Sunwook Kim

Design of Alternating Magnetic Field Stimulator Using Duty Factor

We have developed an alternating magnetic field stimulation system consisting of a switched-mode power supply and a digital control circuit which modulates a duty ratio to maintain a magnetic field intensity of a few mT even while the frequency increases up to 4 kHz with a controllable coil temperature below 30 oC in air. This duty ratio modulation and water circulation are advantageous for cell culture under ac-magnetic field stimulation by preventing the incubator from exceeding a cell-viable temperature of 37 oC. Although the temperature of the coil when subjected to a sinusoidal voltage rapidly increased, that of our system modulated by the duty factor did not change. This is a potentially valuable method to investigate the effects of intermediate frequency magnetic field stimulation on biological entities such as cells, tissues and organs.

Assessment of Pulsed Magnetic Field Stimulus by Using Finger Photoplethysmogram and Pressure Pulse Waveform

Photoplethysmogram (PPG) and pressure pulse waveform (PPW) were compared and evaluated for the efficacy of stimulating knuckles by using the pulsed magnetic field. Both signals were observed simultaneously while the knuckles were exposed for 10 min to the pulsed magnetic field, with maximum field intensity of 0.8 T and transition time of 0.126 msec. After 5 min stimulation of the knuckles, the results showed that the aging indexes calculated from the second derivative of the PPG were increased from ?1.913 to 0.072, and that of the PPW from ?0.063 to 0.387. However, for the relatively long-term stimulation for 10 min, we found that the values of both the aging indexes of the second derivatives and augmentation index of the PPW returned to the starting level. The changes observed in characteristic factors such as the aging indexes of the second derivatives and augmentation index of the PPW indicate the potential of pulsed magnetic field stimulation as a therapeutic method for the treatment of patients with peripheral vascular disease.

Pt Thickness Dependence of Oscillatory Interlayer Exchange Coupling in [CoFe / Pt / CoFe] / IrMn Multilayers with Perpendicular Anisotropy

The interlayer exchange coupling (IEC) in glass/[Buffer:NiO/Pt]/Co/sub 90/Fe/sub 10//Pt/Co/sub 90/Fe/sub 10//Ir/sub 22/Mn/sub 78/ multilayers within an out-of-plane easy axis is studied. The observed oscillatory behavior of the IEC as a function of the nonmagnetic metallic layer (Pt) is thought to be related to the antiferromagnetic ordering induced by the antiferromagnetic IrMn layer.

Partial magnetization reversal in a perpendicular exchange-biased [Pd∕Co]∕FeMn film through laser annealing

Partial magnetization reversal on the perpendicular exchange-bias characteristics in the [Pd∕Co]×5∕FeMn bilayered and [Pd∕Co]×2∕Cu∕Co∕[Pd∕Co]×2∕FeMn spin valve thin films was investigated using laser. The hysteresis loops were measured using the extraordinary Hall effect and magneto-optical Kerr effect (MOKE). By increasing the output intensity of the laser, the exchange-bias fields in the [Pd∕Co]×5∕FeMn bilayered thin film were gradually changed from negative to positive. The distribution of partial magnetization reversal in the vicinity of the laser spot was measured by using MOKE. Although the magnetic moment of the pinned layer was gradually reversed by using laser annealing in the MOKE loops of the [Pd∕Co]×2∕Cu∕Co∕[Pd∕Co]×2∕FeMn spin valves’ structure, that of the free layer was not changed. When the pattern size was decreased to 5×5μm2, the direction of exchange biasing was fully reversed through laser annealing without distorting the hysteresis loop. All the patterned samples below 100×100μm2 have ...

Optimizing the geometry of an in vitro tunneling magnetoresistance biosensor using an immobilized ferrimagnetic nanoparticle agent

Spatial interactions of the magnetic field produced by an immobilized ferrimagnetic nanoparticle agent on the sensing layer have been numerically calculated considering the longitudinal and transverse components of the magnetic field. Based on the calculations, the sensor geometry to obtain the maximum sensing performance of an in vitro exchange biased tunneling magnetoresistance (TMR) based biosensor can be determined prior to fabrication. Numerical analysis considering the one-dimensional (longitudinal) component of the magnetic field along the magnetization of the ferrimagnetic nanoparticle agent found that the geometrical parameters of the biosensor can be determined by simply considering the effective distance (δ). The effective distance (δ) is determined by the remnant magnetization and radius of the ferrimagnetic nanoparticle agent and the detectable field limit (BDL) relevant to the exchange bias field of the TMR sensor. The optimized sensor geometry in terms of the critical length (lc) and width ...

Effects of Co80Fe20 insertion layer on perpendicular exchange bias characteristics in [Pd∕Co]5∕FeMn bilayered thin films

Effects of Co80Fe20 insertion layer on the perpendicular exchange bias characteristics in [Pd∕Co]5∕FeMn bilayered thin films have been investigated by changing the Co80Fe20 insertion layer thickness. Hysteresis loops for both in-plane and perpendicular to the film plane were measured by using a vibrating sample magnetometer and an extraordinary Hall effect. It was found that the thin Co80Fe20 insertion layer was very effective in improving the perpendicular exchange bias characteristics in [Pd∕Co]5∕FeMn bilayered thin films. By increasing Co80Fe20 insertion layer thickness from 0to1.24nm, the exchange bias field was significantly increased up to 332Oe and then decreased down to 60Oe, while the pinned coercivity was monotonically decreased. The effective magnetic anisotropy was kept at positive up to 0.6nm thick of Co80Fe20 insertion layer. It was experimentally confirmed that the perpendicular exchange bias field was promisingly larger than the pinned coercivity when the Co80Fe20 insertion layer had the t...

Correlation Between the Degradation of Perpendicular Anisotropy and the Double Hysteresis Behavior in [Pd/Co]

We clearly detected double hysteresis by increasing Co layer thickness and decreasing the number of bilayers in perpendicular exchange biased [Pd(0.6)/Co(t)] n/FeMn(11.6 nm) thin films. In-plane tensile stress calculations confirmed that the appearance of double hysteresis is closely related to the degradation of stress-induced perpendicular anisotropy in the [Pd/Co] multilayers. Furthermore, annealing at the magnetic field applied perpendicular to the film plane directly verified that the enhancement of thermally induced perpendicular anisotropy, K eff-induced, in the [Pd/Co] multilayers is the main physical reason for removal of the double hysteresis. All our experimental and theoretical results demonstrated that perpendicular anisotropy is the dominant factor in controlling the double hysteresis behavior of perpendicularly magnetized [Pd/Co]n/FeMn exchange biased thin films.

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Compared to acupuncture, the pulsed magnetic field (PMF) stimulus is a useful tool for treatment of many physical conditions and health maintenance due to its advantages as a noninvasive and nontoxic medical treatment. The purpose of this study was to investigate the effect of PMF stimulus direction at PC9 on the alpha activity of electroencephalogram (EEG) and vascular aging calculated from photoplethysmograph (PPG). It can be concluded that the direction of PMF stimulus affects the increase of alpha activity of EEG and PPG, indicating the vascular stiffness and the sclerosis level of blood vessels weakly relevant to the direction of PMF stimulus.

/FeMn Exchange Biased Thin Films

The dependence of magnetic shield layer thickness, permeability of the shield materials, and shielding gap width on the sensing performance of an in vitro exchange biased tunneling magnetoresistance (TMR) spin-valve biosensor with immobilized ferrimagnetic sensor agents has been numerically analyzed by using a finite element method. It was numerically demonstrated that the specially designed magnetic shield layer on the surface of the TMR biosensors is considerably effective to improve the switching field uniformity and to eliminate an undesirable undershoot magnetic field formed on the sensor surface.

The Effect of Magnetic Field Direction on the EEG and PPG Obtained from Pulsed Magnetic Stimulus at Acupoint PC9

We have investigated the effects of pulsed magnetic field (PMF) stimulus on acupoint KI1 using electroencephalography (EEG) analysis. Compared with the control group, the PMF stimulus group showed a decrease of SEF-50, an increase of relative theta power, and activation of alpha power in brain map. This means the impulse of stimulus on acupoint KI1 might pass through the sensory nerve following meridian and approach the cerebral cortex, causing the autonomic nervous system to be activated for relaxed mental state. The percentage (PCT) showing asymmetry pattern and correlation coefficient among channels calculated from EEG data also showed proper PMF stimulus to induce the relaxation. Therefore, PMF stimulus on appropriate acupoint is thought to be one of the candidates for alternative non-invasive acupuncture therapy.