鉍鐵氧-鈷鐵氧磊晶奈米複合薄膜之多鐵特性研究

Abstract

當利用雷射蒸鍍技術成長鉍鐵氧(BFO)與鈷鐵氧(CFO)薄膜於鈦酸(STO)基板時，透過改變其鍍膜條件，可以成功製備出垂直排列之 BFO-CFO 奈米複合結構。從 X 射線繞射分析儀可以得知，STO 基板會對 BFO 與 CFO 薄膜造成應變效應。特別是在 BFO-CFO/STO 的垂直排列奈米複合結構(VAN)中，CFO 晶柱將使得 STO 基板對 BFO 造成之應變效應產生垂直方向上的應力鬆弛，進而讓磁相變溫度產生位移，其中 BFO 粉末的磁相變溫度約在55K 與 200K；BFO/STO 薄膜的磁相變溫度則在 30K 與 160K 附近。而從BFO/CFO/STO 雙層薄膜及 BFO-CFO/STO VAN 的 M-T 與 C-T 量測中我們可以得知，反鐵磁-鐵磁(BFO-CFO)的耦合強度較反鐵磁-鐵電(BFO)的耦合強度為強，使得此系統之其磁化強度及電容的相變不易被觀察到。此外，BFO/STO 與 BFO/CFO/STO 的 C-T 特性十分相似，兩者間僅有細微的差異， 推測為淬火時 BFO 與 CFO 的鐵磁耦合使得電子在 BFO 中發生自旋的重新取向導致，這個現象在 CFO 晶柱埋於 BFO 環境中時則更為複雜。最後，從R-T 量測中，高密度的 BFO-CFO/STO VAN 薄膜在 30K 附近表現出了明顯的絕緣體-金屬轉變，而這個溫度也低於其他研究者在外加強力磁場下所觀察到 BFO 薄膜的轉變溫度。由此結果我們推測，當晶柱密度足夠大時，將有可能產生一個足夠大的局部磁場，並藉此調變 BFO 環境中的電域結構。更深入的介紹與探討將會在本文中呈現。

BFO-CFO vertically align nanocomposite (VAN) was successfully made by utilizing the different wetting conditions from BFO and CFO film when growing on STO substrate at the same time in the PLD system. From the XRD result, there was strain effect from STO substrate to BFO and CFO film. Especially for BFO-CFO/STO VAN, CFO pillar relaxed the strain of BFO film, and shifted magnetic phase transitions on BFO/STO thin film to around 30 K and 160 K as compared to that of BFO powders where the transitions occured around 55 K and 200 K, respectively. From M-T and C-T measurements on BFO/CFO/STO bilayer and BFO-CFO/STO VAN, antiferromagnetic-ferromagnetic coupling (BFO-CFO) is stronger than antiferromagnetic-ferroelectric coupling (BFO). making the magnetization and capacitance anomalies unobservable in those systems. Furthermore, the C-T beharviors of BFO/STO and BFO/CFO/STO are in general similar, but different in subtle details, which presumably originates from the quenching of spin reorientation in BFO due to ferromagnetic coupling from CFO. The situation is even more complicated in the CFO pillar embedded in BFO matrix sample. Finally, from R-T measurement, the high-density BFO-CFO/STO VAN film showed an apparent insulator-metal transition around 30 K, which is similar to that observed in BFO film under strong external magnetic fields which done by another reseach before. The result suggests that when the pillar density is large enough it may generate strong enough local magnetic field to modify the ferroelectric domain structures in BFO matrix. Further investigations are certainly in order to delineate the interisting emergent phenomena observed in the present study.