Fe3O4 나노입자와 S=1/2 쩔쩔매는 스핀시스템에서의 자기전기 효과에 관한 연구

학위논문 (박사)-- 서울대학교 대학원 자연과학대학 물리·천문학부, 2017. 8. 김기훈.Magnetoelectric (ME) effect is the phenomenon in which the electric polarization (P) is controlled by magnetic field (H) or the magnetization is controlled by electric field. From early 2000s, the magneto-electric properties have attracted tremendous interest due to fundamental interests on the physics of ferroelectricity induced by spin order and great application potential for numerous low-power electronic devices. Therefore, thousands of reports on extensive experimental results and theoretical models have well established the understandings on the ME effect. However, there has been lack of extensive and quantitative investigations on the ME effect in nano-sized particles and spin S = 1/2 systems in which the high quantum effects are expected. In this thesis, I focus on the nanoparticles of Fe3O4 and two new discovered ME materials with Cu2+ (S = 1/2) ions, PbCu3TeO7 and Cu3TeO6. First, we investigate quantitatively the magnetoelectric coupling of spherical Fe3O4 nanoparticles with uniform diameters from 3 to 15 nm embedded in an insulating host, using a sensitive ME susceptometer. The intrinsic ME susceptibility is measured, exhibiting a maximum value of ∼0.6 ps/m at 5 K for d = 15 nm. We found that the ME susceptibility is reduced with reduced d but remains finite until d = ∼5 nm, which is close to the critical thickness for observing the Verwey transition. Moreover, with reduced diameter the critical temperature below which the ME susceptibility becomes conspicuous increased systematically from 9.8 K in the bulk to 19.7 K in the nanoparticles with d = 7 nm, reflecting the core−shell effect on the ME properties. In addition, we report the observation of H induced P in a Cu2+ (S=1/2) based staircase kagome compound PbCu3TeO7, in which anisotropic magnetic exchange interaction and spin frustration result in two Neel temperatures at TN1=35 K and TN2=24 K. Below TN2, both pyroelectric and ME current measurements reveal that a finite P//a up to 15 µC/m2 develops under H//c of ~ 8.3 T at which the field induced spin flop transition occurs as verified by the magnetization curve. Furthermore, measurements under H//a uncover that the P =14 µC/m2 appears at ~16 T and disappears at ~38 T. Monte-Carlo simulations reveals that the two antiferromagnetic spin ordering exhibit a sinusoidal modulation below TN1 and an incommensurate proper screw type spin rotation below TN2. The simulation results uncover an ab plane type spiral order under H//a and H//c, while the successive transition to form spiral spin order rotating in bc-plane for H//a. In combination with the experimental and theoretical results, we propose field-induced phase diagram along the two applied H directions. Finally, we study the ME properties in cubic Cu3TeO6 which is first discovered linear ME compound with Cu2+ (S=1/2). The P increases linearly under H below TN=62 K and the sign of P does remains same upon changing the H direction. The spin structure without spatial inversion symmetry support the linear ME effect. These results are meaningful since they opened new possibility of successive researches on the ME effect by providing new technique of ME measurement and discovering new ME materials.Chapter 1. Introductions 1 1.1 Multiferroics and magnetoelectric(ME) effect 1 1.1.1 Linear magnetoelectric coupling 3 1.1.2 Phase controlled multiferroicity and magnetoelectric coupling 7 1.2 Microscopic origins of the spin-mediated magneto-electric effect 8 1.2.1 Exchange striction model 9 1.2.2 Spin current model (Inverse DM model) 10 1.2.3 Spin-dependent p-d hybridization model 15 1.3 Magnetoelectric effects in Spin ½ (Cu2+) systems 16 1.4 Magnetoelectric and magneto-dielectric effects in nanostructures 21 1.5 Purpose 23 1.6 References 24 Chapter 2. Experimental details 28 2.1 Physical property measurements in static field 28 2.1.1 Dielectric constant 29 2.1.2 Pyrocurrent/magnetoelectric (ME) current 30 2.1.3 AC Magnetoelectric susceptibility (MES) 33 2.1.4 Magnetic property 34 2.2 Physical property measurements in pulsed field 34 2.2.1 Electric polarization 35 2.2.2 Magnetization 36 2.3 References 38 Chapter 3. Quantitative measurements of size-dependent magneto-electric coupling in Fe3O4 nanoparticles 39 3.1 Introduction 39 3.2 Experimental methods and results 41 3.2.1 Sample preparation for the measurements 41 3.2.2 Dielectric constant 43 3.2.3 ME and magnetic properties in 15 nm Fe3O4 nanoparticles 46 3.2.4 ME properties in Fe3O4 particles with various diameters 51 3.2.5 Evidence of short range magnetoelectric interaction 57 3.2.6 Magnetoelectric property of a Fe3O4 polycrystalline pellet 58 3.2.7 In-phase and out-of-phase of magnetoelectric property 60 3.2.8 Magnetoelectric effect and oxygen stoichiometry 62 3.3 Summary 63 3.4 References 63 Chapter 4. Magnetically-induced ferroelectricity in the S=1/2 staircase kagome compound PbCu3TeO7 66 4.1 Introduction 66 4.2 Lattice structure 69 4.3 Experimental methods and results 69 4.3.1 Sample preparation 69 4.3.2 Magnetoelectric properties in PbCu3TeO7 poly crystal 71 4.3.3 Magnetoelectric properties in PbCu3TeO7 single crystal 74 4.3.4 Magnetoelectric properties in PbCu3TeO7 in pulsed field 80 4.4 Discussion 83 4.4.1 Phase diagram of PbCu3TeO7 83 4.4.2 Monte Carlo calculation on PbCu3TeO7 84 4.5 Summary 91 4.6 References 92 Chapter 5. Linear magnetoelectric coupling in Cu3TeO6 94 5.1 Introduction 94 5.2 Lattice structure 95 5.3 Experimental methods and results 97 5.3.1 Sample preparation 97 5.3.2 Lattice structure characterization of Cu3TeO6 98 5.3.3 Magnetic properties in Cu3TeO6 99 5.3.4 Magnetoelectric properties in Cu3TeO6 100 5.4 Discussion 105 5.4.1 Analysis based on the magnetic point group 106 5.4.2 Analysis based on the toroidal moment 108 5.4.3 Analysis based on the symmetry of magnetic structure 110 5.4.4 Monte Carlo calculation on Cu3TeO6 111 5.5 Summary 112 5.6 References 112 Appendix A. Set up development a pulse magnet system 114 A.1 Introduction 114 A.2 Pulse magnet system 119 A.2.1 Principle of the pulsed field generation 120 A.2.2 High voltage power supply and reed relay 124 A.2.3 Capacitor bank 125 A.2.4 Switching unit of the pulse magnet 127 A.2.5 High voltage probe 128 A.2.6 Ground resistors and ground connection switch 129 A.2.7 Silicon Controlled Rectifier (SCR) 130 A.2.8 Snubber diode and snubber circuit 133 A.2.9 Crowbar circuit 133 A.2.10 Pulse magnet 134 A.2.11 Control circuit and control program 135 A.2.12 Items for safely 138 A.3 Cryogenic systems 139 A.3.1 Liquid nitrogen/liquid helium 4 cryostat 139 A.3.2 Helium 3 cryostat 143 A.3.3 Gas handling systems (GHSs) 147 A.3.3.1 Helium 4 gas handling system 147 A.3.3.2 Helium 3 gas handling system 149 A.4 Measurement systems in the pulsed field 152 A.4.1 Transport probe 152 A.4.2 Digitizer 155 A.4.3 Software for measurements 156 A.4.4 Ground connection 158 A.4.5 Preparation for the electric resistivity measurement 159 A.5 Experiment procedure in the pulsed field 161 A.5.1 Preparation for the experiment 161 A.5.2 Preparation of the probe and cryostats 162 A.5.3 Magnet test 162 A.5.4 Cool down the sample 165 A.5.5 Measurement and finish of the experiment 168 A.6 Preliminary experiment results 169 A.6.1 Upper critical field of the YBCO single crystal 169 A.6.2 Upper critical field of the Nb2Pd1.3Se5 single crystal 171 A.7 Maintenance of the pulse magnet system 173 A.7.1 Pulse magnet 173 A.7.2 Switching unit 174 A.7.3 Gas handling system 176 A.8 References 177 국문 초록 182Docto

유경준

When the definition of income and the equivalence scale was applied just as it was in the LIS (Luxembourg Income Study), the adjusted disposable income inequality calculated by Gini coefficients in Korea was 0.358 for the year 2000. Compare to the 1996 figure of 0.298 the increase of income inequality has skyrocketed. In addition, the adjusted market income inequality increased from 0.302 in 1996 to 0.374 in 2000. The disposable income inequality ranked the third and the market income inequality ranked at the mid level in 2000 among OECD countries. One significant finding in this paper was that the difference between the disposable income inequality and market income inequality in Korea is very small compared to those of other OECD countries. The relative poverty ratio, which is calculated by using 40% of the medium income bracket of the disposable income was calculated at 7.6% in 1996 and 11.5% in 2000. The poverty ratio in 2000 for Korea ranked one of the highest in OECD countries, also.Ⅰ. 문제의 제기 　Ⅱ. 국제비교 방법 및 기존의 연구결과에 대한 평가 　Ⅲ. 소득분배 수준의 국제비교 　Ⅳ. 한국의 절대빈곤율 추이 　Ⅴ. 결론 및 정책적 시사점 　참고문

A Survey of Relationship between Growth, Distribution and Poverty

The relationship between growth and distribution, which has been revealed through many empirical evidences, is that growth is distribution neutral on average and high asset inequality could be a hindrance of economic growth. The growth versus distributionⅠ.서론 　Ⅱ. 성장·분배와 관련된 논의 　　1. 성장이 분배에 미치는 영향 　　2. 분배가 성장에 미치는 영향 　　3. 소결 　Ⅲ. 성장과 분배 및 빈곤의 관계에 대한 논의 　　1. 성장과 분배 및 빈곤의 관계에 대한 이론적 근거 　　2. 성장과 분배 및 빈곤의

[고용동향] 2003년 고용전망

2002년의 실업률은 10월 현재 2.6%이고, 실업자수는 60만 5천명에 이르고 있다. 2002년의 실업률추이를 간단히 살펴보면, 1/4분기에 3.6%를 기록한 후 예상보다 다소 빠른 속도로 감소하여 2/4분기에 2.9%를 기록하였으며, 3/4분기에는 2.7%로 전반적인 하락세를 보이고 있다. 보통의 경우 4/4분기에는 계절적인 요인과 졸업을 앞둔 신규 노동시장 진입자의 증가로 실업률이 다소 상승하는 것이 일반적이다. 2002년 4/4분기의 경우 실업률이 2.8%정도를 기록할 것으로 예상되어 2002년의 전체 실업률은 3.0% 정도로 예상되고 있다. ..

The Impacts of Vocational Training on Earnings in Korea: Evidence from the Economically Active Population Survey

This paper examines whether and how much vocational training raises an individual's earnings in Korea, using the Economically Active Population Survey. To overcome endogeneity of training, we apply fixed-effects and propensity-score matching (PSM) methodsⅠ. 서 론 Ⅱ. 계량분석모형 　1. 고정효과(Fixed Effects) 모형 　2. 성향점수 매칭법 (Propensity-score matching) Ⅲ. 분석 자료 Ⅳ. 교육훈련의 효과 　1. 임금방정식 추정을 통한 효과 분석 　2. 매칭법을 통한 직업훈련의 효과 분석 　　가.

Analysis on the Change and Its Cause of Income Distribution before and after the Financial Crisis：Income Mobility Perspective

Income inequality in Korea has increased after the economic crisis, and the main reason for the widening of income distribution is due to the increase of the unemployed when analyzed using the Urban Household Report(UHR). However, income inequality has noI. Introduction 　Ⅱ. Compositions of the KDI Income Panel (Urban Workers) 　III. Trends of Income Distribution Before and After the Financial Crisis 　IV. Income Mobility 　V. Analysis on the Cause of Worsening Income Inequality after the Fina

Estimation and Verification for Polarization of Income Distribution in Korea

Upon analyzing several Korean Income data sets, it is confirmed that the relationship between Gini coefficient and bi-polarization measure (EGR) is empirically not different although they each come from different theoretical basis. Furthermore, it is diffⅠ. 서 론 Ⅱ. 기존 연구 결과의 검증 　1. 검증의 내용 　2. 추정 및 검증 결과 Ⅲ. DER지수 　1. DER지수의 공리체계 　2. DER지수의 해석 　3. DER지수의 추정 결과 Ⅳ. 요약 및 결

외환위기 전후 소득분배의 변화추이 및 원인분석 : 소득이동을 중심으로

Income inequality in Korea has increased after the economic crisis, and the main reason for the widening of income distribution is due to the increase of the unemployed when analyzed using the Family Icome and Expenditure Survey(FIES). However, income ineⅠ. 연구의 목적 Ⅱ. KDI 소득패널(도시근로자)의 구성 Ⅲ. 외환위기 전후 소득분포의 변화추이 Ⅳ. 소득이동 Ⅴ. 외환위기 이후 소득불평등도 상승의 원인분석 Ⅵ. 소득분배개선을 위한 과제 참고문헌 부