• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.370.1-370.1
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• 2014

Giant magnetoresistance (GMR), tunneling magnetoresistance (TMR), and magnetic random-access memory (MRAM) are currently active areas of research. Magnetite, Fe3O4, is predicted to possess as half-metallic nature, ~100% spin polarization (P), and has a high Curie temperature (TC~850 K). On the other hand, Spinel ferrite CoFe2O4 has been widely studies for various applications such as magnetorestrictive sensors, microwave devices, biomolecular drug delivery, and electronic devices, due to its large magnetocrystalline anisotropy, chemical stability, and unique nonlinear spin-wave properties. Here we have investigated the magneto-transport properties of epitaxial CoxFe3-xO4 thin films. The epitaxial CoxFe3-xO4 (x=0; 0.4; 0.6; 1) thin films were successfully grown on MgO (100) substrate by molecular beam epitaxy (MBE). The quality of the films during growth was monitored by reflection high electron energy diffraction (RHEED). From temperature dependent resistivity measurement, we observed that the Werwey transition (1st order metal-insulator transition) temperature increased with increasing x and the resistivity of film also increased with the increasing x up to $1.6{\Omega}-cm$ for x=1. The magnetoresistance (MR) was measured with magnetic field applied perpendicular to film. A negative transverse MR was disappeared with x=0.6 and 1. Anomalous Hall data will be discussed.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2004.07a
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• pp.543-546
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• 2004

The melting temperature and critical temperature(Tc) of $YBa_2Cu_3O_x$ with deferent content impurities of PbO and $BaPbO_3$ were studied. When the PbO was used as addition in $YBa_2Cu_3O_x$, although the melting point could be reduced, the superconductivity(the transition width, ${\Delta}T_c$) became poor. From the XRD pattern of the sintered mixture of $YBa_2Cu_3O_x$ and PbO it was known that there is a reaction between $YBa_2Cu_3O_x$ and PbO, and the product is $BaPbO_3$. In the process of the reaction the superconducting phase of $YBa_2Cu_3O_x$ was decreased and in the sample $BaPbO_3$ became the main phase. Therefore the superconductivity was reduced. $BaPbO_3$ was chosen as the impurity for the comparative study. The single phase BaPbO3 was synthesized by the simple way from both mixtures of $BaPbO_3$ and PbO, $BaPbO_3\;and\;PbO_2$. Deferent contents of $BaPbO_3$(10%, 20%, 30%) were added in the $YBa_2Cu_3O_x$. By the phase analysis in the XRD patterns it was proved that there werenot reactions between $YBa_2Cu_3O_x$ and $BaPbO_3$. When $BaPbO_3$ was used as impurity in $YBa_2Cu_3O_x$ the superconductivity was much better than PbO as impurity in $YBa_2Cu_3O_x$. But the melting point of $YBa_2Cu_3O_x$ with $BaPbO_3$ could not be found when the temperature was lower than $1000^{\circ}C$ in the DTA measurement.

• Proceedings of the Materials Research Society of Korea Conference
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• 1992.05a
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• pp.17-17
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• 1992

The ceramic high transition temperature superconducting materials present many interesting characteristics that will be analysed from two points of view: physical behavior, and chemical aspects. From the first point of view, these materials display an enormous variety of different physical properties. At low doping levels the normal state shows antiferromagnetism and insulating behavior. At intermediate doping levels, an anomalous metallic state appears and, the optimum Tc in the superconducting state is generated. With increasing doping a normal metallic state develops and superconductivity starts to disappear. Many of the physical phenomena that describe the overall behavior when doping levels are changed will be discussed. From the poing of view of the chemical aspects. we well discuss some of the problems involved in the methods of preparation with particular emphasis on defects, crystal structures, critical currrents, and applications in technology.

• Journal of the Korean Ceramic Society
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• v.31 no.10
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• pp.1123-1132
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• 1994

Bi-Sr-Ca-Cu-O superconductor thin films were prepared on MgO and LaAlO3 substrates by MOCVD technique. The films deposited on MgO and LaAlO3 substrates became superconducting at 64 K and 70 K respectively. The measured critical current density of thin film deposited on LaAlO3 substrate was around 104 A/$\textrm{cm}^2$. After annealing at $700^{\circ}C$ for 3 hours, the critical transition temperature(Tc) of films deposited on LaAlO3 was changed from 70 K to 74 K.

• Transactions on Electrical and Electronic Materials
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• v.5 no.5
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• pp.185-188
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• 2004

Phase transitions from the amorphous to crystalline states, and vice versa, of GST(GeSbTe) and AST(AsSbTe) thin films by applying electrical pulses have been studied. These materials can be used as nonvolatile memory devices. The thickness of ternary chalcogenide thin films is approximately 100 nm. Upper and lower electrodes were made of AI. I-V characteristics after impressing the variable pulses to GST and AST films. Tc(crystallization temperature) of AST system is lower than that of the GST system, so that the current pulse width of crystallization process can be decreased.

• Journal of the Korean Ceramic Society
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• v.25 no.2
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• pp.154-160
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• 1988

Almost single phase of Y1Ba2Cu3Ox, which exhibits the onset of superconducting transition at 90K, has been prepared via a conventional ceramic processing route followed by an anneal for a period of 10 hours at 45$0^{\circ}C$ in an atmosphere of 100% O2. The mean value for the oxygen content has been determined as x=6.85$\pm$0.02 by an iodometric titration technique. X-ray photoemission spectra are found to be consistent with a mixture of the d8(Cu3+) and d9(Cu2+) as the ground state and reveals that the local concentration of Cu3+ decreases with depth from the surface where x corresponds to 7.0. This is explained as being associated with the intercalation of oxygen during cooling from the annealing temperature.

• Progress in Superconductivity and Cryogenics
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• v.15 no.2
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• pp.9-11
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• 2013

We have investigated the effect of thickness of the MgB2 film on the grain growth direction as well as on their superconducting properties. $MgB_2$ films of various thicknesses were fabricated on c-cut $Al_2O_3$ substrates at a temperature of $540^{\circ}C$ by using hybrid physical-chemical vapor deposition (HPCVD) technique. The superconducting transition temperature ($T_c$) was found to increase with increase in the thickness of the $MgB_2$ film. X-ray diffraction analysis revealed that the orientation of grains changed from c-axis to a-axis upon increasing the thickness of the $MgB_2$ film from 0.6 to 2.0 ${\mu}m$. $MgB_2$ grains of various orientations were observed in the microstructures of the films examined by scanning electron microscopy. It is observed that at high magnetic fields the 2.0-${\mu}m$-thick film exhibit considerably larger critical current density ($J_c$) as compared to 0.6-${\mu}m$-thick film. The results are discussed in terms of an intrinsic-pinning in $MgB_2$ similarly as intrinsic-pinning occurring in high-Tc cuprate superconductors with layered structure.

• Journal of the Korean Chemical Society
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• v.33 no.3
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• pp.326-331
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• 1989

Superconducting fiber of high Tc $YBa_2Cu_3O_{7-{\sigma}}$ was successfully prepared by the sol-gel method. The stoichiometric mixture of Y, Ba and Cu nitrates was dissolved in water, to which citric acid solution was added. Then ammonium hydroxide was added to the mixture in order to increase the pH to a value between 5.8 and 6.2. The mixture was heated carefully to form a homogeneous colloid sol at 358 K. The solution was viscous enough to draw a long gel fiber, which was then heated at 1223K for 8 hrs. under oxygen partial pressure of $Po_2$ = 1 atm., cooled slowly and annealed at 723K for 13 hrs.. The reacted superconducting sample maintained a fiber form with some shrinkage of its volume. The superconducting transition temperatures (Tc, onset and Tc, offset) of $YBa_2Cu_3O_{7-{\sigma}}$ have been determined to be approximately 95K and 82K, respectively. The superconducting phase was also confirmed by Meissner effect at liquid nitrogen boiling temperature.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.36 no.5
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• pp.513-519
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• 2023

It was reported that a tetragonal phase can be stabilized with maintaining good piezoelectric properties when Na_0.5K_0.5NbO₃ (KNN) is modified with 0.06 mol SrTiO₃. However, such a high amount of SrTiO₃ leads not only to poor sinterability but low Curie temperature (T_C). To maintain high TC with good piezoelectric properties in KNN-based lead-free piezoelectric ceramics, this study investigates the effect of Li-doping on the dielectric and piezoelectric properties of 0.96Na_0.5K_0.5NbO₃-0.04SrTiO₃ (KNN-4ST) ceramics. As a result, the orthorhombic-tetragonal phase transition was observed at 2 mol% Li₂CO₃ modified KNN-4ST ceramics, whose T_C, d₃₃ and k_p values are 328℃, 165pC/N and 0.33, respectively.

• Proceedings of the KIEE Conference
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• 2006.07a
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• pp.115-116
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• 2006

Since the discovery of the high-temperature superconductors, many researches have been performed for the practical applications of superconductivity technologies in various fields. As results, significant progress has been achieved. Especially, Superconducting Fault Current Limiter (SFCL) offers an attractive means to limit fault current in power systems. The SFCLS, in contrast to current limiting reactors or high impedance transformers, are capable of limiting short circuit currents without adding considerable voltage drop and energy loss to power systems during normal operation. Under fault conditions, a resistance is automatically inserted into the power grid to limit the peak short-circuit current by transition from the superconducting state to the normal state, the quench. The advantages, like fail safe operation and quick recovery, make SFCL very attractive, especially for rapidly growing power systems with higher short-circuit capacities. In order to verify the effectiveness of the SFCL, in this paper, the analysis of fault current and voltage stability assessment in a sample distribution system and a transmission system are performed by the PSCAD/EMTDC based simulation method. Through the simulation, the advantage of SFCL application is shown, and the effective parameters of the SFCL are also recommended for both distribution and transmission systems. A resistive type component of SFCL is adopted in the analysis. The simulation results demonstrate not only the effectiveness of the proposed simulation scheme but also SFCL parameter assessment technique.