三角(四面)型的自旋不穩態和低維度系統常被認為具有磁介電效應的特性，因此具有四面自旋不穩態和低維度系統的銅鍗氧溴(2252)樣品也被猜測極有可能具有磁介電的特性。此銅鍗氧溴樣品的結構是由四個銅離子在一平面座標形成四角形群集的結構。在結構中由四個銅離子團構成的四面體之間在a-b平面上具有較弱的作用力，因此這樣品是一個研究在四面型自旋不穩態的交互作用、伴隨低能量激發和四面體間的集體磁誘發的理想材料。在銅鍗氧溴樣品中發現了在To ~ 11.5 K的反鐵磁磁躍遷和在Tm ~ 30 K的磁交互作用力。並且在施加最高磁場為90 kOe、頻率變化從1 kHz至2 MHz的變場介電係數量測中發現低溫區間裡11.5 K及40 K 的峰值皆隨著磁場而增加，所以我們猜測磁介電耦合的提升和四面型不穩態群集裡的交互作用力及Te4+的孤對電子所產生的群集間交互作用力有關。

An intriguing magnetodielectric behavior is observed in triangular or tetrahedral frustrated and low-dimensional system. Therefore, the spin-tetrahedral and low-dimensional compound copper-tellurides (Cu2Te2O5Br2) is suggested that has magnetodielectric behavior. Tetragonal Cu2Te2O5Br2 contains clusters of four Cu2+ (S = 1/2) in a planar coordination. These tetrahedral form weakly coupled sheets within the crystallographic a-b plane. Therefore, this system is ideal to study the interplay between the spin frustration on a tetrahedron with localized low-energy excitations and collective magnetism induced by inter-tetrahedral couplings. In this material a strongly reduced magnetic transition temperature To = 11.5 K in comparison with a dominant magnetic exchange of 30 K is found. Low-dimensional systems with triangular geometries are considered as prominent candidates for applications using novel magnetoelectric materials. At the highest applied magnetic field 90 kOe, the temperature dependent dielectric behavior with almost frequency independent well defined maxima at Tm ~ 30 K and To ~ 11.5 K are enhanced compared with that at zero field. We suggest that the observed magnetodielectric coupling can arise from exchange striction involving frustrated tetramer clusters and inter-cluster exchange bridges with polarizable lone-pair electrons on Te4+ ions.

Content……………………………………………………………………………………4
List of Figure…………………………………………….......................................……...6
Chapter 1 Introduction
1.1 General introduction…………………………………………………………….9
1.2 Spin frustrated system…………………………………………..……………11
1.3 Magnetoelectric effects and multiferroicity……………………………………14
1.4 Cu2Te2O5Br2 system……………………..………………………..….………....15
1.4.1 Structural details..…..…………………………………………………..16
1.4.2 Magnetic measurement………………………………………..………..17
1.4.3 Specific heat measurement……………………………………………...18
1.4.4 Neutron Diffraction studies………………………….………………….20
1.5 Motivation……………………………………………………………………...20

Chapter 2 Experimental instrumentations and techniques
2.1 Magnetic properties measurement system...........……………………………...22
2.1.1 Instrumentations………………………… ………………….….22
2.1.1.1 Temperature control…...………………………………………....24
2.1.1.2 Magnet…………………………………………………………...26
2.1.2 Signal detection………………………………….……………….30
2.1.2.1 Superconducting Quantum Interference Divice (SQUID)……….30
2.1.2.2 Transverse system in MPMS…………………………………….31
2.1.3 Reciprocating sample option (RSO)……………………………………33
2.1.4 Field cooled (FC) and Zero field cooled (ZFC) measurements………...36
2.2 Electric measurements…………………………………………………………37
2.2.1 Principle………………………………………………………………...37
2.2.2 Dielectric measurements in magnetic field……………………………..42
2.2.3 Electric polarization…………………………………………………….45
Chapter 3 Results and discussion
3.1 Magnetic properties…………………………………………………………….46
3.2 Dielectric properties……………………………………………………………51
3.2.1 Frequency and temperature dielectric behavior and Polarization.……...51
3.2.2 Magnetodielectric effect……….………………………………………..56
Chapter 4 Conclusion………………………………………………………………….58

References …………………………………………………………………………….59

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