對半摻雜錳氧化物Pr0.5Ba0.5MnO3作了磁學及傳輸性質的量測，發現A位置原子排序程度的改變，對系統的磁學及傳輸性質有相當程度的影響。此系統中有兩種結構扭曲，分別來自於原子尺寸不匹配及結構拉撐效應，與A位置原子之排序密切相關。當A位置原子混亂時，將會使長程A-型的反鐵磁排列遭到壓制；在A位置原子完全有序的Pr0.5Ba0.5MnO3中，二階磁相轉變可能來自於結構扭曲；在混合相的樣品中，由磁相分離而來的集體鍵長擾動效應會引進一個任意場，這個任意場會抑制磁相轉變，此種效應使得磁矩間的行為類似偶極矩間的交互作用，這會使在反平行的磁矩間產生一個應力，這可能為磁化強度與電阻產生階梯式行為的主要原因。此應力會被磁場釋放抵消，故在磁場下，會有鐵磁分量在反鐵磁團塊中析出，進而給出非常陡峭之磁化強度與電阻的躍遷。此種躍遷只發生在低溫，最主要的原因是由於熱擾動之能量遠大於實驗上所能供給的磁能。

The half-doped manganese oxide Pr0.5Ba0.5MnO3 exhibits anomalous magnetic and transport properties with changing the amounts of A-site randomness. There are two kinds of structure distortion related to the fraction of the A-site randomness which arise from size-mismatch and structural stretching effect. The long-range A-type antiferromagnetic ordering is suppressed by the A-site randomness. The structure distortion might be the main factor of the second-order magnetic phase transition in the A-site totally ordered Pr0.5Ba0.5MnO3. The cooperative bond length fluctuation due to magnetic phase segregation introduces a random field that suppresses the magnetic phase transition in the samples with mixed phase. In addition, the stepwise behavior is only observed at low temperatures in the samples with mixed phase. The bond length fluctuation between separated AFM and FM phases makes the interaction of spin moments be dipole-dipole interaction which introduces a strain acting on a pair of antiparallel spins. As the stress is released at the threshold magnetic field Hthreshold, FM regions abruptly precipitate in the AFM clusters instead of gradually growing at the expense of the AFM clusters, giving rise to the extraordinarily sharp steps in magnetization and resistivity.

Acknowledgements　……………………...………….……..…..…..…　i

Chinese Abstract　…………………………..………….……………..　ii

English Abstract　……………………………………….….……..…　iii

Contents　…………………………………………….…………………　iv

List of Figures　………………………………...………………………　vi

List of Tables　……………………………………...…………………..　xi

Exordium　…………………………………………..……………..……　1

Chapter 1	Introduction  ……………………………………………..　3

1.1 The Discovery of Manganites and the Colossal Magnetoresistance Effect  ……………………………………………………..…  3

1.2	Phase Diagram of Half-doped Manganites and Related Issues  ………………………………………………………… 8

1.3 Motivation  …………………………………………………  11

Chapter 2	Theoretical Background  …………………………….…　13

2.1 The Rietveld Method for Crystal Structure Refinement　…..  13

2.2 Double Exchange Mechanism　…………………………..…　16

2.3 Jahn-Teller Distortion　…………………………….……..…　22

2.4 Sperexchange Mechanism　……………………..……..……　26

2.5 Magnetic Phase Transition　……………………………..…　28
Chapter 3	Experimental Techniques ………………..………………　33

3.1 Setup of Tube Furnace　……………....………….…………　33

3.2 Sample Preparation　……………..…………………….……　34

3.3 The Powder X-ray Diffraction　…………………………..…　38

3.4 Measurement of Resistance　………..………………………　40

3.5 Physical Properties Measurement System (PPMS)　…..……　45

3.6 Field Emission Scanning Electron Microscope　……………　50

Chapter 4	Results and Discussion  ……………….……..…………　51

Chapter 5	Conclusion  ………………………………………………　90

References　……………………………………………..…………..…　92

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