Effect of magnetic field–current annealing on the magnetoimpedance of Co-based ribbons

The roles of applied magnetic field during the current annealing of Co_68.15Fe_4.35Si_12.5B₁₅ soft magnetic amorphous ribbons have been studied. The Joule heating process causes a complex domain structure which is related to the circular field generated by the current. We have shown that magnetic field–current annealing modifies the transverse permeability and under suitable conditions, enhances the GMI response.     

Temperature dependence of biased hysteresis loops in hard-soft nanocrystalline Co-based ribbons

The low field hysteresis loops of hard-soft magnetic materials can be biased as a consequence of the magnetic coupling of both phases. In hard-soft nanocrystalline materials, magnetostatic and exchange interactions between the hard crystalline phase and the much softer embedding amorphous matrix coexist and the relevance of their effect on the hysteresis is difficult to distinguish. In this work, the thermal evolution of the biased magnetic hysteresis loops of hardsoft devitrified Co₆₆Fe₄Mo₂Si₁₆B₁₂ ribbons has been studied between room and Curie temperature. The nature of the coupling between the two magnetic phases is discussed in terms of the influence of the temperature on the magnetic properties of the soft phase, mainly on its saturation magnetization and exchange constant, and on the switching field distribution of the hard phase. The results allow to conclude that the effect of the exchange interaction predominates over the magnetostatic one.     

Microwave power absorption analysis in the devitrification process of Co-based amorphous ribbons

Melt-spun Co₆₆Fe₄B₁₂Si₁₃Nb₄Cu soft magnetic ribbons were devitrified at low annealing temperatures (623 K), for times 5–20 min. Microwave power absorption measurements at 9.4 GHz (X-band) were carried out in two geometries. In geometry 1, the ribbon’s plane was oriented parallel to AC magnetic field. For the orientation 2, the ribbon’s plane was normal to the AC magnetic field. In both cases, the ribbon’s axis was parallel to the DC magnetic field. For both orientations, two absorptions were observed: the first corresponds to a low field microwave absorption (LFA) centered in zero dc magnetic field, and a higher field absorption corresponding to the ferromagnetic resonance (FMR). In the geometry 1, a single FMR spectrum was observed for all the samples, with a shift in resonant field as annealing increased. For geometry 2, evidence of the superposition of two FMR signals was observed. FMR spectra are therefore due to a combination of two different magnetic phases corresponding to the amorphous matrix and nanocrystallites. Deconvolution calculations were carried out on FMR spectra to separate the contributions. Their behavior as a function of annealing time was in good agreement with the magnetic softening, also obtained with LFA results. The differences in microwave absorption, for both geometries, can be explained by differences in the electromagnetic wave propagation volume.     

Fabrication and magnetic properties of Cu50(Fe69Si10B16C5)50 thin microwires

In this work, we study the granular samples of mixed Cu₅₀(Fe₆₉Si₁₀B₁₆C₅)₅₀ composition where half of alloy composition is commonly used amorphous soft magnetic material and the other half-Cu. Glass covered Cu₅₀(Fe₆₉Si₁₀B₁₆C₅)₅₀ microwires were produced and their magnetic properties were studied. The evolution of the structure after the annealing was observed using X-ray diffraction with Cu K_α radiation. The as-prepared Cu₅₀(Fe₆₉Si₁₀B₁₆C₅)₅₀ microwires present a relatively low coercivity of about 5 Oe and exhibit non-regular hysteresis loop typical behavior for two-phases systems. Annealing resulted in magnetic hardening of the samples with coercivity of about 50 Oe. The variation of the coercivity and remanent magnetization with the temperature at 5–300 K were obtained from those curves. Temperature dependence of magnetization at 5–300 K exhibits significant difference between field-cooled and zero-field cooled behaviour. Observed dependences interpreted in terms of two-phase structure of as-prepared samples and evolution of the structure under annealing.     

Magnetic behavior and microstructure of Finemet-type ribbons in both,surface and bulk

Different kinds of magnetic anisotropies have been induced during the nanocrystallization process of Co- and Ni-rich amorphous ferromagnetic (Finemet) ribbons using diverse procedures like the application of a constant stress or an axial magnetic field during the annealing process. Magnetization measurements have evidenced the anisotropy of the treated samples. The main goal of this work has been the structural and microstructural analysis of the treated ribbons using X-ray Diffraction (XRD) and Atomic Force Microscopy (AFM), detecting substantial differences in the crystallization state and grain size of the samples depending on the treatment that was carried out. Moreover, AFM measurements revealed in all the treated samples a strong nanocrystallization of the surface without evidences of amorphous matrix, which contrast with XRD measurements that have shown a high content of amorphous phase in the bulk of the ribbons. Magneto-optical Kerr effect measurements have been performed with the aim to elucidate the complex magnetic behavior that is expected for the surface of the ribbons, measuring surface hysteresis loops that show much higher coercive field values than in the bulk.     

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.     

Thermal embrittlement of Fe-based amorphous ribbons

Thermal embrittlement of amorphous Fe_78?xNi_xSi₁₀B₁₂ and Co₇₈Si₁₀B₁₂ alloys has been studied. The majority of Fe-based amorphous alloys became brittle after annealing to temperatures significantly lower than their crystallization temperatures, whereas the non-Fe-based amorphous alloys remained ductile after annealing up to their crystallization temperatures. The present results emphasize on the key difference between the embrittlement behavior of Fe-based and non-Fe-based amorphous alloys. We point out a possible correlation between different phenomena like Invar effect, compositional inhomogenity, and residual stress playing a critical role in the thermal embrittlement of Fe-based amorphous alloys.     

X-Ray diffraction study of the evolution of phase and structural state and macroscopic stress during annealing of soft magnetic Fe95 − x Zr5N x films prepared by ion-plasma deposition

The evolution of the phase composition, nanostructure parameters, and macroscopic stress in soft magnetic Fe_{95 ? x} Zr₅N _x films (prepared by ion-plasma deposition onto quartz substrates) during their annealing has been investigated by X-ray diffraction. During deposition, depending on the N content, either a mixed structure composed of an X-ray amorphous phase enriched in Zr and N and a crystalline phase (α-Fe(N) solid solution) or an X-ray amorphous phase enriched in Fe, Zr, and N is formed in the films. During annealing, depending on the temperature and nitrogen content, different combinations of crystalline phases (α-Fe(N) and Zr(N) solid solutions, α-Fe, Fe₄N, Fe₂N, ZrO₂) are formed in the films. The large compressive stress formed in the films during deposition changes to a lower tensile stress during annealing.     

Structural,compositional and annealing effects on magnetic properties in R1−xCox(R = Er,Tb, Sm) amorphous thin film alloys

Amorphous alloys of the rare earths and transition metals series possess unique magnetic properties which are highly dependent on the alloy compositions and the microstructure. The magnetic properties have been investigated in various sputtered R_1−xCo_x amorphous thin film alloys presenting highly anisotropic magnetic systems. Samples have been annealed for different temperatures. The magnetic changes, at various composition and various temperatures, have been measured by vibrating sample magnetometer (VSM); the structural and compositional properties have been investigated utilizing Rutherford back scattering (RBS) and scanning electron microscopy (SEM) analysis. The most significant results are: (1) Curie temperatures T_c above room temperature and coercivity H_c are strongly dependent on the alloys composition; moreover, the shapes of hysteresis loops are affected and (2) field annealing (before temperature of crystallisation) makes the samples magnetically softer (reducing H_c) and reduce the internal stress induced during the deposition by inducing a well defined anisotropy.     

Structure and thermomagnetic properties of powders produced from melt spun FeNbBCu ribbons

Amorphous ribbons of Fe₇₇Nb₇B₁₅Cu₁ prepared by melt-spinning and powders produced from them by ball-milling were characterized by means of calorimetry, X-ray diffraction, scanning electron microscopy and vibrating sample magnetometry. Upon thermal treatment the amorphous alloy experiences a primary crystallization that leads to bcc-Fe nanocrystals dispersed in an amorphous matrix. Magnetic measurements indicate that this alloy in the amorphous and nanocrystalline state is a good soft magnetic material. Values of saturation magnetization and coercivity are 120 Am²/kg and 5 A/m respectively, for the alloy in the nanocrystalline state. Pre-annealing, post-relaxation and nanocrystallization as well as various milling parameters were explored and the structural and magnetic changes induced have been studied. The analysis of the particle size distribution and morphology of the powders show that the brittleness resulting from pre-annealing of the ribbons is very effective in reducing the particle’s size. Recovery of the high coercitive field induced by milling is achieved by post-annealing to an extent that depends mostly on the milling conditions.     

Magnetostatic properties of amorphous and nanostructured Fe73.5Si13.5B9Cu1Nb3 wires

We present the results of experimental investigations of magnetostatic properties of Fe-based amorphous wires with diameter about 100 μm. The samples were annealed at different conditions. The as-quenched sample had a weak increase (about 5%) of the magnetization with magnetic field decrease – the negative differential magnetic permeability could be observed in its hysteresis loops. The coercive force was about 0.2 Oe. The conventional annealing decreased the value of the saturation field down to 5 Oe without change of the coercive force value. When the annealing was accompanied with torsion stress the coercive force became less than 50 mOe. After the current annealing any peculiarities disappeared in magnetization in a weak magnetic field.     

Magnetic properties of Fe-Co-Nd-Y-B magnet prepared by suction casting

The magnetic properties of Fe_67−xCo₁₀Nd₃Y_xB₂₀ (x = 0, 2, 6, 10) alloys prepared by suction casting had been investigated. It was found that a small addition of Y (6%) was very effective in improving glass-forming ability (GFA) of Fe-Co-Nd-Y-B alloys. The as-cast Fe₆₁Co₁₀Nd₃Y₆B₂₀ alloy presented soft magnetic behavior. However, the alloy showed the hard magnetic behavior due to exchange coupling between soft and hard magnetic phases after annealing at 948 K for 30 min. The results provide a promising approach for the production of bulk magnets by the simple process of copper mold casting and subsequent heat treatment.     

Synthesis and magnetic properties of the SiO2–BaFe12O19 glass–ceramic composites

Samples of the magnetic glass–ceramics SiO₂–BaFe₁₂O₁₉ were obtained from mixtures of Fe₂O₃–BaSO₄–SiO₂–Na₂CO₃–CaCO₃ following a two-step route. The samples were characterized using XRD analysis and scanning electron microscopy, and their room temperature magnetization hysteresis loops were measured. It was found that the magnetic phase was completely developed in the precursor BaFe₁₂O₁₉ material when it was calcinated above 1000 °C. The size of the particles of the BaFe₁₂O₁₉ phase depends on the thermal treatment conditions and on the SiO₂ concentration. The magnetic data showed that the high value of the coercive magnetic field is preserved despite the fact that both the remanent and the saturation magnetizations diminish substantially.     

Asymmetric magnetization reversal of partially devitrified Co66Si15B14Fe4Ni1

Anomalous hysteresis has been observed in annealed Co₆₆Si₁₅B₁₄Fe₄Ni₁ amorphous ribbons consisting of asymmetrically distorted and horizontally shifted loops. Although the magnetic hysteresis loops present remarkable similarities with those produced by exchange bias uniaxial anisotropy, this is discarded in these samples, so the comprehension of the origin of these characteristics is an interesting topic of research. Transmission electron microscopy, selected area diffraction and X-ray diffraction experiments have been carried out in order to investigate the structural properties of the samples. A very low dilution of nanocrystallites, with sizes of about 20 nm, has been detected coexisting with some bigger crystals of around 0.4–1.0 μm. The hysteretic particularities and their evolution with different magnetic treatments are then analyzed as a consequence of the dipolar interaction between the crystalline particles and the residual amorphous matrix.     

Hall resistivity of a magnetic amorphous alloy — Effects of thermal cycling and annealing

We have studied the effects of thermal cycling and annealing on the Hall resistivity ?_H and electrical resistivity ? of an amorphous magnetic material, Metglas 2826A. Thermal cycling up to 600 K has no significant effect on ? but changes ?_H, especially at T > 300 K. Annealing at higher temperatures T_a affects ?_H more strongly that it influences ?. For T_a = 650 K the Curie temperature jumps by ～300 K and this, combined with X-ray data, is regarded as being symptomatic of the onset of crystallization. It is demonstrated that, for a fixed T, the variation of the extraordinary Hall coefficient R₁ is simply correlated with that of ? for both the amorphous material and the annealed (crystallized) specimens. Furthermore, for a given T_a, the variations of R₁ and ? as functions of temperature exhibit the same formal relationship. The temperature dependence of ? in the annealed specimens is rendered plausible in terms of recent theories of disordered materials. Room-temperature studies of electron spin resonance and the field dependence of the magnetization are also reported.     

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.     

Magnetic properties of field annealed Ni-rich microwires

We report on the temperature evolution of the magnetic behavior of Ni-riched microwires obtained by the well-known Taylor–Ulitovsky method, with nominal compositions of Ni₈₀Si₆B₁₄ and Ni₇₈Si₈B₁₄, respectively. We study the Differential Scanning Calorimetry curves in the temperature range between RT to 600 °C, and the thermomagnetic evolution up to 800 °C at the two applied magnetic field values of 0.1 and 0.5 T for both, glass-coated microwires and uncovered microwires with the coating removed by selective etching in HF solution. We also correlate our measurements with magnetic domain pattern obtained by means of Bitter technique and RT VSM axial hysteresis loops. The evolutions of the magnetic and hysteresis properties for the as-quenched and annealed microwires are compared as a function of their thermo-magnetic history.     

Effects on the structural and magnetic properties of amorphous ribbons of (Co0.94Fe0.06)72.5Si12.5B15 caused by 4 MeV Cl2+ ion irradiation

The use of energetic ion irradiation to modify magnetic materials has attracted increasing interest in recent years. The possibility of patterning surfaces on these materials offers a wide range of potential applications particularly in technologies related to magnetic storage media, sensing devices and electromagnetic shielding materials. In this work, ultrasoft non-magnetostrictive (Co_0.94Fe_0.06)72.5Si_12.5B₁₅ amorphous ribbons, 50 μm thick and 0.85 mm wide, fabricated by the chilly block melt spinning technique are irradiated, in their amorphous state, by 4 MeV Cl²⁺ ions with a fluence of 5 × 10¹³ cm⁻². The hysteresis properties of both irradiated and non-irradiated samples are characterized by means of a vibrating sample magnetometer while surface magnetic domain structure is observed by Bitter technique. The presence of an induced magnetic anisotropy in irradiated samples is ascribed to the local damage, caused by ion irradiation treatment, which results in modified coercive field and permeability of the samples. X-ray diffraction results are presented to confirm the amorphicity of the structure even after irradiation with ions.     

Magnetostatic bias in soft/hard bi-phase layered materials based on amorphous ribbons and microwires

Bi-phase soft/hard magnetic systems have been prepared by electroplating CoNi alloy with relative hard magnetic character (10² Oe coercivity) onto ultrasoft (coercivity less than 1 Oe) FeCo-based amorphous alloys prepared by rapid solidification techniques. Experiments have been performed in planar (ribbons) and cylindrical (microwires) configuration. Low-field hysteresis loops after premagnetizing to saturation are ascribed to the magnetization reversal process of the soft inner phase, while harder component remains close to its remanence. Such low-field loops exhibit a shift typical of magnetic bias phenomena which in this case is ascribed to the magnetostatic coupling originated through the stray fields created by uncompensated charges at the ends of the harder phase.     

Co-based magnetic microwire and field-tunable multifunctional macro-composites

Structural, magnetic and mechanical properties of Co-based magnetic microwires and their composites had been investigated. It was found that annealing amorphous microwires at 600 °C caused a drastic variation in the amorphous structure due to crystallization and consequently degraded the soft magnetic properties of the microwires. The tensile tests on the single microwires of different size with and without glass-coated layer revealed a coherent correlation between the mechanical properties and the wire geometry. When compared with single magnetic microwires, the magnetic and magneto-impedance properties of composites were much improved. The strong field dependence of the effective permittivity and transmission/reflection parameters in the Gigahertz range of the composites containing short wires or arrays of continuous wires indicated that these new composites are promising candidate materials for a variety of self-sensing applications.