S.S. Yoon

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.

Effect of annealing on anisotropy field in Fe84Zr7B8Cu1 amorphous ribbons evaluated by giant magnetoimpedance

The magnetoimpedance (GMI) profiles were measured in Fe84Zr7B8Cu1 amorphous ribbons annealed at various temperatures. The anisotropy field evaluated from GMI profiles changed sensitively with annealing temperature. The decreasing Hk and increasing magnitude of GMI with the annealing temperature were ascribed for by the increase of magnetic softness, while the increasing Hk and decreasing magnitude of GMI for the further increment of annealing temperature was associated with the microstructural change due to the annealing.

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.

Giant magnetoimpedance in composite wires with insulator layer between non-magnetic core and soft magnetic shell

Abstract A method for calculation of the magnetoimpedance in composite wires having an insulator layer between non-magnetic core and soft magnetic shell is described. It is assumed that the magnetic shell has a helical anisotropy and the driving current flows through the core only. The distribution of eddy currents and expressions for the impedance are found by means of a solution of Maxwell equations taking into account the magnetization dynamics within the shell governed by the Landau–Lifshitz equation. The effect of the insulator layer on the magnetoimpedance is analyzed.

Decomposition of magnetization processes in complex permeability spectra of 3% SiFe

Complex permeability spectra were measured as a function of the ac field amplitude h0 and the cut angle α with respect to the [001] direction in a rectangular 3% SiFe sample. The spectra for α=0° at a relatively low field amplitude showed Debye-type dispersion with a relaxation frequency of 26.3 kHz. However, at a relatively high field amplitude, the spectra changed to a superposition of two dispersions, one with a relaxation frequency at 3.3 kHz and the other with a relaxation frequency at 26.3 kHz. Irreversible and reversible 180° domain wall motions were considered as mechanisms responsible for the h0 dependent relaxation at 3.3 kHz and the h0 independent relaxation at 26.3 kHz, respectively. From the α dependence of the spectra, we obtained another relaxation of the magnetization rotation, with a relaxation frequency of 0.16 MHz, in the original 180° domains for α⩾70°.

Permeability and magnetoimpedance in Fe77.5Si7.5B15 wire under torsional stress

The complex permeability spectra and the magnetoimpedance (MI) are measured as a function of torsional stress and torsion angle in Fe77.5Si7.5B15 amorphous wire. The permeability spectra show irreversible and reversible relaxation with relaxation frequencies of 40 and 300 kHz, respectively. The static permeability resulted from the irreversible magnetization decreases with torsion angle for a negative direction, but increases with a positive angle. The MI profile shows one symmetric peak for the positive torsion angle, which is same MI behavior as in the case of zero torsion. However, there is a hysteresis in the MI profile for negative torsion, that is, there are two asymmetric peaks for decreasing field and one peak for increasing field.

Off-diagonal magnetoimpedance in field-annealed Co-based amorphous ribbons

The off-diagonal magnetoimpedance in field-annealed CoFeSiB amorphous ribbons was measured in the low-frequency range using a pick-up coil wound around the sample. The asymmetric two-peak behavior of the field dependence of the off-diagonal impedance was observed. The asymmetry is attributed to the formation of a hard magnetic crystalline phase at the ribbon surface. The experimental results are interpreted in terms of the surface impedance tensor. It is assumed that the ribbon consists of an inner amorphous region and surface crystalline layers. The coupling between the crystalline and amorphous phases is described through an effective bias field. A qualitative agreement between the calculated dependences and experimental data is demonstrated. The results obtained may be useful for development of weak magnetic-field sensors.

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.

Depth profile of transverse permeability spectrum in an annealed Co-based amorphous ribbon

In this article, we investigate the depth profile of transverse permeability spectra extracted from MI spectra in annealed ribbons under weak magnetic field in order to clarify the effect of crystalline layer on the magnetisation process.

Asymmetric giant magnetoimpedance curve due to antiferromagnetic coupling

The asymmetric giant magnetoimpedance curves have been measured under the rotation of magnetic field on the weak-field annealed Co-based amorphous ribbons to analyze the characteristics of bias field. The bias field from the hard magnetic phases is expected to be of an order of 100 Oe along the annealing field direction. The effective field of soft magnetic phase in the field region parallel to the bias field is less than that antiparallel to the bias field, indicating that an antiferromagnetic exchange coupling exists between the magnetization of the soft phase and the bias field of the hard magnetic phases.