Seok Soo Yoon

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.

Magnetic Sensor System Using Asymmetric Giant Magnetoimpedance Head

Recently sensitive micro-magnetic sensors are strongly required in various technologies such as BT and IT. This paper gives the comprehensive analysis of the Asymmetric Giant MagnetoImpedance (AGMI) sensors performance with negative feedback. Asymmetrical behavior of the GMI is required for linear magnetic field sensors as the sensitivity and linearity for magnetic field are the most important parameters in the practical application of GMI to magnetic sensors, and this has been realized by magnetic field annealing in amorphous ribbon. A novel AGMI sensor was developed and the performance of the sensor was carefully studied with and without applying negative feedback. The sensor uses 10 mm times 1 mm times 20 mum Co66Fe4Si15B15 ribbon as a sensing element. On applying the negative feedback the sensor shows excellent linearity free from hysteresis, independent of temperature variations. The sensitivity AGMI sensor was found to be 0.27 V/Oe in dynamic range of -2 ~ 2 Oe.

Nanowires array modified electrode for enhanced electrochemical detection of nucleic acid

The gold nanowires array electrode (AuNWsA) was synthesized by two step electrodeposition, which provided well oriented vertically aligned nanowires. The dimensions of the nanowires were determined by scanning electron micrograph and found to be around 1.5 μm in length with 200 nm diameter. Each nanowire was separated by a distance of 2-3 times the diameter of the nanowire itself. The electrochemical performance of the AuNWsA electrode was evaluated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Using these analytical tools, this AuNWsA electrode was shown to have a high effective surface area and excellent electron transfer surfaces compared with flat bare Au electrode. The AuNWsA electrode was then used as an electrochemical biosensor electrode by immobilizing probe DNA and analyzed by CV, EIS and Fourier transform infrared measurements. The results of this analysis suggested that the AuNWsA electrode provides good surfaces for the immobilization and hybridization of DNA. The selectivity of the probe DNA immobilized AuNWsA electrode was tested using non-complementary and one base pair mismatching DNA. The detection limit of the AuNWsA electrode was determined to be 6.78×10(-9) M, which is two times smaller than the bare Au electrode.

Electrochemical biosensor for Mycobacterium tuberculosis DNA detection based on gold nanotubes array electrode platform

The template assisted electrochemical deposition technique was used for the synthesis of gold nanotubes array (AuNTsA). The morphological structure of the synthesized AuNTsA was observed by scanning electron microscopy and found that the individual nanotubes are around 1.5 μm in length with a diameter of 200 nm. Nanotubes are vertically aligned to the Au thick film, which is formed during the synthesis process of nanotubes. The electrochemical performance of the AuNTsA was compared with the bare Au electrode and found that AuNTsA has better electron transfer surface than bare Au electrode which is due to the high surface area. Hence, the AuNTsA was used as an electrode for the fabrication of DNA hybridization biosensor for detection of Mycobacterium Tuberculosis DNA. The DNA hybridization biosensor constructed by AuNTsA electrode was characterized by cyclic voltammetry technique with Fe(CN)6(3-/4-) as an electrochemical redox indicator. The selectivity of the fabricated biosensor was illustrated by hybridization with complementary DNA and non-complementary DNA with probe DNA immobilized AuNTsA electrode using methylene blue as a hybridization indicator. The developed electrochemical DNA biosensor shows good linear range of complementary DNA concentration from 0.01 ng/μL to 100 ng/μL with high detection limit.

The role of exchange coupling on the giant magnetoimpedance of annealed amorphous materials

The characteristics of giant magnetoimpedance (GMI) profiles were measured in annealed amorphous Co66Fe4B15Si15 ribbons in the open air as functions of annealing time, temperature and field. The GMI profile measured at 0.1 MHz exhibits a drastic step-like change, the so-called ‘‘GMI-valve’’ in samples produced with the optimum annealing parameters: time taX8 h, temperature TaE3801C and in the weak-field range, 0: 5O epHap 3O e: The GMI-valve is related to an exchange coupling of a bias-field with the magnetization of the soft amorphous phase, where the bias-field is caused by HCP-Co, FCC-Co and/or Co2Si crystalline phases in a B and Si depleted layer. r 2002 Elsevier Science B.V. All rights reserved.

Separation of reversible domain-wall motion and magnetization rotation components in susceptibility spectra of amorphous magnetic materials

The reversible susceptibility spectra are measured for rectangular Co66Fe4NiB14Si15 samples with various easy-axis angles, α, relative to the sample axis. A phenomenological method is proposed for the reversible spectra to separate the relaxation processes of domain-wall motion and magnetization rotation. The separation provides a method for measuring the static susceptibilities and the relaxation frequencies for the two reversible magnetization processes. The α and the longitudinal stress dependence show that the separated spectra with relaxation frequencies near 360 kHz and 1.6 MHz correspond to relaxations of domain-wall motion and to magnetization rotation, respectively.

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.

Prototype Milli Gauss Meter Using Giant Magnetoimpedance Effect in Self Biased Amorphous Ribbon

In our present work, we developed a GMI (giant magnetoimpedance) sensor system to detect magnetic fields in the milli gauss range based on the asymmetric magnetoimpedance (AGMI) effect in Co-based amorphous ribbon with self bias field produced by field-annealing in open air. The system comprises magnetoimpedance sensor probe, signal conditioning circuits, A/D converter, USB controller, notebook computer, and program for measurement and display. Sensor probe was constructed by wire-bonding the cobalt based amorphous ribbon with dimensions 10 ㎜ × 1 ㎜ × 20 ㎛ on a printed circuit board. Negative feedback was used to remove the hysteresis and temperature dependence and to increase the linearity of the system. Sensitivity of the milli gauss meter was 0.3 V/Oe and the magnetic field resolution and environmental noise level were less than 0.01 Oe and 2 mOe, respectively, in an unshielded room.

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...

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.