在本論文中，我們探討三種幾何自旋頓挫態系統的新穎特性，分別是Cu2OSeO3共享角四面體、Cu2OCl2焦綠石結構及Cu3Bi(SeO3)2O2Cl2籠紋晶格。

新穎材料Cu2OSeO3在特定溫度及磁場下，其磁結構呈現渦流狀的Skyrmion態，透過外加物理壓力(壓縮)、化學壓力(膨脹)與摻雜磁性鎳元素和非磁性鋅元素用以探討對Skyrmion的相變化及背後物理機制。藉由交流磁化率的測量中發現Cu2OSeO3在摻雜非磁性鋅元素下，其Skyrmion態分裂成兩個Skyrmion態，從摻雜磁性鎳元素的系統中，鎳元素佔據銅(I)的位置並誘發姜-泰勒效應，因而使磁場-溫度相圖中Skyrmion態的存活區塊變巨大，在施加高壓的系統中發現Skyrmion態的區塊隨著外加物理壓力下變大，這個現象可歸咎於自旋-軌道耦合與鐵磁交換作用力的比值增大。Cu2OSeO3系統內自旋-軌道耦合與鐵磁交換作用力的比值隨著碲元素取代下變小，導致Skyrmion態漸漸消失。

利用變溫變磁場磁化強度、比熱、介電及極化率的量測中，Cu2OCl2焦綠石晶格及Cu3Bi(SeO3)2O2Cl2籠紋晶格皆被觀測到有第二類多鐵特性，Cu2OCl2多鐵特性的機制與反對稱交換有關，另外新多鐵材料Cu3Bi(SeO3)2O2Cl2的機制為在外加磁場下誘發自旋翻轉，因而破壞bc平面的磁二重對稱，最後在c方向產生極化。

The purpose of this dissertation is to study three geometrical spin-frustrated systems, such as the corner-sharing of tetrahedron in Cu2OSeO3, pyrochlore structure in Cu2OCl2, and Kagome lattice in Cu3Bi(SeO3)2O2Cl.

Novel skyrmion phase in Cu2OSeO3 is a remarkable characteristic as a vortex-like spin configuration in specific magnetic field and temperature region. Using the effects of compression pressure, chemical expansion, non-magnetic (Zn) and magnetic (Ni) element doping on Cu2OSeO3, outstanding and interesting research findings were investigated. The skyrmion phase region becomes split upon Zn doping concentration. Large enhancement of skyrmion area upon Ni-doped could be ascribed to the magnetic anisotropy of the Jahn-Teller active Ni2+ ion in Cu(I) site. Under the external pressure, the skyrmion zone is enlarged owing to the enhancement of strengths of competing Dzyaloshinsky–Moriya (DM) and Heisenberg exchange (J) interactions. Skyrmion zone is gradually suppressed with Te doping, which could be related to the decrease of D/J parameter.

Temperature and magnetic field dependent magnetization, specific heat, dielectric, and polarization properties in pyrochlore Cu2OCl2 and Kagome single crystal Cu3Bi(SeO3)2O2Cl were established as new type-II multiferroics. The mechanism of multiferroic behavior in Cu2OCl2 is related to the inverse Dzyaloshinskii–Moriya (DM) interaction. Furthermore, the key mechanism of the anisotropic spin-flip induced multiferroicity in Cu3Bi(SeO3)2O2Cl can be ascribed to the breaking of magnetic two-fold symmetry in the bc plance above Hc.

摘要 i
Abstract ii
Content iii
List of figures v
List of tables xiv
Chapter 1: Introduction 1
1.1: Skyrmion 1
1.1.1: Skyrmion research in B20 systems 1
1.2: Multiferroics 4
1.3: Motivations 7
Chapter 2: Splitting of skyrmion phase in Zn doped Cu2OSeO3 10
2.1: Sample synthesis and characterization in Zn doped Cu2OSeO3 10
2.2: Magnetic properties and H-T phase diagram in Zn doped Cu2OSeO3 12
Chapter 3: Enhancement of skyrmion stability in Ni doped Cu2OSeO3 24
3.1: Sample synthesis and characterization in Ni doped Cu2OSeO3 24
3.2: Magnetic properties and H-T phase diagram in Ni doped Cu2OSeO3 31
3.3: Electrical properties 35
Chapter 4: Pressure effects on the skyrmion phase in Cu2OSeO3 38
4.1: Sample synthesis and characterization in Cu2OSe1-xTexO3 38
4.2: Magnetic properties and H-T phase diagram under high pressure in Cu2OSeO3 40
4.3: Magnetic properties and H-T phase diagram in Cu2OSe1-xTexO3 43
Chapter 5: Multiferroics in spin frustrated system Cu2OCl2 49
5.1: Sample synthesis and characterization in Cu2OCl2 49
5.2: Magnetic properties 49
5.3: Electrical properties 50
5.4: Temperature dependent synchrotron X-ray diffractions 51
Chapter 6: Anisotropic spin-flip induced multiferroic behavior in Cu3Bi(SeO3)2O2Cl 60
6.1: Magnetic properties 60
6.2: Electrical properties 74
Chapter 7: Anisotropic pressure effects on the Kagome Cu3Bi(SeO3)2O2Cl 71
7.1: Sample synthesis and characterization in Cu3Bi(Se1-xTexO3)2O2Cl 72
7.2: Magnetic properties 75
Chapter 8: Conclusions 82
References 93
List of publications 108

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