Cheol Gi Kim

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 380u200a°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.

Effect of neutron irradiation on magnetic properties in the low alloy Ni-Mo steel SA508-3

The B-H hysteresis loop and Barkhausen noise have been measured in the neutron irradiated SA508 steel of 45 μm thickness. The coercive force of B-H loop showed a slow change up to a neutron dose of 1014 n/cm2 and increased by 15.4% for a 1016 n/cm2 dose sample compared with that of the unirradiated one, related to the domain wall motion hindered by the increased defects. However, the amplitude of Barkhausen noise reflecting the wall motion decreased slowly up to 1014 n/cm2 irradiation, followed by a rapid decrease of 37.5% at 1016 n/cm2.

Effect of annealing field on asymmetric giant magnetoimpedance profile in Co-based amorphous ribbon

Abstract We prepared weak-field annealed amorphous samples in the open air and present the change of giant magnetoimpedance (GMI) profiles as functions of annealing field, H a . The profile for 0.1xa0MHz measuring frequency shows two typical peaks in the sample annealed in a field of H a H a to 500xa0mOe, GMI peak in the region where the applied field is antiparallel to the annealing field decreases with H a , showing an asymmetric GMI profile. As the annealing field increases over 500xa0mOe, the GMI in the antiparallel field region disappears and a GMI-valve in the parallel field region is revealed due to the exchange coupling of domains in amorphous core with the unidirectional anisotropy field in a crystalline layer. However, in the sample with H a of 15xa0Oe, the GMI peak in the antiparallel region is the observable again.

Permeability and giant magnetoimpedance in Co69Fe4.5X1.5Si10B15 (X=Cr, Mn, Ni) amorphous ribbons

The magnetoimpedance (MI) has been measured in the amorphous ribbons of the soft ferromagnetic alloy Co69Fe4.5X1.5Si10B15 (X=Cr, Mn, Ni) as functions of frequency (fu200a). For all of the three samples, at low frequency, f⩽5u200aMHz, the MI ratio increases with increasing frequency, but the MI ratio decreases at high frequency, f⩾5u200aMHz. The MI profiles are not changed at low frequency regions of f⩽1u200aMHz in the amorphous ribbons. The MI ratio at high frequency of f=5u200aMHz becomes 57% in Co69Fe4.5Cr1.5Si10B15, but the MI ratio becomes 30% in Co69Fe4.5Mn1.5Si10B15 and Co69Fe4.5Ni1.5Si10B15. The MI ratio at f=10u200aMHz becomes 45% in Co69Fe4.5Cr1.5Si10B15 and the MI ratio becomes 23% in Co69Fe4.5Mn1.5Si10B15 and Co69Fe4.5Ni1.5Si10B15, respectively. The maximum values of field sensitivity are 2.7(X=Cr), 2.5(X=Mn), 2.2(X=Ni)%/Oe for f=5u200aMHz.

Asymmetric GMI characteristics in current-biased amorphous (Co0.94Fe0.06)72.5Si12.5B15 wire

Abstract The giant magnetoimpedance (GMI) has been measured in as-quenched (Co0.94Fe0.06)72.5Si12.5B15 amorphous wire as a function of DC-biased current, Ib, along sample axis and measuring frequency f. The GMI profile during half a cycle of magnetization for Ib=0 shows a typical behavior of GMI, that is, single peak for f =0.1 MHz and symmetric two peaks for f ⩾0.5 MHz . Asymmetric GMI profile appears for I b ⩾0.85 mA in the frequency range between 0.1 and 10xa0MHz, due to the suppression of wall motion by the induced circular field. The field sensitivity amounted to 170%/Oe for 0.1xa0MHz with I b =3.4 mA and 150%/Oe for f ⩾0.5 MHz with 12.7xa0mA.

Decomposition of susceptibility spectra in a torsion-stressed Fe-based amorphous wire

The complex susceptibility spectra are measured as functions of the alternating-current field amplitude and the torsion angle in an Fe77.5Si7.5B15 amorphous wire. The susceptibility spectra show dispersion with a relaxation frequency of 40 kHz due to irreversible motion of the inner core domain walls when the driving field is larger than the threshold field of 10 mOe. The spectra for a small driving field can be decomposed into two relaxation dispersions by using the nonlinear curve fitting, one originating from reversible wall motion of the inner core domains and with a relaxation frequency of 0.36 MHz, and the other originating from reversible magnetization rotation in the outer shell domains and with relaxation frequency of 1.82 MHz. The static susceptibilities resulting from the reversible and the irreversible magnetization processes show an asymmetric change with positive and negative torsion angles.

Validity of the Stoner-Wohlfarth model in hysteretic giant magnetoimpedance of annealed amorphous materials

The hysteretic characteristics of giant magnetoimpedance (GMI) profiles have been measured in Co-based amorphous ribbon with various anisotropy angles θk, and they have been analyzed by using the Stoner–Wohlfarth model. A two-peak behavior with a dip near zero field is revealed in the measured GMI profile at 10 MHz, irrespective of θk. The negligible hysteresis of the field for the dip is in close agreement with that calculated assuming a magnetization jump from a metastable to a stable state. However, the hysteretic asymmetries for the increasing and decreasing fields in the samples with the angle range of 20°⩽θk<60° are well described by a divergence in the calculation without a magnetization jump. The asymmetry for the sample with θk⩾60° may also be due to the divergence. But the two peak of the measured profiles, which are due to the anisotropy distribution in the actual materials, are different from the calculated profiles, which have a single peak near zero field. This indicates that the Stoner–Wohl...

Evaluation of anisotropy field in amorphous Fe71+xNb7B22−x alloys by GMI measurement

Abstract We have measured the variation of magnetoimpedance (MI) ratio on Fe 71+ x Nb 7 B 22− x ( x =0, 2, 6, 8) amorphous ribbons for the evaluation of anisotropy field H k . MI at a frequency of 10xa0MHz is related to the transverse permeability from rotational magnetization. MI varies sensitively with the various Fe-contents, reflecting the magnetic softness. The field interval of the MI peaks corresponds to the anisotropy field ( H k ) and the estimated H k decreases with the Fe-contents.