在本論文中，我們有系統地研究了稀磁性半導體氧化鋅摻雜釩中鐵磁性的來源與多鐵錳酸釔中非正規鐵電性的來源。這項研究的目的是為瞭解稀磁性半導體與多鐵材料的機制與其應用的可能性，由於這兩種材料是新穎自旋電子元件的候選材料。
在第一部分，我們對氧化鋅摻雜釩的氫化退火效應做有系統地檢驗，藉以釐清以氧化鋅為基礎的稀磁性半導體中鐵磁性的來源。X光繞射與釩原子K-edge之近緣吸收光譜的結果指出在此研究中的所有樣品具有良好的置換結構以及提高氫化退火溫度能夠增加微觀結構的缺陷。在氧化鋅摻雜7.5 %釩中，當提高氫化退火溫度時，我們觀察到一磁性的相轉變從順磁性為主的相變成弱鐵磁性的相而且其室溫的飽和磁化量得到了加強。這個磁性的相轉變可以歸因於結構缺陷的增加導致形成較大的等效束縛磁極化子。進一步，我們分析這些樣品的電性傳輸與交流導電率。結果顯示這些樣品的電性傳輸行為呈現埃弗羅斯可變範圍跳躍以及增加氫化退火溫度能夠增大微觀結構缺陷的量和提高導電率。這結果證明了在氧化鋅摻雜釩中磁化強度的變化是與缺陷濃度密切相關的。
另一方面，氧化鋅摻雜2.5 %釩之室溫的鐵磁性機制也有系統地被討論。其磁性與霍爾效應的量測結果顯示出當自由載子濃度達到每立方公分約4.57E18個電子時，其室溫的鐵磁性將被抑制。根據上述的結果，我們推斷以氧化鋅為基礎的稀磁性半導體其鐵磁性的來源不僅與缺陷濃度有關連而且與束縛磁極化子之間的電荷傳輸與交互作用有相關。這項工作對於稀磁性半導體中鐵磁性的機制提供了一個深入的理解。
在第二部分，對於仔細退火下的多鐵錳酸釔之結構相與其晶格的扭曲進行了研究藉以闡明其多鐵性的來源。當退火溫度達到約攝氏1100度時，觀察到一結構的相變從正交晶相轉變為六角晶相。此結構的相變導致了磁性的轉換從三維錳-氧-錳變成二維錳-氧-錳的超交換耦合，此為觀察到反鐵磁到磁性的調制之原因。藉由提高退火溫度，隨著結構扭曲的增加其鐵電性得到了提升。結合氧原子K-edge之近緣吸收光譜與鐵電性的分析結果顯示六角晶的錳酸釔中非正規鐵電性的來源既不是來自於釔原子d0-ness模型所建議的，由Y-Op(在平面上的氧原子)位置之大的偏離中心移動所導致，也不是由於一維的錳原子d0-ness模型所預測的，由未佔用的錳a1g軌域所引起。在六角晶的錳酸釔中非正規的鐵電性其驅動力是從Y-OT(頂端的氧原子)鍵結到OT-Mn鍵結的電荷移轉。這項工作對於稀土錳氧化物中多鐵性的機制提供了一個新的認識。
最後，我們基於稀磁性半導體與多鐵材料的特殊物理特性提出一些可能的應用在於自旋電子元件上。

In this dissertation, we systematically investigated the origin of ferromagnetism in V-doped ZnO diluted magnetic semiconductors (DMSs) and the origin of improper ferroelectricity in YMnO3 multiferroics. The purpose of this research is to understand the mechanisms of DMSs and multiferroics and their possibilities of applications because both materials are the candidate materials for novel spintronic devices.
In the first section, we systematically examined hydrogenated annealing effects on V-doped ZnO to clarify the origin of ferromagnetism in ZnO-based DMSs. The results of x-ray diffraction and x-ray absorption near-edge spectra (XANES) on V K-edge identify that all samples studied here have good substitutional structures and increases of hydrogenated annealing temperature (Tha) could raise the micro-structural defects. In 7.5 % V-doped ZnO, we observed a magnetic transformation from a paramagnetism dominated to a weak ferromagnetic phase and their room temperature (RT) saturation magnetization was enhanced, as increasing Tha. This magnetic transition could be attributed to the increases of structural defects result in forming larger effective bound magnetic polarons (BMPs). Further, we analyzed the electrical transport and ac conductivity for these samples. The results suggest that the electrical transport behaviors of these samples exhibit an Efros’s variable range hopping and increases of the Tha can enlarge the amount of micro-structural defects and enhance their electrical conductivities. This result demonstrates that the variation of magnetization is strongly correlated with the defects density in V-doped ZnO.
In the other hand, the mechanism of RT ferromagnetism for 2.5 % V-doped ZnO was systematically discussed, as well. The magnetic and Hall effect measurements indicate that the RT ferromagnetism was suppressed when the free carrier concentration reaches to about 4.57E18 1/cm3. According above results, we conclude that the origin of ferromagnetism of ZnO-based DMSs is not only associated with the defects concentration, but also correlated to charge-transport and interaction between the BMPs. This work provides a deep understanding for the mechanism of ferromagnetism in DMSs.
In the second section, the structural phase of YMnO3 multiferroics and their lattice distortions upon careful annealing were investigated to elucidate the origin of their multiferroicity. A structural transition from an orthorhombic to hexagonal phase was observed when the annealing temperature (Ta) reached around 1100 degree C. This structural transformation results in a magnetic transition from 3D Mn-O-Mn to 2D Mn-O-Mn superexchange coupling, which is responsible for the observed antiferromagnetic to magnetic modulation. The ferroelectricity was enhanced as escalating of the structural distortion by rising Ta. A combination of XANES on O K-edge and ferroelectric analyses suggests the origin of improper ferroelectricity in hexagonal YMnO3 is neither caused by the large off-center movements of the Y-Op (in-plane oxygen) sites, as suggested by the Y d0-ness model, nor by the unoccupied Mn a1g orbitals, as predicted by the 1D Mn d0-ness model. The charge-transfer from Y-OT (apical oxygen) bonds to OT-Mn bonds is the driving force of improper ferroelectricity in hexagonal YMnO3. This work provides a new understanding for the mechanism of multiferroicity in rare-earth manganites.
Finally, we suggest some possible applications for spintronic devices based on special physical properties of DMSs and multiferroics.

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