Dc Joule Heating Research Articles

Nanometric crystallization of amorphous ribbons by DC Joule heating

We report the results of studies on the development of nanocrystalline phase by DC Joule heating. Structural and magnetic properties, after various annealing conditions, are reported. We show that DC Joule heating is an effective technique for control of nanometric crystallization and it is also compared with respect to conventional heat treatment and laser annealing. In Fe 73.5 Nb 3 Cu 1 Si 13.5 B 9 , the average grain size of Joule heated samples is significantly lower than in the furnace and laser annealed alloy.

Nanometric crystallization of amorphous ribbons by DC Joule heating

Abstract We report the results of studies on the development of nanocrystalline phase by DC Joule heating. Structural and magnetic properties, after various annealing conditions, are reported. We show that DC Joule heating is an effective technique for control of nanometric crystallization and it is also compared with respect to conventional heat treatment and laser annealing. In Fe 73.5 Nb 3 Cu 1 Si 13.5 B 9 , the average grain size of Joule heated samples is significantly lower than in the furnace and laser annealed alloy.

Effect of the amorphous-to-nanocrystalline transformation on the high-frequency magneto-impedance in Fe63.5Cr10Cu1Nb3Si13.5B9 melt-spun ribbons

The evolution from the amorphous to the nanocrystalline state has been studied on Fe 63.5 Cr 10 Cu 1 Nb 3 Si 13.5 B 9 ribbons by means of high-frequency (30 kHz – 6 GHz) magneto-impedance measurements. Furnace annealing and DC Joule-heating have been applied and have been shown to induce similar effects to the samples, although current annealed ones display enhanced magneto-impedance properties.

Influence of stress-annealing on magneto-transport properties in Co-based amorphous ribbons

Abstract In this work, the influence of both conventional annealing (performed with or without stress) and dc Joule-heating on magneto-transport properties of Co67Fe4Mo1.5Si16.5B11 amorphous ribbons has been studied in the low (up to 10 MHz) and high-frequency region (15 MHz to 6 GHz). The giant magnetoimpedance (GMI) response has been shown to be enhanced in the lower limit of the studied frequency range. All the studied samples displayed a maximum in the impedance behaviour as a function of applied magnetic field usually related to the magnetic anisotropy field. Magneto-transport measurements performed at high frequency confirm the behaviour of displaying a higher GMI response at low-frequency values for all the studied samples.

Spectral range of the ac Josephson effect in [001]-tilt YBa/sub 2/Cu/sub 3/O/sub 7-x/ bicrystal junctions

We have studied the ac Josephson effect in YBa/sub 2/Cu/sub 3/O/sub 7-x/ bicrystal junction by monitoring the voltage dependence of the dc current response /spl Delta/I(V) induced by low-power monochromatic radiation with the frequencies f/sub i/. The intensity of odd-symmetric resonances in the function /spl Delta/I(V) near the voltages V/sub i/=hf/sub i//2e is proportional to the amplitude of Josephson oscillations at the frequency f/sub i/. The resonance intensities have been mapped as a function of the Josephson frequency f/sub i/ in the range from 5 GHz to 5 THz, the junction resistance R/sub n/ in the range from 0.4 to 80 Ohm and junction characteristic voltages I/sub c/R/sub n/ in the range from 5 /spl mu/V to 1.8 mV. The central frequency of the range was scaled with the I/sub c/R/sub n/-product of the junction and it could be shifted from GHz-range to the THz-range by a temperature decrease. The spectral range was limited by an enhanced noise broadening of the Josephson linewidth at low frequencies and by increased dc Joule heating at high frequencies.

Crystallization study during DC joule heating in amorphous ribbons

A study of the temperature dispersion during current annealing of amorphous ribbons was done. The local temperature was obtained by measuring the local infrared emission during the thermal treatment. The results exhibit a nonuniform macroscopic distribution of temperatures during the current annealing. In this work, we have developed a theoretical model of the crystallization in the sample during the annealing that confirms the results obtained.

Induced anisotropy and magneto-impedance measurements in Fe 73.5Nb 3Cu 1Si 13.5B 9 nanocrystalline alloys

The effect of induced magnetic anisotropy on GMI has been studied in Fe 73.5Nb 3Cu 1Si 13.5B 9 melt-spun amorphous and nanocrystalline ribbons. The amorphous-to-nanocrystalline transformation has been induced by means of furnace annealing and DC joule heating. The correlation between average magnetic anisotropy calculated from quasi-static hysteresis loops and GMI will be discussed in detail.

Magnetic permeability relaxation in amorphous Fe 62.5Co 6Ni 7.5Zr 6Cu 1Nb 2B 15

The high-frequency initial magnetic permeability and its relaxation have been studied in Fe 62.5Co 6Ni 7.5Zr 6Cu 1Nb 2B 15 after heat treatment (both in furnace or via DC Joule heating). The magnetic permeability steadily increases in correspondence of both structural relaxation and nanocrystallisation of the alloy. The intensity of the permeability relaxation (measured between fixed times after demagnetisation) is reduced by structural relaxation but exhibits a sharp maximum immediately before the onset of nanocrystallisation. The nanocrystalline alloy is characterised by a nonzero, if reduced, permeability relaxation.

Crystallization of the Fe-Cu-M-Si-B (M=Nb, V) amorphous alloys by direct-current Joule heating

The effects of dc Joule heating on structural, electrical and magnetic properties of Fe-Cu-M-Si-B (M=Nb, V) amorphous alloys have been investigated. Amorphous ribbons of these two systems were resistively heated by direct currents from 1.5 to 2.5 A. X-ray diffraction analysis has shown that changes in the relative electrical resistance correlate with the onset of the crystallization process. The F shape of hysteresis loops indicates the induction of transverse magnetic anisotropy as a result of applied treatments.

Nanocrystallization-controlled magnetoresistance of amorphous ribbon Fe 86Zr 7Cu 1B 6

Magnetoresistance (MR) of amorphous ribbons of Fe 86Zr 7Cu 1B 6 have been investigated on the nanocrystallization by direct DC Joule heating. 3-D road map of these samples was illustrated by the crystal sizes, coercivities and magnetizations, started from as-cast to the total annealing. The largest negative MR of 0.5% was found for this system at T = 10 K.

Giant magnetoimpedance effect in melt-spun Co-based amorphous ribbons and wires with induced magnetic anisotropy

GMI in Co 68.25Fe 4.5Si 12.25B 15 melt-spun amorphous wires and ribbons has been studied. Selected samples have been submitted to DC Joule-heating to induce circular and transverse magnetic anisotropy. Hysteresis loops have been measured using a fluxmetric technique. The results were interpreted in terms of circumferential and transverse permeability correlated with the magnetic domain structure.

Nanocrystalline magnetic materials obtained by flash annealing

The aim of the present work was to produce enhanced-remanence nanocrystalline magnetic material by crystallizing amorphous or partially amorphous Pr4.5Fe77B18.5 alloys by the flash annealing process, also known as the dc-Joule heating process, and to determine the optimal conditions for obtaining good magnetic coupling between the magnetic phases present in this material. Ribbons of Pr4.5Fe77B18.5 were produced by melt spinning and then annealed for 10-30 s at temperatures 500 - 640 °C by passing current through the sample to develop the enhanced-remanence nanocrystalline magnetic material. These materials were studied by X-ray diffraction, differential thermal analysis and magnetic measurements. Coercivity increases of up to 15% were systematically observed in relation to furnace-annealed material. Two different samples were carefully examined: (i) a sample annealed at 600 °C which showed the highest coercive field Hc and remanence ratio Mr/Ms and (ii) a sample annealed at 520 °C which showed phase separation in the second quadrant demagnetization curve. Our results are in agreement with other studies which show that flash annealing improves the magnetic properties of some amorphous ferromagnetic ribbons.

Study of Magnetic Properties of Joule Heated Granular Co<sub>x</sub>Cu<sub>100-x</sub> Ribbons

Granular Co-Cu alloy ( Co10Cu90) produced by melt-spinning and subsequently annealed in vacuum by DC Joule heating at different current densities have been investigated by DC and AC susceptibility measurements. High values of the heating current density lend to promote grain growth and to favore the increase of intergrain interactions strength, determining a variety of magnetic propeties from a collective fi freezing in random directions of moments of nanosized Co particles to an inhomogeneous and pl progressive blocking of moments of particles with increasing size. Meanwhile, significant changes of the giant magnetoresistance exhibited by the system were also observed. The correlation between microstructure, magnetic properties and GMR is discussed.

Influence of Nanocrystallization on the High-Frequency Magnetoimpedance in Ultrathin Fe83Cu1Nb7B9 Melt-Spun Ribbons.

ABSTRACTUltrathin ribbons of amorphous Fe83Cu1Nb7B9 obtained by planar flow casting, have been submitted to dc Joule heating in vacuum to develop a nanocrystalline phase. Variations in the initial magnetic permeability as high as 1000 times the as-quenched value have been measured in a wide range of nanocrystalline samples. The magnetoimpedance behavior in the microwave region has been studied as a function of the nanocrystalline state. In this work the intensity of the impedance variation has been related to the samples degree of softness.

Description of magnetic hardening in nanocrystalline FeZrBCu alloy using the Preisach model of hysteresis

In this work we studied temperature dependent magnetization processes in nanocrystalline Fe 86 Zr 7 B 6 Cu 1 . Ribbons produced by melt-spinning in air, were submitted to fast dc Joule heating in vacuum to induce a nanocrystalline α-Fe phase. In order to investigate the untrivial interplay between structural and magnetic properties, hysteresis curves were measured as a function of temperature, with particular attention to the magnetic hardening effect.Details of the magnetization process were analyzed by using the Preisach model of hysteresis.

Magnetic and structural properties of nanocrystalline ultrathin Fe83Cu1Nb7B9 submitted to Joule heating

Ultrathin (- 10 μm) ribbons of the amorphous alloy Fe 83 Cu 1 Nb 7 B 9 obtained by single roll melt spinning have been submitted to dc Joule heating in vacuo in order to develop a nanocrystalline phase characterised by a very reduced grain size. The initial permeability of the nanocrystallized alloy was measured on open samples by means of a conventional fluxmetric technique with air-flux compensation. The interval of magnetising frequency was 100 Hz - 10 kHz. All samples were submitted to X-ray diffraction to evaluate the mean grain size, composition and strain level of the nanocrystalline phase. A substantial improvement in the magnetic permeability of Joule-heated nanocrystalline samples with respect to the conventionally annealed ones has been always found. Variations as high as 1000 times the as-quenched value have been observed for a wide range of electrical-current densities (4 10 7 ≤ j < 6 10 7 A/m 2 ).

Influence of the distribution of magnetic moments on the magnetization and magnetoresistance in granular alloys

In granular solids, the magnetoresistance is directly related to the macroscopic magnetization, but this relationship is extremelly complex due to the distribution of grain sizes and the intergranular magnetic interactions. The dependence of the magnetoresistance on the magnetization is here investigated by means of a theoretical model that is developed taking explicitly into account the magnetic moment distribution and the spin-dependent electron-impurity scattering within magnetic grains and at the interface between the grains and the metallic matrix. Using this model, one can explain large experimental deviations from the parabolic behavior of the magnetoresistance vs magnetization curves that are typically expected for equal noninteracting superparamagnetic grains. The expressions for the magnetization and magnetoresistance, obtained for general distribution funtions, are tested considering a log-normal-type distribution function by fitting on data obtained from melt-spun Cu${}_{90}$Co${}_{10}$ ribbons after annealing by dc Joule heating. The experimental data are well traced using just three parameters that determine the particle size distribution, the particle density, and the ratio of the scattering cross section at the boundaries of the grains to the scattering cross section within the grains.

Giant magnetoresistance in magnetic granular Co 15Cu 85 alloys annealed by direct-current Joule heating

We have studied the structure and giant magnetoresistance (GMR) in the Co 15Cu 85 granular alloys prepared by melt-spinning and annealed by the direct-current (dc) Joule heating. At T = 10 K, GMR as large as 50.0% (in a magnetic field up to 30 kOe) has been observed in a sample annealed with I = 6 A. The Joule-heated samples show higher GMR than those annealed by the conventional method. It is attributed to less content of residual Co in the Cu matrix for the dc Joule-heated samples than that for the samples annealed in a furnace.

A model of the DC Joule heating in amorphous wires

en

Giant magnetoresistance in melt-spun granular Cu/sub 100-x/Co/sub x/ alloys with correlated magnetic moments

A study of giant magnetoresistance (GMR) has been performed on melt-spun ribbons of Cu/sub 100-x/Co/sub x/ (5/spl les/x/spl les/20) subjected to DC Joule heating in order to optimise the GMR response. The GMR results, plotted as functions of the reduced magnetisation, exhibit significant deviations from the parabolic behavior predicted in the case of no correlation among magnetic moments. A simple theory of the GMR in Cu-Co granular systems, taking explicitly into account the correlation existing among the magnetic particles, is highlighted. The effect of both Co content and degree of magnetic correlation on the maximum GMR values attained for each composition is briefly discussed.