在爲數不多的複鐵式材料中，鐵酸鉍(BiFeO3)是目前唯一被證實居禮溫度(TC)與尼爾溫度(TN)兩個轉變溫度都在室溫以上的複鐵式材料。BiFeO3同時擁有反鐵磁特性與相當大的鐵電極化量，並具備磁-電耦合特性，因此擁有製作成元件的潛力。我們的目標就是運用BiFeO3為釘札層製作全氧化物自旋閥元件，其他各層材料為：Zn0.7Ni0.3Fe2O4/ LaNiO3/ Zn0.7Ni0.3Fe2O4/ BiFeO3/ LaNiO3/ SrTiO3。本論文詳細討論這種新穎元件的製作過程及其所遇到的問題，其中包括自製氧化物靶材，射頻磁控濺鍍成長各層氧化物薄膜，以及各層膜的物理特性與異質磊晶引起的晶體結構變形等問題，主要成果概括如下。
LaNiO3金屬氧化物導電特性與晶體結構中氧離子化學劑量比有關，氧空位的形成會造成單位晶胞體積增加，以及結構中部分Ni3+還原成Ni2+而讓具有近似立方的鈣鈦礦結構扭曲，導電率下滑。此外，過多的氧空位讓Ni變得更容易擴散進入相鄰的BiFeO3薄膜，使BiFeO3的漏電流增加。作爲鐵磁層的Zn0.7Ni0.3Fe2O4，其磊晶薄膜成長於BiFeO3表面時，晶體結構會因受到壓縮應力而使氧離子空位減少，晶包體積縮小，但磊晶成長仍造成其晶體結構從立方晶系扭曲變形為四方晶系。此結構扭曲會迫使一部分處於四面體配位中的Zn離子到八面體配位中去，與Fe離子互換，造成飽和磁化量(MS)的降低。Zn0.7Ni0.3Fe2O4/ BiFeO3接面在磁場5k Oe下從400 oC冷卻到室溫後，室溫下量測可觀察到交換偏磁(exchange bias)效應，偏磁場為20 Oe，矯頑力為30 Oe。而偏磁場與矯頑力也會隨著量測溫度呈現大幅度的變化，5K溫度下量測，偏磁場與矯頑力分別增加到150 Oe與900 Oe。另外我們也觀察到，以低於BiFeO3尼爾溫度的 300 OC度退火，依舊能建立交換偏磁，偏磁場與矯頑力分別8 Oe與19 Oe。最終完成的全氧化物自旋閥，在室溫環境下和100 Oe的低磁場範圍內，磁阻變化率為0.86%。但受限於BiFeO3的漏電流，藉由電場控制磁阻的變化的目標尚無法達成。
最後章節討論到，在BiFeO3 B-site上共摻雜Ti4+與Mg2+可以抑制BiFeO3中Fe2+的形成，降低漏電流。且經由不具磁矩的Ti4+與Mg2+稀釋BiFeO3中反鐵磁有序排列的Fe3+離子，會使尼爾溫度(TN)隨摻雜量提高而降低，有利於避免磁場退火時的化學反應。最後完成的Zn0.7Ni0.3Fe2O4/ BiFe0.9(Mg Ti)0.1O3異質磊晶結構，在外加磁場5k Oe下從290 oC冷卻到室溫後，室溫下量測，交換偏磁場為9 Oe，矯頑力為15 Oe。

The aim of this work was to fabricate an all-oxide spin valve using multiferroic BiFeO3 as the antiferromagnetic pinning layer. The proposed architecture was Zn0.7Ni0.3Fe2O4/ LaNiO3/ Zn0.7Ni0.3Fe2O4/ BiFeO3/ LaNiO3/ SrTiO3. We have grown such a multilayered heterostructure epitaxially by the RF magnetron sputtering. X-ray diffraction showed that the films were indeed biaxially aligned with reasonable in-plane and out-of-plane textures, under the growth conditions optimised for achieving good ferroelectric and magnetic properties. Some key physical and material issues for building up such a novel device were addressed, in particular the heteroepitaxy-induced strain effects on the electrical and magnetic properties of each layer and the establishment of exchange bias between BiFeO3 and Zn0.7Ni0.3Fe2O4. The strains caused a significant increase in the coercivity but a decrease in the saturation magnetization of the ferrimagnet used. The all-oxide architecture allowed the spin valve to be field-annealed from a temperature above the Neel point of BFO (380 OC), after which a clear exchange bias was observed. Magnetoresistance was also achieved in such all-oxide spin valves, which was switchable magnetically in a similar way as the conventional metallic spin valves.

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