Kil Jae Jang

Asymmetric giant magnetoimpedance in field-annealed Co-based amorphous ribbon

The giant magnetoimpedance (GMI) has been measured in weak-field-annealed Co-based amorphous ribbon as functions of both the annealing field, Ha, and the measuring frequency. The GMI profile measured at 0.1 MHz shows a typical two-peak behavior for the sample with Ha less than 50 mOe. The GMI peak in the region where the applied field is antiparallel to the annealing field decreases with Ha, showing an asymmetry in the GMI profile. Eventually, the GMI peak in the antiparallel-field region disappears, and a drastic step-like change in the GMI peak is revealed for an applied field which is parallel to the annealing field of 500 mOe. The asymmetric GMI phenomenon for the 0.1 MHz measuring frequency, the so-called “GMI valve,” is due to a peculiar domain structure, which occurs in the sample surface during the field annealing. The GMI peak in the parallel-field region appears again for measuring frequencies over 0.5 MHz due to the remarkable contribution of the magnetization rotation to the GMI.

Analysis of asymmetric giant magnetoimpedance in field-annealed Co-based amorphous ribbon

The asymmetric giant magnetoimpedance (GMI) profile has been realized in weak-field-annealed Co-based amorphous ribbon at the annealing temperature of 380 °C in open air. Asymmetric GMI profiles with respect to applied field become profound as the annealing field increases over 500 mOe. The asymmetric GMI profile at the frequencies of ac current over 0.5 MHz is well ascribed for by the rotational transverse magnetization of single domain under a uniaxial anisotropy in amorphous core and a unidirectional anisotropy due to the exchange coupling with the bias field in the crystalline layer, underlying surface oxidation layer developed during the annealing in open air.

Annealing effect on microstructure and asymmetric giant magnetoimpedance in Co-based amorphous ribbon

We investigated the modification of microstructure and the change in GMI profile in a weak field-annealed amorphous ribbon as a function of the annealing time. The FWHM of x-ray diffraction patterns decreased in 8 h annealed samples, suggesting the reduction of the amorphous phase in the surface layer. Auger electron spectroscopy represented the compositional depth profile, where the oxide layer was heavily enriched in B and Si contents, balanced by a depletion of these elements in the underlying materials. The reduction of B and Si content decreases the crystallization temperature of the depletion layer, resulting in the formation of crystal or crystallites during annealing in air. The sharp change of GMI profiles was observed for the annealed samples over 1 h, but there was hysteresis in the GMI profile for increasing and decreasing field for the 1 h annealed sample. As the annealing time increased, the peak in the antiparallel field region to annealing field disappeared. Eventually, the GMI-valve in parallel field region was revealed in a 8 h annealed sample.

Frequency characteristics of permeability in weak-field annealed Co-based amorphous ribbon

The Co-based amorphous ribbons were annealed under the weak-field of a few hundred A/m at a temperature of 380C for 8 h in an air atmosphere. The annealing field Ha was applied along the sample axis during the annealing to develop the bias field in crystalline layer on the sample surface. Two relaxation due to wall motion and rotational magnetization are revealed in low and high frequency region, respectively, for as-quenched and annealed samples in vacuum under zero field. For the annealed samples in air, the low frequency relaxation due to domain wall motion drastically decreases, while the high frequency relaxation due to rotational magnetization remains irrespective of Ha These results indicate that the wall of longitudinal domains are strongly coupled with the bias field, while the rotational magnetization is enhanced in the annealed samples under the field.

Annealing temperature dependence of permeability spectra and asymmetrical giant magnetoimpedance in Co-based amorphous ribbon

The permeability spectra and giant magnetoimpedance (GMI) profiles have been measured as a function of annealing temperature, Ta in field-annealed Co-based amorphous ribbon. The permeability spectra show the superposition of two dispersions of Debye type for Ta⩽200 °C. As Ta increases over 250 °C, the low frequency dispersion begins to decrease, and eventually disappears for Ta⩾350 °C due to the exchange coupling with the bias field formed in surface crystalline layer. However, the high frequency dispersion increases with Ta up to 350 °C and then becomes nearly constant. The GMI profile measured at 0.1 MHz shows a one peak in the samples of Ta⩽300 °C. As Ta increases over 350 °C, the GMI-valve is revealed due to the influence of bias field on wall motion. As the Ta increases over 450 °C, the asymmetric two peaks of GMI profile are shown instead of the GMI valve due to the damping of wall motion after the crystallization.

Analysis of giant magneto-impedance in terms of anisotropy field in annealed amorphous Co66Fe4NiB14Si15 ribbons

Abstract The magneto-impedance (MI) profiles of Co66Fe4NiB14Si15 amorphous ribbons changed sensitively with the annealing temperature, reflecting the change in anisotropy field, which determines the field dependence of transverse permeability. The effective anisotropy field, evaluated from these profiles, were discussed in terms of the increase of magnetic softness and microstructural change due to the annealing.

Effect of annealing field orientation on GMI profiles in Co-based amorphous ribbon

The giant magnetoimpedance (GMI) is studied as function of annealing-field angle /spl theta//sub k/ in Co/sub 66/Fe/sub 4/NiB/sub 14/Si/sub 15/ ribbon with respect to transverse direction. The GMI profile at 10 kHz shows two peaks located at negative and positive external field H. The interval between two peaks, about 2.1 Oe for /spl theta//sub k/=0/spl deg/, slightly increases with /spl theta//sub k/ up to /spl theta//sub k/=30/spl deg/ and then decreases down to 1.0 Oe for /spl theta//sub k/=90/spl deg/. However, two peaks are asymmetric and hysteresis exists during cyclic magnetization. The asymmetry ratio, fractional difference of two peaks, is maximum about 2% for /spl theta//sub k/=10/spl deg/ and its sign is reversed from /spl theta//sub k/=40.

Relaxation of rotational magnetization in annealed Co-based amorphous ribbons

Abstract The permeability spectra for various amplitudes of AC magnetic field h 0 were measured in an annealed Co 66 Fe 4 NiB 14 Si 15 amorphous ribbon. The rotational magnetization was involved only for low-driving amplitude of h 0 ⩽ 0.4 mOe. The DC permeability and relaxation time, obtained from the spectra of rotational relaxation, were discussed in terms of the change of restoring constant by the magnetic softening and nucleated crystalline sites during annealing.

Effect of microstructure on asymmetric giant magneto-impedance Co-based amorphous ribbon

Much work has been done on the giant magneto-impedance (GMI) in soft magnetic Co-based amorphous Wins and ribbons tecause they exhibit a highly m i t i v e MI effect for magnetic sensor applications [l]. Recently, asymmeUic GMI profile has been reported during magnetization in w&-field-anoealed amorphous ribbon, without the elaboration of microshucture [Z]. lo this work we prepare weak-field-anneal+ amorphous samples as a funetion of the annealing time for the micmstnrctural modification and discuss the change in GMI profile in term of microstructure in specimen surface.