• Journal of the Korean Magnetics Society
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• v.19 no.1
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• pp.22-27
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• 2009

Reduction of the critical current density ($J_c$) for STT magnetization switching is most important issue of magnetic tunnel junctions (MTJs) based MRAM. This report describes how to decrease the Jc and will introduce the recent research progresses of STT-MRAM devices with material engineering and structural improvement, respectively.

• The Transactions of the Korean Institute of Power Electronics
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• v.27 no.6
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• pp.464-470
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• 2022

A new type of soft-switching forward converter is proposed in this study. By adding only a few components, the inductor, diode, switch, and capacitor exhibit higher efficiency than the conventional forward converter. Therefore, the switching losses of the proposed forward converter are considerably reduced compared with those of the conventional forward converter. In addition, the reset winding is not used because of the capacitor employed in the auxiliary circuit. The auxiliary capacitor is adopted for zero-current-transition operation and for dissipating magnetization energy. The performance of the proposed forward converter is validated using experimental results from a 60 W, single-output, forward converter prototype, and design guidelines are presented.

• Journal of Power Electronics
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• v.12 no.4
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• pp.615-622
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• 2012

The switching frequency of the power electronic devices used in large synchronous motor drives is usually kept low (less than 1 kHz) to reduce the switching losses and to improve the converter power capability. However, this results in a couple of problems, e.g. an increase in the harmonic components of the stator current, and an undesired cross-coupling between the magnetization current component ($i_m$) and the torque component ($i_t$). In this paper, a novel complex matrix model of electrically excited synchronous motors (EESM) was established with a new control scheme for coping with the low switching frequency issues. First, a hybrid observer was proposed to identify the instantaneous fundamental component of the stator current, which results in an obvious reduction of both the total harmonic distortion (THD) and the low order harmonics. Then, a novel complex current controller was designed to realize the decoupling between $i_m$ and $i_t$. Simulation and experimental results verify the effectiveness of this novel control system for EESM drives.

• Journal of the Korean Magnetics Society
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• v.10 no.5
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• pp.232-236
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• 2000

CoSm thin films with a Cr underlayer have received continuous attention as a potential material for a high density longitudinal magnetic recording media. In this study the Ax gas sputtering pressure effects on the magnetic properties of CoSm thin films, which were fabricated by using a dc magnetron sputtering machine, were investigated. The magnetic switching volume is especially important parameter to understand the thermal stability of the written information, magnetization reversal process and media noise. Therefore, in this paper the effects of sputtering pressure on the magnetic switching volume of CoSm thin films grown on Cr underlayer with the same sputtering conditions was studied. As the Ar sputtering pressure during sputtering of the CoSm magnetic layer increases from 5 to 30 mTorr, the measured switching volumes decreased from 9.0 to 5.2$\times$10$^{-18}$ cm$^3$. The calculated diameter of switching unit from V＊ was less than 22 nm, which satisfies the Sharrock's requirement on the thermal stability of the high density longitudinal magnetic recording media.

• Journal of Magnetics
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• v.7 no.1
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• pp.4-8
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• 2002

The magnetoresistance (MR) and the magnetization reversal of a lateral spin-injection device based on a spin-polarized field effect transistor (spin FET) have been investigated. The device consists of a two-dimensional electron gas (2DEG) system in an InAs single quantum well (SQW) and two ferromagnetic $(Ni_{80}Fe_{20})$ contacts: all injector (source) and a detector (drain). Spin-polarized electrons are injected from the first contact and, after propagating through the InAs SQW are collected by the second contact. By engineering the shape of the permalloy contacts, we were able to observe distinct switching fields $(H_c)$ from the injector and the collector by using scanning Kerr microscopy and MR measurements. Magneto-optic Kerr effect (MOKE) hysteresis loops demonstrate that there is a range of magnetic field (20~60 Oe), at room temperature, over which the magnetization in one contact is aligned antiparallel to that in the other. The MOKE results are consistent with the variation of the magnetoresistance in the spin-injection device.

• Journal of the Korean Magnetics Society
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• v.17 no.3
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• pp.124-127
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• 2007

Magnetic tunnel junctions (MTJs), which consisted of amorphous CoFeSiB layers, were investigated. The CoFeSiB layers were used to substitute for the traditionally used CoFe and/or NiFe layers with an emphasis given on understanding the effect of the amorphous free layer on the switching characteristics of the MTJs. CoFeSiB has a lower saturation magnetization ($M_s\;:\;560\;emu/cm^3$) and a higher anisotropy constant ($K_u\;:\;2800\;erg/cm^3$) than CoFe and NiFe, respectively. An exchange coupling energy ($J_{ex}$) of $-0.003\;erg/cm^2$ was observed by inserting a 1.0 nm Ru layer in between CoFeSiB layers. In the Si/$SiO_2$/Ta 45/Ru 9.5/IrMn 10/CoFe 7/$AlO_x$/CoFeSiB 7 or CoFeSiB (t)/Ru 1.0/CoFeSiB (7-t)/Ru 60 (in nm) MTJs structure, it was found that the size dependence of the switching field originated in the lower $J_{ex}$ using the experimental and simulation results. The CoFeSiB synthetic antiferromagnet structures were proved to be beneficial for the switching characteristics such as reducing the coercivity ($H_c$) and increasing the sensitivity in micrometer size, even in submicrometer sized elements.

• The Transactions of the Korean Institute of Power Electronics
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• v.24 no.4
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• pp.259-267
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• 2019

An active clamp flyback (ACF) converter applies a clamp circuit and circulates the energy of leakage inductance to the input side, thereby achieving a zero-voltage switching (ZVS) operation and greatly reducing switching losses. The switching losses are further reduced by applying a gallium nitride field effect transistor (GaN-FET) with excellent switching characteristics, and ZVS operation can be accomplished under light load with boundary conduction mode (BCM) operation. Optimal design is performed on the basis of loss analysis by selecting magnetization inductance based on BCM operation and a clamp capacitor for loss reduction. Therefore, the size of the reactive element can be reduced through high-frequency operation, and a high-efficiency and high-power-density converter can be achieved. This study proposes an optimal design for a high-efficiency and high-power-density BCM ACF converter based on GaN-FETs and verifies it through experimental results of a 65 W-rated prototype.

• Journal of the Institute of Electronics Engineers of Korea SC
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• v.39 no.6
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• pp.71-78
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• 2002

A method of soft switching in the half bridge converter using a small saturable core is presented. For the soft switching, a small dead time of both switch off is made in a pair of MOSFET switch. The saturable core is of rectangular magnetization characteristics and the core flux swings from the minus to the plus saturation during ON time of the switch. The soft switching is realized in dead time by a resonance between the saturating inductance of the core and the stray capacitance of MOSFET. As an extension of this concept, instead of the saturable core, we propose a new soft switching circuit using a linear inductance and two switches, which is especially suitable for regulating the output and also for high frequency switching. A theoretical principle of soft switching presented here was confirmed by experiments on a half bridge converter of 1.25 KW.

• Proceedings of the Korean Magnestics Society Conference
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• 2006.11a
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• pp.86-88
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• 2006

• Proceedings of the Korean Magnestics Society Conference
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• 2007.05a
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• pp.237.1-237.1
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• 2007