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- 학과정보 : 물리학과 프런티어융합 (검색결과 3 건)
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Cha Seunghun Kyung Hee University Graduate School 2023 국내석사
The Eu4Ga8Ge16 is a homologous structure compound comparable with a cage structured clathrate Eu8Ga16Ge30 which investigated in respect of magnetocaloric effect (MCE). The Eu4Ga8Ge16 has an orthorhombic structure (Cmcm) with the charge-balancing Ga/Ge framework encompasses the Eu2+ atomic chains, resulting in the one-dimensional caged spin chain structure. It shows metamagnetic transition from antiferromagnetic state by low magnetic field which is thought to be beneficial to MCE. We examined the magnetic, electrical properties of single and polycrystalline Eu4Ga8Ge16 and its MCE. Also, hydrogen storage capacity of Eu4Ga8Ge16 is investigated since caged structure is beneficial to storing gas molecules. We found its unique properties that may come from its one-dimensional spin chain structure such as inverse magnetic hysteresis at T = 2 K, non-normal behaviors in electrical resistivity such as the Kondo-like effect, strong electron-phonon coupling. The magnetic entropy change of Eu4Ga8Ge16 is evaluated as -ΔSM = 14.6 J kg-1 K-1 and adiabatic temperature change is evaluated as ΔTad = 7.8 K at TN along the [100] direction, which are superior to previously reported type-I clathrate Eu8Ga16Ge30 compound. We also observed hydrogen is stored in the Eu4Ga8Ge16. Therefore, the giant magnetocaloric materials can be extendable to the antiferromagnets with low-magnetic field metamagnetic transition and caged magnetic materials such as Eu4Ga8Ge16 can be the multi-functional materials for magnetocaloric and hydrogen storage application. Moreover, we suggest that unique properties of Eu4Ga8Ge16 are worthy of further magnetic and electrical investigations.
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BaTiO₃ 박막의 구조적, 광학적 특성 및 강유전 특성 연구
BaTiO3 박막의 구조적, 광학적 특성 및 강유전 특성 연구 김은지 지도교수 이호선 물리학 석사 경희대학교 일반대학원 본 연구에서는 Si(111) 기판에 300 ℃이하에서 RF magnetron sputtering을 이용하여성 장한 강유전체 BaTiO3(BTO) 박막의 광학적, 구조적 특성들을 확인하였다. XRD분석을 통해 박막의 결정성이 정방정계(tetragonal)인 것을 확인하고 SEM, AFM 분석을 통해 박막의 표 면과 단면 특성을 확인하였다. Ellipsometry분석을 통해 Optical gap energy를 구한 결과 두께가 얇을수록, 주입 가스의 산소 유량 비율이 늘어날수록 4.58 eV에서 4.42 eV까지 optical gap이 감소하는 것을 확인하였다. 또한 위의 측정을 통하여 확인한 BaTiO3 박막의 성장 조건을 바탕으로 Pt/BTO/Pt 구조의 MFM (metal-ferroelectric-metal) 커패시터를 제작하여 박막의 강유전성을 확인하였다. O2가스의 유량의 변화에 따른 강유전체의 잔류분극 값의 변화를 확인해본 결과 Ar:O2 = 1:1(10sccm:10sccm)에 해당할 때 5.150μC/cm-2으 로 가장 큰 값을 나타내는 것을 확인하였고 PFM 측정을 통해 전압 인가 영역에 따라 도메 인 형성을 나타내는 위상 사진을 확인하였다. 또한 두께가 얇아질수록, 산소 유량이 높을수 록 BTO 박막에서 더 강한 강유전성을 확인하였다.
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HIDAYATI RAHMATUL 경희대학교 대학원 2024 국내박사
Superconducting materials are a unique class of functional materials that exhibit no electrical resistance when they are cooled below a specific critical transition temperature. The exploration and development of High-Entropy Alloys (HEAs) as innovative functional materials represent a rapidly evolving research area within the field of materials science. A number of HEA superconductors have been discovered so far, each demonstrating a range of intriguing properties. The field of HEA superconductors has been a hotbed of research activity since the first superconducting HEA was discovered in 2014. This dissertation delves into the synthesis and superconducting properties of High-Entropy Alloys (HEAs), specifically the Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 compound, using planetary ball milling and the Spark Plasma Sintering (SPS) method. The sintered HEA sample exhibited a BCC crystal structure with a small amount of secondary phases (Fe- based impurities). The superconducting phase transition was observed in the electrical resistivity and magnetic susceptibility measurements, with a Tc similar to that of an arc- melted (AM) sample. The study also revealed that the zero-temperature limit of the upper critical magnetic fields and coherence length were slightly decreased or comparable to the AM HEA sample. The field-dependent isothermal magnetization hysteresis exhibited typical type-II superconducting behavior with magnetic flux avalanches in the low magnetic field region. Notably, the calculated critical current density (Jc) was significantly increased compared to the AM HEA sample, attributed to enhanced pinning force due to point pinning as well as surface pinning effects. The vortex relaxation measurements showed stable remanent magnetization for over 10,000 s after magnetic fields were turned off, which is unlike conventional superconducting vortex relaxation behavior. The strong pinning force in the SPS sintered Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 compound is beneficial for practical applications because of the significantly enhanced Jc with stable remanent magnetization. The dissertation also explores the superconducting properties of Medium-Entropy Alloy (MEA), specifically Nb2/5Hf1/5Zr1/5Ti1/5 , synthesized by AM and powder metallurgical SPS methods. The SPS-MEA demonstrates a substantial improvement in vortex pinning, leading to an increase in the Jc when contrasted with AM-MEA. The research suggests that powder metallurgical processes for HEA synthesis enhance Jc and vortex pinning force while maintaining high mechanical strength, which is beneficial for practical applications in superconducting magnets. The study concludes that HEA and MEA have emerged as promising classes of materials due to their versatility and potential for various applications. In this study, we also discovered an unprecedented phenomenon of diamagnetically aligned spin-triplet superconductivity in an Fe-based high entropy alloy compound, NbTaTiZrFe. A significant diamagnetic signal was observed below 42 K, indicating a superconducting Tc of 7 K under zero-field-cooled magnetic susceptibility conditions. However, this transitioned to a ferromagnetic state under field-cooled conditions. The diamagnetic spin-triplet pairing is substantiated by a variety of experimental and theoretical evidence. This includes a significant diamagnetic signal at low fields coupled with a strong ferromagnetic coercive force (Hco) of 1800 Oe, a ferromagnetic spin flip signal exclusive to the diamagnetic state, and metallic ferromagnetism as confirmed by scanning magnetic force microscopy and spin-resolved density of states. Interestingly, the spin-triplet diamagnetic/ferromagnetic superconductivity can be manipulated by offset field conditions (direct- and oscillation- off) to eliminate a residual magnetic field at room temperature. This discovery of diamagnetically aligned spin-triplet superconductivity is novel and provides an excellent platform for exploring Majorana fermions and their potential application in quantum computation. Keywords High entropy alloy, superconductivity , critical current densities, pinning force, flux jump, vortex pinning, spin-triplet, ferromagnetic superconductivity, diamagnetic superconductivity, itinerant ferromagnet superconductor.
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