在自旋不穩態系統的研究中，由於其內秉不穩、複雜的競爭交換作用和本身獨特的結構造成磁陀具有序無序的現象，如此引發很多有趣的物理特性。為了探討其背後的物理機制，極限條件下的實驗遂成為一有力的工具，其包括在強磁場及高壓下的實驗環境。
在本論文中，我們探討兩種自旋不穩態系統，分別為準一維雙鏈反鐵磁材料鈉釩氧和立方亞鐵磁材料硒銅氧。鈉釩氧的反鐵磁相變TN為140 K，於外加磁場 介於1至5特斯拉下有兩個子相被發現且其對應不同的溫度TN1和TN2。在強磁場下，TN和TN1幾乎不受影響，而TN2與外加磁場為一非線性的關係。同樣的，壓力下的研究顯示TN和TN1隨著壓力而下降，且TN2亦與壓力呈現非線性關係。另一方面，硒銅氧的亞鐵磁相變為58 K，其在磁場及壓力下磁化率對溫度的關係顯示原亞鐵磁相變往高溫區移動。有趣的是在高壓實驗中，無論在常壓或高壓下的條件我們觀察到溫度低於亞鐵磁相變的磁滯曲線有自旋反轉的現象。以上顯示硒銅氧在高磁場及壓力下會使磁陀提早排列有序。總而言之，本論文中探討的兩個樣品鈉釩氧和硒銅氧，無論在高磁場抑或是高壓力的環境下其物理特性都有相同的趨勢。

In the geometrical spin frustrated systems, order-disorder phenomena are interesting, for their intrinsic fluctuation, complicated completing interactions, and lattice structure leading to many intrigue physical behavior. To clarify the mechanism of such systems, experiments under extreme conditions (diverse magnetic fields and hydrostatic pressures) are powerful tools to meet the needs.
In this dissertation, two kind of interesting materials are investigated. One is quasi-1D double chain antiferromagnet NaV2O4, the other is cubic like ferrimagnet Cu2OSeO3. In the polycrystalline compound NaV2O4, it exhibits an antiferromagnetic transition TN at 140 K, together with two field dependence subphases at 1 T≤H≤5 T. The two characteristic temperature TN1 and TN2 are associated to two subphases, which is determined by the peak position in the derivative of magnetization with respect to temperature. Under magnetic field, TN and TN1 remain almost unchanged (linear behavior), while TN2 acts in a nonlinear behavior with the application of magnetic field. Further, TN1 and TN2 are found to decrease roughly linear with applied pressure, while TN2 follows a nonlinear relation with applied pressure. On the other hand, the cubic single crystal Cu2OSeO3 exhibits a ferrimagnetic transition at 58 K, which is shifting to high temperature range with increasing magnetic fields. The peak values (from the mutual inductance measurements) associated with the ferrimagnetic transitions also increase with applied hydrostatic pressures. Moreover, the spin-flipped transitions are observed below transition temperature at ambient and applied pressure (12.67 kbar). The measurements above strongly suggested the ferrimagnetic spin configurations order earlier, i.e. transition temperatures increase with applied magnetic fields and pressures. In summary, the investigated frustrated spin systems (NaV2O4 and Cu2OSeO3) behave with the same trend with applied magnetic fields and hydrostatic pressures. It is possibly induced by the external DC magnetic field and the structure change and/or deformation under pressure.

Abstract…………………………………………………………………………..…1
摘要…………………………………………………………………..……………..…3
Content……………………………………………………………………………...4
List of Figures………………………………………………………………………...6
Chapter 1: Introduction
1.1: Geometrically frustrated spin system…………………………………………9
1.2: NaV2O4 and Cu2OSeO3 systems…………………………………………...14
Chapter 2: Experimental instruments
2.1: High pressure AC susceptibility……………………………………………..22
2.1.1: Principle……………………………………………………………...…22
2.1.2: Instrumentation……………………………………………………….23
2.1.3: Perform the experiment…………………………………………………27
2.2: SQUID magnetometer……………………………………………………….29
2.2.1: Principle………………………………………………………………...29
2.2.2: Measurements…………………………………………………………..33
2.2.3: Homemade pressure apparatus at SQUID…………………………….. 38
2.3: Pulsed magnetic field experiment…………………………………………...41
Chapter 3: Theory
3.1: Mechanism of exchange interactions………………………………………..43
3.1.1: Magnetic dipole interaction………………………………………….…44
3.1.2: Direct exchange………………………………………………………...44
3.1.3: Indirect exchange……………………………………………………….46
3.2: Pseudospin-lattice coupled-mode model (PLCM)…………………………..50
3.2.1: Definitions……………………………………………………………....50
3.2.2: Spectral representation………………………………………………....52
3.2.3: An example of the application by PLCM.................................................53
Chapter 4: Results and discussion
4.1: NaV2O4 system……………………………………………………………..55
4.1: Cu2OSeO3 system…………………………………………………………..61
Chapter 5: Conclusion
5.1: NaV2O4 system……………………………………………………………..68
5.1: Cu2OSeO3 system…………………………………………………………..69
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