低維自旋系統及幾何不穩態系統目前受到很大的關注，若是同時符合上述兩種條件，則在低溫環境下可能會出現與傳統的磁性有序不同之磁性相，其允許結構有序之自旋無序的基態出現，像是量子自旋液體(quantum spin liquid)、量子自旋玻璃(quantum spin glass)。此論文中，我們做了準二維三角反鐵磁鎳鎵硫NiGa2S3.85 之磁性和介電常數的測量與討論，以研究其在低溫下的磁狀態和介電特性。
在變溫下的磁化率測量中，發現凍結溫度出現在溫度6 K，推測此缺硫(S)樣品之化學成分應接近NiGa2S3.85。由施以磁場7 T 的磁化率圖形可見到ZFC(zero-field-cooling，零場降溫曲線)與FC(field-cooling，加場降溫曲線)在低於相變溫度有明顯的分岔現象，這是長程有序的情形，也可能NiGa2S3.85 此樣品在相變溫度以下之磁排列比spin liquid 態更為有序。
變溫之介電常數的測量顯示了一個有趣現象，由於在相鄰層之間的接面的界面極化（稱為馬克士威 - 華格納鬆弛，Maxwell-Wagner relaxation），而造成在變頻率的情況下介電常數相當大的提升。在不同外加磁場下（高達 9 T），介電常數與溫度關係圖形顯示出磁介電 (magnetodielectric)行為可以忽略不計，是磁電阻效應的結果。

Spin systems with low dimensionality and geometrical frustration have attracted interest because of the possible emergence of novel magnetic phases at low temperatures by suppressing conventional magnetic order, and allow for a novel spin-disordered ground state, such as quantum spin liquid and glass without any apparent structural disorder. In this thesis, we have discussed magnetic and dielectric measurements on the quasi-two-dimensional triangular antiferromagnet NiGa2S3.85, in order to investigate its magnetic state and dielectric property at low temperatures.
In the measurement of susceptibility In the measurement of susceptibility χ(T), we found that the freezing temperature is near 6 K. This suggests that the sample with the sulfur deficiency should be close to the stoichiometric NiGa2S3.85. Under 7 T magnetic filed ZFC(zero-field-cooling) and FC(field-cooling) still bifurcate indicating more magnetic order state than spin-liquid.
Temperature-dependent dielectric measurement shows an interesting colossal enhancement of dielectric constant with frequency dispersion and is ascribed to the interfacial polarization (termed as Maxwell-Wagner relaxation) at the interface between adjacent layers. Temperature-dependent dielectric measurement under applied magnetic field (up to 9 T) shows negligible magnetodielectric behavior because of the sample is not magneto resistive.

致謝 i
論文摘要 ii
Abstract iii
目錄 v
圖目錄 vi
表目錄 viii
第一章 :簡介 1
1.1:簡介 1
1.1.1: 幾何自旋不穩態系統(Geometrical spin frustrated system) 1
1.1.2: 鎳鎵硫系統(NiGa2S4 system) 4
1.1.3: 自旋液體(Spin-liquid) 系統 7
1.2:動機 8
第二章 :實驗儀器與方法 9
2.1: 量子超導干涉儀 9
2.1.1: 原理 9
2.1.2: 量測 14
2.2: 介電係數量測 16
2.2.1: 儀器原理 16
2.2.2: 量測方法 17
第三章 :結果與討論 18
3.1: 磁特性 18
3.2: 介電特性 26
第四章 :結論 41
參考文獻 42
附錄 45

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