Magneto-optical study of perovskite oxides using Sagnac interferometer = 사그낙 간섭계를 이용한 페로브스카이트 산화물의 자기-광학 연구

국문 초록 (Abstract)

저잡음 및 고분해능 측정 기술은 새로운 상의 발견을 가능하게 하고, 복잡한 물리적 현상에 대한 이해를 가능하게 합니다. 특히, 물질의 대칭성(symmetry)을 정확히 판별하는 것은 응집물질계에서 나타날 수 있는 가상의 상의 존재를 검증하는 데 필수적입니다. 예를 들어, 일부 일반적이지 않은 초전도체(unconventional superconductivity) 및 고온 초전도체에서 제안된 고리 전류 상(loop current order)은 시간반전 대칭성을 위배하는 것으로 알려져 있습니다. 이러한 미세한 신호를 검출하기 위해, 연구자들은 자기-광학 효과 측정을 위한 초정밀 사그낙 간섭계를 개발해 왔습니다. 사그낙 간섭계는 모든 가역적 효과(reciprocal effects)를 제거함으로써, 기존 편광판 기반 방식보다 수백 배 더 미세한 신호 검출이 가능합니다.
본 학위논문에서는 사그낙 간섭계를 민감한 자기장 센서로 활용하는 한편, 시간에 따라 변하는 신호를 측정할 수 있도록 사그낙 간섭계를 개발한 결과를 제시합니다. 먼저, 강자성체 및 초전도체 구조에서의 마이크로미터 스케일 자기장 이미징 결과를 소개합니다. 고감도 사그낙 간섭계를 통해, 얇은 니켈(Ni) 박막이 덮인 초전도 나이오븀(Nb) 박막 내에서 단일 양자 볼텍스(quantum vortex) 신호까지 검출할 수 있음을 검증하였습니다. 이어서, 사그낙 간섭계를 기반으로, 시간 분해능을 가지는 자기-광학 측정 방법을 개발하였습니다. 연속파 광원 대신 펄스 광원을 도입함으로써, 수백 피코초 수준의 시간 분해능을 구현하였습니다.
이러한 기술적 개발 이외에도, 페로브스카이트 산화물 Sr2Ir1-xRhxO4의 자기 응답 특성을 정밀하게 측정하고 분석하였습니다. 민감한 자기-광학 커 효과 측정과 스핀 해밀토니안 모델 계산을 통해, Sr2Ir1-xRhxO4가 기존 반강자성 이론으로 설명할 수 없는 독특한 자기 도메인 구조를 지님을 확인하였습니다. 이러한 도메인 구조는 일반적이지 않은 자기 응답을 동반하며, 스핀 네마틱 상(spin nematic phase)과 반강자성 상이 상호작용함을 시사합니다.

다국어 초록 (Multilingual Abstract)

A low-noise and high-resolution measurement enables the discovery of new phases and deepens our understanding of complex physical phenomena. In particular, the accurate determination of symmetry of a matter is crucial for verifying hypothetical phases in condensed matter systems. For instance, certain unconventional superconducting orders or loop current order phase in high-temperature superconductors are believed to brake time-reversal symmetry (TRS). To detect the subtle signals associated with such exotic orders, researchers have developed state-of-the-art techniques such as the Sagnac interferometer for magneto-optical measurements. The Sagnac interferometer’s ability to reject all reciprocal effects allows it to detect signals that are hundreds of times weaker than those accessible with conventional polarizer-based methods.
In this thesis, we propose the utilization of the Sagnac interferometer as a highly sensitive magnetic field sensor and further develop it for time-resolved measurements. First, we demonstrate micron-scale magnetic field imaging of ferromagnetic and superconducting structures. Owing to the high sensitivity of the Sagnac interferometer, we successfully detected the magnetic signature of a single Abrikosov vortex in a superconducting Nb thin film covered with a thin ferromagnetic Ni layer. Second, we introduce a time-resolved magneto-optical Kerr effect measurement technique based on the Sagnac interferometer. By replacing a continuous wave light source to a pulsed one, we achieved a temporal resolution on the order of hundreds of picoseconds.
Beyond these technical advancements, we performed a comprehensive study of the magnetic response of a perovskite oxides Sr2Ir1-xRhxO4 and observed unconventional magnetic behavior. Using sensitive magneto-optical Kerr effect measurements and spin Hamiltonian model calculations, we show that Sr2Ir1-xRhxO4 hosts non-trivial domain structures that cannot be explained by conventional antiferromagnetic theory. These anomalous domain structures give rise to unconventional magnetic responses, suggesting the interplay of a spin nematic order and a canted antiferromagnetic order.

목차 (Table of Contents)

• 1. Introduction 1
• 1.1 Magneto-optics for condensed matter physics 1
• 1.2 Magneto-optical Kerr effect (MOKE) 2
• 1.3 Sagnac interferometer for MOKE measurement 3
• 1.4 Loop-less Sagnac interferometer 5
• 1.4.1 Schematic of the loop-less Sagnac interferometer 5
• 1.4.2 Optical modulation 7
• 1.5 Jones calculus 9
• 2. Experimental methods 16
• 2.1 Polar Kerr effect 16
• 2.1.1 Normal-incidence Sagnac interferometer with off-the-shelf optics 16
• 2.1.2 Normal-incidence Sagnac interferometer in cryogenic refrigerator 17
• 2.2 Longitudinal and transverse Kerr effect 18
• 2.3 Test samples 20
• 2.3.1 Normal-incidence Sagnac interferometer 20
• 2.3.2 Oblique-incidence Sagnac interferometer 24
• 2.4 Noise and stability of the setup 25
• 3. Micron-scale field imaging of ferromagnetic and superconducting structures 28
• 3.1 Magnetic field imaging using magneto-optics 28
• 3.2 Sample fabrication and basic characterization 30
• 3.3 Magnetic field imaging of Nb structures 32
• 3.4 Magnetic field measurement of Nb disk 34
• 3.5 Vortex manipulation via optical heating 36
• 3.6 Experimental details 37
• 3.6.1 Calibration of the conversion factor 37
• 3.6.2 Extracting the vortex contribution 39
• 3.6.3 Background subtraction procedure 39
• 3.6.4 Hysteresis loop of the disk 40
• 4. Magneto-optical study of antiferromagnetic Sr2Ir1-xRhxO4 42
• 4.1 Background 42
• 4.2 Determination of local magnetization vector using the Sagnac interferometer 45
• 4.3 MOKE measurement of undoped Sr2IrO4 45
• 4.4 MOKE measurement of Rh-doped Sr2IrO4 51
• 4.5 Interplay of canted antiferromagnetism and nematic order 53
• 4.6 Model Hamiltonian 54
• 4.7 Appendix 56
• 4.7.1 Magnetic responses at P2, P3, and P6 in undoped Sr2IrO4 56
• 4.7.2 The Hxy term with a phenomenological account of nematicity in the system 57
• 4.7.3 Calculated magnetic responses for undoped Sr2IrO4 59
• 4.7.4 Magnetic responses of the position P0 in undoped Sr2IrO4 60
• 4.7.5 MOKE vs Temperature for various doping levels 61
• 4.7.6 Differential susceptibility of undoped Sr2IrO4 62
• 4.7.7 MOKE image and magnetic responses of the 5% Rh-doped sample 63
• 4.7.8 MOKE measurement and model calculation of the 3% Rh-doped sample 65
• 4.7.9 Background subtraction procedure for the MOKE data 66
• 4.7.10 Table for the various magnetic ground states and its energy 67
• 5. Time-dependent phenomena 69
• 5.1 Background 69
• 5.2 Setup of time-resolved Sagnac interferometer 71
• 5.3 Basic characterization 74
• 5.4 Ferromagnetic resonance of Permalloy thin film 75
• 5.5 Conclusion and outlook 80
• References 81
• Publication List 88
• Korean abstract 89