Exploring Novel Transport Effects in Magnetic Weyl Semimetal Co₃Sn₂S₂: A Doping Perspective = 자기적 웨일 반금속 Co₃Sn₂S₂에서의 새로운 수송 현상 탐구: 도핑 관점에서의 연구

국문 초록 (Abstract)

토폴로지 자기 물질은 전자 위상, 스핀-궤도 결합, 자기 구조 간의
상호작용을 탐구할 수 있는 독특한 플랫폼을 제공하며, 스핀트로닉스 및 양자 수송
분야에서 유망한 응용 가능성을 지닌다. 본 연구에서는 카그메
자기체(Co3Sn2S2)에서 원소(Bi) 도핑과 비정량적 결함(non-stoichiometric
defect) 조절을 통해 새로운 수송 현상 조절 경로와 외래 스핀 구조(exotic spin
texture) 안정화 가능성을 탐구하였다. 먼저, Co₃Sn₂₋ₓBiₓS₂ (0 ≤ x ≤ 0.15)
단결정을 대상으로 국소 반전 대칭 파괴(local inversion symmetry breaking,
LISB)를 활용하여 국소적으로 Dzyaloshinskii–Moriya 상호작용(DMI)을
유도하였다. 그 결과, 낮은 온도 영역에서 기존의 내재적(intrinsic) 및
외재적(extrinsic) 기여 한계를 넘어서는 비정형 스케일링 관계를 보이며 이상 홀
효과(anomalous Hall effect, AHE)가 강화되었다. 이러한 비정상적 AHE 강화는
두 개의 자기 모멘트와 비자성 불순물이 관여하는 다중 산란 과정에 의해 유발되는
스큐 스캐터링(skew scattering)과 관련이 있는 것으로 해석된다. 강화된 스큐
스캐터링은 약 70 K 이하에서 나타나는 스핀 키랄리티 상관(spin chirality
correlation, Tχ)과 연관되며, 높은 자기장에서도 지속된다. 이러한 결과는
LISB 가 스핀-궤도 결합 현상에 강력하게 영향을 미칠 수 있음을 보여주며,
토폴로지 자기체에서 강건한 스핀 구조 효과를 실현할 수 있는 새로운 경로를
제시한다.
둘째, Co₃₊ₓSn₂₋ₓS₂ (0 ≤ x ≤ 0.3)에서 비정량적 치환을 통한 전자
도핑을 구현하여 카그메 격자를 유지하면서 수송 특성을 향상시켰다. 이를 통해
페르미 준위가 와일 노드(Weyl nodes) 근처로 상승하며, 2 K 에서 전하 운반자가
정공에서 전자로 전환되는 크로스오버 현상이 관찰되었다. 특히 x = 0.27 샘플은
1,150%의 거대 횡자기저항(transverse magnetoresistance, MR)과 18,000
cm²V⁻¹s⁻¹의 초고이동도(carrier mobility)를 기록하여 기존 자기 와일 준금속을
능가하였다. 이러한 결과는 비정량적 도핑이 토폴로지 전자 상태 최적화에 중요한
역할을 함을 보여주며, Co₃₊ₓSn₂₋ₓS₂가 스핀트로닉스 및 양자 수송 응용에
유망한 후보임을 입증한다.
마지막으로, 본 연구는 중심대칭 카그메 자기체 Co₃Sn₂S₂ 단결정에서
비정량적 도핑을 통한 DMI 구현이라는 벌크 물질 엔지니어링 경로를 제시한다.
밀도 범함수 이론(DFT) 계산 결과, 비정량적 Co₃₊ₓSn₂₋ₓS₂에서 층간(Co/Sn)
자리바꿈 결함(out-of-plane anti-site defect)이 DMI 를 유발하는 주요
원인임을 확인하였다. 또한, Co₃₊ₓSn₂₋ₓS₂의 홀 측정 및 자기력 현미경(MFM)
관찰에서 강건한 토폴로지 홀 효과(topological Hall effect, THE)와 버블 형태의
키랄 자기 도메인이 나타났다. THE 의 출현은 비정량적 도핑이 단순히 전자
충만도를 변화시키는 것뿐만 아니라 국소 반전 대칭을 깨고 스핀-궤도 결합
경로를 강화하여 DMI 를 유도함을 의미한다. 종합하면, 본 연구는 원소(Bi) 도핑과
비정량적 결함 조절이 Co₃Sn₂S₂ 카그메 자기체에서 스핀 키랄리티, 전자 위상,
키랄 스핀 구조 및 수송 특성을 조절하는 강력한 전략임을 제시한다. 또한, 무질서
기반 스핀-궤도 현상에 대한 근본적 이해를 제공함과 동시에 차세대 스핀트로닉스
장치를 위한 강건한 토폴로지 스핀 위상 설계에 실질적인 지침을 제시한다.

다국어 초록 (Multilingual Abstract)

Topological magnetic materials offer a unique platform for exploring the interplay
between electronic topology, spin–orbit coupling, and magnetic textures, with promising
applications in spintronics and quantum transport. In this study, we investigate the elemental (Bi)
and non-stoichiometric defect engineering in the Kagome ferromagnet Co3Sn2S2 to uncover new
pathways for tuning novel transport phenomena and stabilizing exotic spin textures. Firstly, we
utilize the local inversion symmetry breaking (LISB) in Co3Sn2-xBixS2 (0 ≤ x ≤ 0.15) single
crystals to induce the Dzyaloshinskii-Moriya interaction (DMI) locally. As a consequence, the
anomalous Hall effect (AHE) is enhanced with an unconventional scaling relation of
conductivities at lower temperatures, beyond the conventional limits of intrinsic and extrinsic
origin. We ascribe this unanticipated enhancement of AHE with an unconventional scaling relation
to the skew scattering induced by the multiple scattering processes that involve two magnetic
moments and a non-magnetic impurity. The enhanced skew scattering correlates with the
emergence of spin chirality correlation below ~ 70 K, defined as Tχ, and persists under high
magnetic fields. These findings highlight how disorder-driven LISB can strongly influence
spin–orbit phenomena, offering a new route to realize robust spin texture effects in topological
magnets.
Secondly, we achieve controlled electron doping in Co3Sn2S2 via non-stoichiometric
substitution in Co3+xSn2-xS2 (0 ≤ x ≤ 0.3), preserving the Kagome lattice while enhancing
transport properties. This method shifts the Fermi level upward close to the Weyl nodes, leading to a crossover from hole to electron conduction at 2 K. The x = 0.27 sample exhibits a giant
transverse magnetoresistance (MR) of 1,150% and ultra-high carrier mobility of 18,000 cm²V⁻¹s⁻¹,
surpassing conventional magnetic Weyl semimetals. Our findings highlight the role of
non-stoichiometric doping in optimizing topological electronic states, positioning Co3+xSn2-xS2 as a
promising candidate for spintronic and quantum transport applications.
Finally, we demonstrate a bulk materials-engineering route to DMI through controlled
nonstoichiometric doping of the centrosymmetric Kagome ferromagnet Co3Sn2S2 single crystal.
Our density functional theory (DFT) calculation shows that the out-of-plane anti-site (Co/Sn)
defects are responsible for DMI in the non-stoichiometric Co3+xSn2-xS2. Furthermore, the Hall and
MFM measurements of Co3+xSn2-xS2 exhibit both a robust topological Hall effect (THE) and
bubble-like chiral magnetic domain. The emergence of THE indicates that non-stoichiometry not
only perturbs electronic filling but also induces DMI by breaking local inversion symmetry and
enhancing spin–orbit coupling pathways. Collectively, these results establish the elemental (Bi) as
well as non-stoichiometric defect engineering as a powerful strategy to manipulate spin chirality,
electronic topology, chiral spin texture, and transport properties in Kagome ferromagnet Co3Sn2S2.
This work provides both fundamental insight into disorder-driven spin–orbit phenomena and
practical guidelines for designing robust topological spin phases for next-generation spintronic
devices.

목차 (Table of Contents)

• CHAPTER 1: INTRODUCTION ······································································· 1
• 1.1 Topology of Electronic Bands ····································································· 4
• 1.2 Transport Properties of Topological Weyl Semimetals ····························· 16
• 1.3 Co3Sn2S2 ··································································································· 26
• 1.4 Doping Strategies ······················································································· 31
• CHAPTER 2: Experimental Techniques ························································· 35
• 2.1 Growth of Co3Sn2S2 and Doped Series ···················································· 36
• 2.2 Single crystal characterizations ·································································· 39
• 2.3 Magnetization Measurement ······································································ 47
• 2.4 Electrical Transport Measurement ····························································· 49
• CHAPTER 3: Enhancing Anomalous Hall Effect and Spin Chirality
• Correlation in Co3Sn2-xBixS2 through Local
• Dzyaloshinskii-Moriya Interaction Engineering ····················· 55
• 3.1 Introduction ································································································ 55
• 3.2 Results and Discussion ··············································································· 58
• 3.3 Conclusion ·································································································· 81
• CHAPTER 4: Enhancing Magnetoresistance and Mobility in Co3+xSn2-xS2
• via Non-Stoichiometric Doping for Fermi Level Engineering 82
• 4.1 Introduction ································································································ 82
• 4.2 Results and Discussion ··············································································· 84
• 4.3 Conclusion ································································································ 104
• CHAPTER 5: Stoichiometry Control as a Route to Chiral Spin Texture in
• the Centrosymmetric Kagome Magnetic Weyl Semimetal
• Co3+xSn2-xS2 ··········································································· 105
• 5.1 Introduction ······························································································ 105
• 5.2Results and Discussion ·············································································· 108
• 5.3 Conclusion ································································································ 123
• CHAPTER 6: Conclusion ················································································ 124
• References ········································································································· 126
• ABSTRACT (Korean) ····················································································· 150