近年來由於被觀察到超巨磁阻的特性，而使的具有鈣鈦礦結構的錳氧化合物 Ln1-xAxMnO3 (Ln=La, Nd, Pr, and A=Ca, Sr, Ba, Pb) 引起相當廣泛的興趣而成為重要的研究主題。基本上，未經摻雜的LaMnO3 具有絕緣和反鐵磁的特性。藉由使用兩價的鹼金屬取代鑭，化合物中的錳形成 Mn 3+ / Mn4+ 的混價狀態，特性轉變為金屬和鐵磁性，同時呈現出超巨磁阻的現象。

本篇論文主要是研究鈣鈦礦結構超巨磁阻材料 La0.7-xLnxPb0.3Mn1-yMeyO3 (Ln=Pr, Nd and Y, Me=Fe and Co) 的磁傳輸與電特性。藉由化合物中鐠、銣和釔摻雜取代鑭位置，鈷和鐵摻雜取代錳位置來研究取代效應所造成的晶格結構、磁傳輸、與電性的影響。研究結果顯示，鑭位置由鐠、銣取代直接造成晶格扭曲，所有材料因摻雜而產生晶格結構的變化。由x光繞射圖形判讀顯示隨著摻雜量的增加樣品可為菱面體結構甚至為正交體結構。晶格扭曲的增加使得Mn3+－O2-－Mn4+ 的鍵角變小，錳的自旋方向傾斜，導致雙重交換作用減弱，終至鐵磁性的減弱，鐵磁轉換溫度的變小,eg電子能帶寬減小，電阻率增加，然而卻也因為在外加磁場下高電阻的大量降低而造成磁阻率增加,同時由於鐠、銣等磁性離子的介入使得飽和磁矩增加。比較鑭位置的取代結果，相較於鐠和銣，釔取代鑭的效應並不相同。因為釔為非磁性離子，故溫度倚變的磁矩數值顯示其飽和磁矩隨釔含量的增加而減小。相同取代量時，由零場冷卻和場冷卻磁性量測結果可得知釔取代時磁序較鐠或銣取代時短。且其離子半徑較小，晶格較易扭曲，使得Mn3+－O2-－Mn4+ 的鍵角較易變小，而造成鐵磁轉換溫度較小和磁阻率較大。

錳位置的取代造成的晶格扭曲較不明顯，這是因為錳、鈷和鐵的離子半徑差異並不大。所有材料的晶格結構因摻雜而產生結構些微變化，同時由x光繞射圖形判讀顯示樣品可為菱面體結構或正交體結構，溫度倚變的磁矩數值顯示摻雜量的增加將導致扮演鐵磁性雙重交換作用的Mn3+－O2-－Mn4+ 含量減少，而反鐵磁性超交換作用的Mn－O－Me (Me=Fe and Co) 的總含量增加，因而鐵磁特性逐漸減弱。同時由零場冷卻和場冷卻磁性量測可得知材料在鐵磁性轉換溫度下原有的長程耦合鐵磁特性漸被短程耦合取代，鈷與鐵離子取代錳離子抑制雙重交換作用，因而導致鐵磁性的衰減和鐵磁轉換溫度的降低。

綜合比較化合物中鐠、銣和釔摻雜取代鑭位置，鈷和鐵摻雜取代錳位置所造成的晶格結構、磁傳輸、與電性的影響其機制並不相同。鐠、銣和釔摻雜取代鑭位置主要是造成晶格扭曲，使得Mn3+－O2-－Mn4+ 的鍵角變小而抑制鐵磁性雙重交換作用，導致諸特性的改變。鈷和鐵摻雜取代錳位置則是直接減少鐵磁性雙重交換作用Mn3+－O2-－Mn4+ 鍵結出現的機率。本論文的研究在藉由各個元素的摻雜以釐清化合物中各元素所扮演的角色，進一步瞭解此類材料的特性，以期將來在製造薄膜元件時可以充分掌握其物理機制，同時在特性改善上有所依據。

The hole-doped perovskite manganese oxide such as Ln1-xAxMnO3 (Ln = La, Nd, Pr, and A = Ca, Sr, Ba, Pb) is one of the most studied topics in the recent years due to the observation of colossal magnetoresistance (CMR). Basically, LaMnO3 has an almost insulating behavior and on antiferromagnetic arrangement. By substituting a divalent cation (A2+) in place of La3+, LaMnO3 can be driven into metallic and ferromagnetic state. Mixed valence of Mn 3+ / Mn4+ is needed for both metallic
behavior and ferromagnetism in these materials. The CMR characteristic occurs in the ferromagnetic state.

A systematic investigation of the structural, magnetic and electrical properties in the perovskite colossal magnetoresistance materials La0.7-xLnxPb0.3Mn1-yMeyO3 (Ln=Pr, Nd and Y, Me=Fe and Co) has presented in this thesis. By subatituting Nd, Pr, Y for the La and Co, Fe for the Mn, the substitution effects on the crystallographic deformation, magnetotransport behavior and electrical properties in these compounds have been studied.

According to the results of this research, crystallographic distortion is induced by the
substitution of smaller ions, Pr or Nd, onto the La-site. Powder $x$-ray diffraction patterns show a crystallographic transition from rhombohedral symmetry (R-3c) to orthorhombic (Pbnm) crystal structure as the doping content is increased. The increase of deformation from R-3c to Pbnm decreases the bond angle of Mn3+－O2-－Mn4+ , increases the cant of Mn spin, weakens the double-exchange interaction and results in decrease of ferromagnetism, low ferromagnetic transition temperature Tc, eg electron bandwidth and conductivity. However, the great quantity of decrease in resistivity by an external field leads to the increase in the magnetoresistance ratio. We also find that the increase of saturation magnetization results from the contribution of magnetic ion of Pr or Nd. In addition. in contrast to substitution La by magnetic ion of Pr and Nd, the saturation magnetization is decreased as Y content is increased. The zero-field-cool (ZFC) and field-cool (FC) magnetic measurements indicate that the range of spin ordering for Y one is shorter than Pr one or Nd one
with the same doping content. It is because of the small ionic radius of Y, which results in larger distortion, increases the bond angle of Mn3+－O2-－Mn4+, and
corresponds low ferromagnetic transition temperature.

The distortion induced by Mn-site substitution is not obvious due to the similar radius of Mn, Co and Fe. Powder x-ray diffraction patterns show a single phase of rhombohedral symmetry (R-3c) for Co doped ststem and a slight crystallographic transition from rhombohedral (R-3c) to orthorhombic (Pbnm) symmetry for Fe doped system. Values of temperature dependence of magnetization indicate that the ferromagnetic double-exchange interaction is gradually substituted by the
superexchange interaction. The ZFC-FC curves also indicate that long-range spin ordering is progressively substituted by the short-range spin ordering. The substitution of Mn by Co and Fe supresses the double-exchange interaction, decreases the ferromagnetism and the ferromagnetic transition temperature.

Due to the synthesis of the substitution of Nd, Pr, Y for La and Co, Fe for Mn, the mechanism of substitution effects are proved different. The substitution of Nd, Pr and Y for La distorts the crystal, decreases the Mn3+－O2-－Mn4+ bond angle, and results in the transition of properties, while the substitution of Co and Fe for Mn decrease the percentage of ferromagnetic Mn3+－O2-－Mn4+. The purpose of this thesis is to clear up the role functions of all elements in these compounds and properties of these compounds. Based on the knowledge of these compounds, it would be helpful to control the physical mechanism and improve the characteristics on preparing their thin film devices.

CONTENTS
中文摘要
ABSTRACT
致謝
CONTENTS
LIST OF FIGURES
LIST OF TABLES
1.INTRODUCTION
2.THEORY
3.EXPERIMENTAL
3.1 Specimens preparation
3.2 Magnetic properties measurements
3.3 Electrical properties measurement
4 RESULTS AND DISSUSSION
4.1 Magnetic and electrical properties of archetype La0.7Pb0.3MnO3
4.2 Substitution e_ect of La-site in La0.7Pb0.3MnO3 system
4.2.1 Characteristic of (La0:7_xNdx)Pb0:3MnO3 system
4.2.2 Characteristic of (La0:7_xPrx)Pb0:3MnO3 system
4.2.3 Characteristic of (La0:7_xYx)Pb0:3MnO3 system
4.2.4 Synthesis of La-site substitutions for x=0.1
4.2.5 Synthesis of La-site substitutions for x=0.7
4.3 Substitution e_ect of Mn-site in La0:7Pb0:3MnO3 system
4.3.1 Characteristic of La0:7Pb0:3(Mn1_xCox)O3 system
4.3.2 Characteristic of La0:7Pb0:3(Mn1_xFex)O3 system
4.3.3 Synthesis of Mn-site substitutions
CONCLUSIONS
REFERENCES

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