論文摘要 Sr2FeMoO6為一室溫低磁場之穿隧式磁阻材料，其優異的磁阻特性使得具雙層鈣鈦礦結構的新穎磁阻材料成為近年來熱門的磁性材料課題。本實驗先以高解析度中子繞射實驗探討當我們以Ca原子取代Sr2FeMoO6樣品中Sr原子位置時，對結構造成影響。結果發現樣品結構由Sr2FeMoO6時的I4/m轉變為Ca2FeMoO6的P21/n，並觀測到其中Fe-O-Mo鍵角發生扭曲的現象，角度從Sr2FeMoO6的180 ?到Ca2FeMoO6的154 ?左右。 接著以高強度中子繞射實驗擬合兩樣品之磁有序結構，發現受到晶體結構扭曲的影響，Fe與Mo之間的直接交換交互作用與Fe-O-Mo之超交換交互作用相互抗衡，Fe磁矩與Mo磁矩的相對位置由Sr2FeMoO6的反方向些許傾斜排列轉成Ca2FeMoO6的同方向的非共線性排列。最後輔以磁化率實驗以及積分強度實驗為樣品的磁有序特性做標定，得Sr2FeMoO6之兩個磁有序相轉變溫度分別為165K(Mo)以及450K(Fe)；Ca2FeMoO6樣品之磁有序相轉變溫度則為150K(Mo)以及380K(Fe)。 Abstract The double provskites A2BB’O6 have attracted lots of interest recently, Sr2FeMoO6 is one of them having great magnetoresistance performances ( high operating temperature and low applied field ). In this experiment, we discuss the influence of replacing the Sr atoms in Sr2FeMoO6 with Ca atoms and found that the structure of Sr2FeMoO6, space group I4/m, changes to P21/n when Ca took the place of Sr. An angle distortion was observed clearly, the angle of Fe-O-Mo changes from 180?( Sr2FeMoO6, I4/m ) to about 154?( Ca2FeMoO6, P21/n ). Then we refine the magnetic structure by fitting the high flux neutron scattering patterns. The direct-exchange interaction of Fe-Mo become stronger because the bonding angle of Fe-O-Mo decreases, and it will balance with the super-exchange interaction of Fe-O-Mo; this makes the relative directions of Fe and Mo moments change from a canted anti-ferromagnetic arrangement to a noncolinear ferromagnetic arrangement. Finally, by measuring the magnetic susceptibility and the integrating intensity vs. temperature, we confirmed that there are two transition temperature in both samples, 165K ( Mo ) and 450K ( Fe ) in Sr2FeMoO6, 150K ( Mo ) and 380K ( Fe ) in Ca2FeMoO6.
