Title Page
ABSTRACT
Contents
Chapter 1. Introduction 13
1.1. Ferromagnetism 14
1.1.1. Background 14
1.1.2. Localized picture 16
1.1.3. Itinerant picture 19
1.1.4. Local vs itinerant picture 20
1.2. Strontium ruthenate (SrRuO₃) 21
1.3. Outline of the Thesis 23
1.4. References 25
Chapter 2. Experimental Methods 27
2.1. Angle-resolved photoemission spectroscopy (ARPES) 28
2.1.1. Introduction to ARPES 28
2.1.2. Polarization-dependent ARPES 33
2.1.3. Spin-resolved photoemission spectroscopy (SARPES) 34
2.2. Pulse laser deposition (PLD) 39
2.2.1. Introduction of PLD 39
2.2.2. Sample growth 40
2.3. References 42
Chapter 3. Spin-dependent dual ferromagnetism in SrRuO₃ 43
3.1. Background 44
3.1.1. Controversy on the origin of ferromagnetism in SrRuO₃ 44
3.1.2. Motivation 45
3.2. Methods 46
3.2.1. Thin-film growth 46
3.2.2. Magnetic property measurement 48
3.2.3. ARPES, and SARPES measurements 48
3.3. Results 52
3.4. Discussion 55
3.4.1. Dual nature of ferromagnetism 55
3.4.2. Spin-dependent correlation strength 56
3.5. Conclusion and Remarks 60
3.6. References 62
Chapter 4. Hund J assisted Mott transition in 2D SrRuO₃ 64
4.1. Background 65
4.1.1. Difficulties in Hund's correlation study 65
4.1.2. Motivation 66
4.2. Methods 69
4.2.1. ARPES measurement 69
4.2.2. Low-energy electron diffraction (LEED) 70
4.3. Results 71
4.3.1. Symmetry-preserving strain engineering in 2D SrRuO₃ 71
4.3.2. Orbital occupancy changes 74
4.3.3. Control of electronic structures with orbital differentiation 78
4.4. Discussion 81
4.5. Conclusion and Remraks 84
4.6. References 86
Chapter 5. Strain-depedent magnetic behavior of SrRuO₃ near coherent-incoherent cross-over 89
5.1. Background 90
5.1.1. Overview 90
5.1.2. Motivation 91
5.2. Results 92
5.2.1. Strain-dependent Mott transition in 10 u.c. SrRuO₃ film 92
5.2.2. Strain-dependent magnetic transition in 10 u.c. SrRuO₃ film 95
5.3. Conclusion and Remarks 97
5.4. References 98
Chapter 6. Summary & Remarks 99
Chapter 7. Publication List 101
국문 초록 105
Figure 1.1. Schematic of an ordered ferromagnetic structure in the Heisenberg models on a simple cubic lattice. 15
Figure 1.2. Schematic band structure of the Stoner model for ferromagnetism. 18
Figure 1.3. Structure, resistivity, and magnetism of single-crystalline SrRuO₃. 22
Figure 2.1. Schematic image of photoemission process. 29
Figure 2.2. Lab-based in-situ ARPES cluster system. 32
Figure 2.3. Schematic presentation of ARPES measurement geometry and corresponding parity of d orbitals. 33
Figure 2.4. Schematic of the VLEED detector. 35
Figure 2.5. in-situ magnetization process in the UHV chamber for SARPES measurement. 36
Figure 2.6. Spin-resolved spectra before/after in-situ magnetization. 37
Figure 2.7. Schematic of pulsed laser deposition method. 40
Figure 3.1. Sample growth and characterization of SRO thin film. 47
Figure 3.2. Schematic Fermi surfaces of SRO thin films without folded bands. 49
Figure 3.3. Electronic structure of SRO thin films measured by ARPES. 50
Figure 3.4. Spin-resolved electronic structures of SRO thin films. 51
Figure 3.5. Temperature-dependent spin-polarized band dispersions. 54
Figure 3.6. Schematic presentation of the interactions involved. 58
Figure 3.7. Temperature-dependent magntic properties of SRO. 59
Figure 3.8. Spin-dependent itinerant and localized ferromagnetism. 61
Figure 4.1. Orbital-selective phase transition in SRO. 67
Figure 4.2. Symmetry-preserved strained-SRO monolayer. 72
Figure 4.3. Interatomic optical transitions and control of orbital occupancy. 76
Figure 4.4. Metal-insulator transition in SRO monolayers. 77
Figure 4.5. Orbital contributions of the bands below the EF in an SRO monolayer. 79
Figure 4.6. Doping-dependent studies of an effective (2-orbital/2-electron) system. 80
Figure 5.1. Strain-dependent Mott transition in SRO. 93
Figure 5.2. Strain-dependent spin-resolved photoemission spectra. 94
Figure 5.3. Strain-dependent spin-splitting energy size. 96