• 천문학회지
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• 제37권5호
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• pp.527-531
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• 2004

It is argued that the key task in understanding magnetic fields in the cosmos is to comprehend the origin of their order or coherence over large length scales in galaxies. Obtaining magnetic fields can be done in stars, whose lifetime is usually $10^{10}$ rotations, while galactic disks have approximately 20 to 50 rotations in their lifetime since the last major merger, which established the present day gaseous disk. Disorder in the galactic magnetic fields is injected on the disk time scale of about 30 million years, about a tenth of the rotation period, so after one half rotation already it should become completely disordered. Therefore whatever mechanism Nature is using, it must compete with such a short time scale, to keep order in its house. This is the focal quest.

• Journal of Magnetics
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• 제22권2호
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• pp.333-343
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• 2017

A rotating magnetic field (RMF) ${\Phi}_1-{\Phi}_2$ model was developed in consideration of the skin effect. The rotating magnetic field's induced three-dimensional flow was simulated numerically, and the influence of the skin effect was investigated. The rotating magnetic field drives the rotating convection in the azimuthal direction, and a secondary convection appears in the radial-meridional direction. The results indicate that ignoring the skin effect results in a smaller azimuthal velocity component and larger radial and axial velocity components, and that the deviation becomes more obvious with the larger dimensionless shielding parameter K.

• Current Applied Physics
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• 제18권11호
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• pp.1201-1204
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• 2018

The effect of electric current pulses on a sub-100 nm magnetic bubble state in a symmetric Pt/Co multilayer was directly observed using a full-field transmission soft X-ray microscope (MTXM). Field-induced evolution of the magnetic stripe domains into isolated bubbles with their sizes down to 100 nm was imaged under varying external magnetic fields. Electric current pulses were then applied to the created magnetic bubbles, and it was observed that the bubbles could be either created or annihilated by the current pulse depending on the strength of applied magnetic field. The results suggest that the Joule heating plays a critical role in the formation and/or elimination of the bubbles and skyrmions. Finally, the schematic phase diagram for the creation and annihilation of bubbles is presented, suggesting an optimized scheme with the combination of magnetic field and electric current necessary to utilize skyrmions in the practical devices.

• 한국분말재료학회지
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• 제30권4호
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• pp.297-304
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• 2023

Iron oxide (Fe₂O₃) nanoclusters exhibit significant potential in the biomedical and pharmaceutical fields due to their strong magnetic properties, stability in solutions, and compatibility with living systems. They excel in magnetic separation processes, displaying high responsiveness to external magnetic fields. In contrast to conventional Fe2O3 nanoparticles that can aggregate in aqueous solutions due to their ferrimagnetic properties, these nanoclusters, composed of multiple nanoparticles, maintain their magnetic traits even when scaled to hundreds of nanometers. In this study, we develop a simple method using solvothermal synthesis to precisely control the size of nanoclusters. By adjusting precursor materials and reducing agents, we successfully control the particle sizes within the range of 90 to 420 nm. Our study not only enhances the understanding of nanocluster creation but also offers ways to improve their properties for applications such as magnetic separation. This is supported by our experimental results highlighting their size-dependent magnetic response in water. This study has the potential to advance both the knowledge and practical utilization of Fe₂O₃ nanoclusters in various applications.

• International Union of Geodesy and Geophysics Korean Journal of Geophysical Research
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• 제22권1호
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• pp.6-23
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• 1994

A disconnection event (DE) of the cometary plasma tail is one of most spectacular phenomena observed in comets. Yet, for years it has remained one of the great unsolved problems I astronomy and space physics. The solar wind is thought to play a major role in the creation of comet plasma tail (type Ⅰ) disconnection events. The goal of this paper is to present a mechanism that explains the disconnection event in terms of the local conditions at the comet. Comparison of the solar wind conditions and 16 DEs in Halley's comet shows that DEs are associated primarily with crossings of the heliospheric sector boundary and apparently not with any other properties of the solar wind, such as a high speed stream[Yi et al., 1994]. A 3-dimensional resistive magnetohydrodynamic simulation in this paper supports this association by showing that only front-side magnetic reconnection between the reversed interplanetary magnetic fields that exist when a comet crosses the heliospheric sector boundary [Niedner and Brandt, 1978] could reproduce the morphology of a DE, including ray formation [Brandt, 1982].

• Journal of Astronomy and Space Sciences
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• 제36권3호
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• pp.105-113
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• 2019

Spectroscopic observations of barium star ${\zeta}$ Capricornus (HD204075) obtained at the 8.2 m telescope of the European Southern Observatory, with a spectral resolving power R = 80,000 and signal to noise ratio greater than 300, were used to refine the atmospheric parameters. We found new values for effective temperature ($T_{eff}=5,300{\pm}50K$), surface gravity ($log\;g=1.82{\pm}0.15$), micro-turbulent velocity ($v_{micro}=2.52{\pm}0.10km/s$), and iron abundance ($log\;N(Fe)=7.32{\pm}0.06$). Previously published abundances of chemical elements in the atmosphere of HD204075 were analyzed and no correlations of these abundances with the second ionization potentials of these elements were found. This excludes the possible influence of accretion of hydrogen and helium atoms from the interstellar or circumstellar environment to the atmosphere of this star. The accretion of nuclear processed matter from the evolved binary companion was primary cause of the abundance anomalies. The young age of HD204075 allows an estimation of the time-scale for the creation of the abundance anomalies arising from accretion of interstellar hydrogen and helium as is the case of stars with low magnetic fields; which we estimate should exceed $10^8$ years.

• Advances in nano research
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• 제5권4호
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• pp.281-301
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• 2017

In this disquisition, an exact solution method is developed for analyzing the vibration characteristics of magneto-electro-elastic functionally graded (MEE-FG) beams by considering porosity distribution and various boundary conditions via a four-variable shear deformation refined beam theory for the first time. Magneto-electroelastic properties of porous FG beam are supposed to vary through the thickness direction and are modeled via modified power-law rule which is formulated using the concept of even and uneven porosity distributions. Porosities possibly occurring inside functionally graded materials (FGMs) during fabrication because of technical problem that lead to creation micro-voids in FG materials. So, it is necessary to consider the effect of porosities on the vibration behavior of MEE-FG beam in the present study. The governing differential equations and related boundary conditions of porous MEE-FG beam subjected to physical field are derived by Hamilton's principle based on a four-variable tangential-exponential refined theory which avoids the use of shear correction factor. An analytical solution procedure is used to achieve the natural frequencies of porous-FG beam supposed to magneto-electrical field which satisfies various boundary conditions. A parametric study is led to carry out the effects of material graduation exponent, porosity parameter, external magnetic potential, external electric voltage, slenderness ratio and various boundary conditions on dimensionless frequencies of porous MEE-FG beam. It is concluded that these parameters play noticeable roles on the vibration behavior of MEE-FG beam with porosities. Presented numerical results can be applied as benchmarks for future design of MEE-FG structures with porosity phases.

• 한국컴퓨터그래픽스학회논문지
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• 제29권5호
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• pp.1-9
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• 2023

애니매트로닉스 제어 기술은 공학, 의학, 예술 등 여러 학문 분야에서 흥미로운 주제이며 관련 연구가 활발히 진행되고 있다. 애니매트로닉스의 움직임을 제어하는 일반적인 방법은 몸체 내부에 장착된 전동 모터를 사용하는 것이다. 하지만 이러한 방식은 몸체 내부의 공간이 좁을 경우 적용하기 어렵다. 본 연구에서는 몸체가 가늘고 긴 촉수 생명체의 움직임을 제어하기 위해 몸체 내부에 기계 장치를 장착하는 대신 외부 자력을 이용하는 방법을 제안한다. 구체적으로 본 연구에서는 애니매 트로닉스의 관절체 몸체를 자성이 있는 금속 소재로 만들어 외부에 설치된 전자석의 인력에 영향을 받도록 하였다. 그리고 전자석의 세기를 PID 제어기로 제어하여 애니매트로닉스 몸체의 위치를 실시간으로 제어할 수 있도록 하였다. 또한 자석이 회전 운동을 하도록 하고 회전의 속도를 변화시켜 다양한 움직임을 연출할 수 있도록 하였다. 본 연구의 실험은 가상 환경에 구현하여 시뮬레이션을 통해 수행되었다. 사용자에 의한 실시간 제어의 결과뿐만 아니라 다양한 스타일의 애니메이션 생성 방법 및 결과를 제시하여 그 효용성을 증명하였다.