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온도 변화에 따른 La1-xSrxMnO3 (LSMO) 박막 열전도도와 LSMO와 SrTiO3 경계면 열저항
강준구,진형진,양임정 한국물리학회 2023 새물리 Vol.73 No.1
La0.88Sr0.12MnO3 (LSMO) thin films with different thicknesses were deposited using pulsed laser deposition. The thermal conductivity of the thin films was measured using the 3ω method from 100 K to 293 K. The thermal conductivity showed film thickness dependence. The interfacial resistance was separated from the measured thermal conductivity, resulting in the thermal conductivity of the LSMO film. The thermal conductivity of the LSMO film increased with temperature from 100 K to 293 K and showed a noticeable anomaly near 250 K. The interfacial thermal resistance between the LSMO thin film and the STO substrate decreased as temperature increased, particularly between 100 K and 150 K. The interfacial effect was found to be important in determining LSMO films up to 500 nm in thickness. 펄스 레이저 증착 방법을 이용하여 두께가 다른 La0.88Sr0.12MnO3 (LSMO) 박막을 증착하여 3ω법으로 100 K에서 293 K 영역에서 열전도도를 측정하였다. 측정한 열전도도는 박막 두께 의존성을 나타내었다. 박막과 기판 경계면 열저항을 측정한 결과에서 분리시키는 분석을 통하여 LSMO 박막의 열전도도와 경계면 열저항을 구하였다. LSMO 박막 열전도도를 나타내는 박막 내부 영역의 열전도도는 온도가 증가함에 따라 증가함을 나타내었고, 250 K에서 열전도도가 상대적으로 다르게 변하는 경향을 나타내었다. LSMO 박막과 STO 기판 사이 열저항은 온도가 증감함에 따라 감소하였고, 특히 100–150K 영역에서 급격히 감소하였다. 온도가 증감함에 따라 LSMO 박막과 STO 기판 경계면 열저항 효과는 줄어들지만 100–500 nm 두께의 LSMO 박막 열전도도에 경계면 열저항이 큰 영향을 미치는 것을 확인하였다.
Highly insulating ferromagnetic cobaltite heterostructures
최우석,강경태,진형진,Zheng Gai,이호녕 한국물리학회 2017 Current Applied Physics Vol.17 No.5
Ferromagnetic insulators are rather rare but possess great technological potential in, for example, spintronics. Individual control of ferromagnetic properties and electronic transport provides a useful design concept of multifunctional oxide heterostructures. We studied the close correlation among the magnetism, atomic structure, and electronic structure of oxide heterostructures composed of the ferromagnetic perovskite LaCoO3 and the antiferromagnetic brownmillerite SrCoO2.5 epitaxial thin film layers. By reversing the stacking sequence of the two layers, we could individually modify the electric resistance and saturation magnetic moment. The ferromagnetic insulating behavior in the heterostructures was understood in terms of the electronic reconstruction at the oxide surface/interfaces and crystalline quality of the constituent layers.
김고운,공현준,김동진,이호수,진형진,이호선,진미정,민성욱 한국물리학회 2017 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.71 No.12
The formation of cation vacancies can be useful for electro-chemical devices. In this regard, an understanding of vacancy formation is an important subject for enhancing current electrochemical devices and for developing next generation energy devices. In this work, we chose the well-known lead-free ferroelectric (K0.5Na0.5)NbO3 (KNN) as a model system to understand both the formation of cation vacancies and the relationship between cation vacancies and the physical properties. We studied sintering-duration dependence of the dielectric properties and the cation contents of KNN ceramics at the temperatures near the melting point of KNN. The difference in sintering duration led to a drastic change in the dielectric property, as well as to the creation of cation vacancies. Interestingly, we observed unequal evaporation of cations during the sintering process, which was confirmed by the data obtained from laser-induced breakdown spectroscopy. In addition, we found more drastic changes in the imaginary dielectric constant, which were likely due to a decrease in ionic conducting species, such as K and Na, in KNN.
Magnetic Modulation by Oxygen Vacancies in Epitaxial Ga0.5Fe1.5O3−δ
Kim Hyun Jung,진형진,서유성,Hwang Jungseek,Hong Seungbum 한국물리학회 2020 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.77 No.12
Oxygen vacancies have often been believed to reduce ferroic properties. However, in this work, we demonstrated that oxygen vacancies caused enhanced magnetism in ferrimagnetic Ga0.5Fe1.5O3-δ by preventing Fe-O hybridization and formation of Fe2+. Epitaxial films of 001 Ga0.5Fe1.5O3-δ (GFO) were synthesized on (001) yttria-stabilized zirconia substrates by using radio-frequency magnetron sputtering, and the films were cooled in a forming gas. To see the effect of the oxygen content, we also prepared post-annealed GFO films in 500 Torr of O2. From the analyses of the structural and the optical properties, the magnetism, and the chemical state of the two prepared samples, we verified that oxygen vacancies induced lattice expansion, reduced p-d hybridization, and importantly enhanced magnetism, i.e., increased magnetic moments and the transition temperature. Our results reveal that the superexchange interaction governing the magnetism of GFO can be controlled by the oxygen vacancy content.
Effect of Lithium Doping on the Transport Properties of MoO3 Ceramics
안유정,김고운,이준혁,김현정,이재광,진형진 한국물리학회 2019 새물리 Vol.69 No.10
In this research, we studied the effects of lithium doping on the transport properties of MoO3 ceramics. A series of lightly lithium-doped MoO3 ceramics was prepared: 2, 4 and 8 mol% Li-doped MoO3. The x-ray diffraction analysis confirmed that the typical MoO3 structures were retained at the low doping. However, from laser-induced-breakdown spectroscopy, the differences in the peak intensities of the doped samples clearly indicates that lithium ions are successfully incorporated into the lattices in different amounts. To see the effects of the lithium doping on the transport properties in the MoO3 lattices, impedance spectroscopy is used in the capacitor geometry. A systematic increase in the overall bulk conductivity and a faster relaxation behavior were observed when the doping level was increased. Lastly, when the doping level was increased the activation energy was drastically decreased, which indicates that the lithium facilitates transport along a 2D path.
GdFe<SUB>12</SUB> 박막의 증착온도에 따른 자기적 특성 변화
조대길(Daegill Cho),류상균(Sangkyun Ryu),진형진(Hyoungjeen Jeen) 한국자기학회 2020 韓國磁氣學會誌 Vol.30 No.6
Alloys of rare earth metal and transition metal with ThMn<SUB>12</SUB> structure are expected to have high saturation magnetization and high magnetic anisotropy. Despite its advantages, it has not been commercialized because of its metastability. GdFe<SUB>12</SUB> is a material having ThMn<SUB>12</SUB> structure. Therefore, it is expected to be difficult to stabilize GdFe<SUB>12</SUB> in bulk form. In this study, GdFe<SUB>12</SUB> was fabricated in the form of epitaxial thin films to stabilize the phase. The films were grown epitaxially from 500 ℃ to 200 ℃. From the results of x-ray diffraction and x-ray absorption, it was confirmed that a (330) GdFe<SUB>12</SUB> thin films were grown. The epitaxial GdFe<SUB>12</SUB> thin film had a saturation magnetization (M<SUB>s</SUB>) = 1130 emu/㎤ and coercivity = 712 Oe at a deposition temperature of 400 ℃.