limit (LOQ) of 19 ppb Cr(VI) in 0.1 mol L−1 H2SO4.