臺灣博碩士論文加值系統

摘要

最近文獻報導Fe3O4/Mn3O4的多層膜呈現補償點及扭轉相(twisted phase) 的現象，顯示Fe3O4與Mn3O4薄膜在介面呈現反平行之耦合現象。本論文延伸這項工作，進一步探討薄膜厚度的效應，由於薄層間如果要維持的反平行耦合，自旋必須形成螺旋結構，因而薄層的厚度可能有臨界厚度的性質。 我們用分子束磊晶作成三組超晶格，分別為 [Fe3O4(x nm)/Mn3O4(y nm)]z 而(x,y,z)=(16,4,10)，(8,2,20)及(4,1,40)。由於這三組超晶格的Fe3O4及Mn3O4的總厚度相同，均為40 nm及160 nm，而總厚度均為200 nm。如果沒有任何界面或低維的效應，此三組樣品的磁性則應保持一樣。
實驗包括X-光繞射，M-T及M-H量測。磁性量測結果(包括飽和磁化量、餘磁、矯頑力)顯示(x,y,z)=(16,4,10)及(8,2,20)二組樣品非常一致。而(x,y,z)=(4,1,40)的結果則大不相同，此結果表示(1)介面的效應並不十分重要，但(2)Mn3O4的厚度及Fe3O4的厚度分別小於4 nm及1 nm 時，則呈現二維的效應。這項二維的效應分別與Mn3O4或Fe3O4的異向能(K)及交換能(A)有密切的關係。原則上螺旋狀的自旋結構與(A/K)1/2的長度有關，初步估計Mn3O4薄膜的(A/K)1/2大約為幾個nm，與目前觀察到的結果極為相符。
本實驗工作展現出氧化物薄膜Fe3O4與Mn3O4可行成高品質單晶之薄膜，單層的厚度可小到4 nm或1 nm，更重要的觀察是Fe3O4/Mn3O4兩鐵磁層的耦合與垂直方向的螺旋結構有密切關聯，當(x,y,z)=(4,1,40)時，螺旋狀的磁結構無法達成穩定狀態。

Abstract

The compensation point and the twisted phase phenomenon that exist in the multilayer of Fe3O4/Mn3O4 indicate an anti-parallel coupling at the interface between Fe3O4 and Mn3O4. This thesis extends previous work to study the thickness effect of Fe3O4/Mn3O4 superlattices. It is assumed that in order to balance anti-parallel coupling between ferrimagnetic layers, a spiral-like spin configuration must exist for lowering the energy. This effect may thus show a critical thickness of ferrimagnetic layer. Below this critical thickness, an anti-parallel coupling may disappear of fundamentally show different (low dimensional) behavior.
We use molecular beam epitaxy (MBE) to make three Fe3O4/Mn3O4 supperlattices. These are: [Fe3O4(x nm)/Mn3O4(y nm)]z , (x,y,z)=(16,4,10), (8,2,20), (4,1,40). The total thickness of Fe3O4 and Mn3O4 is 40 nm and 160 nm, respectively, for all 3 superlattices and total thickness is 200 nm. Note that if there isn’t any interface nor low dimensional effect, these samples should have the same magnetic response. The experiments include x-ray diffraction (XRD), magnetization vs temperature (M-T) and magnetic hysteresis curves ( M-H). The magnetic results, including magnetization saturation (MS), Magnetic romance (Mr), and Coercivity (Hc) for the samples of (x,y,z)=(16,4,10) and (8,2,20) are very similar. But the sample of (x,y,z)=(4,1,40) is very different with others. These results provides direct evidences that the interface effect in these superlattices is minor, but when the thickness of Mn3O4 and Fe3O4 below 4nm and 1nm a 2-dimention effect is clearly showed. This critical thickness of magnetic layer originates by the spiral configuration of spin at the interface which is associated with the anisotropy energy (K) and exchange energy (A) of Mn3O4 and Fe3O4. In principle, the critical thickness is related to (A/K)1/2. We estimates the (A/K)1/2 of Mn3O4 ~ a few nm, which is consistent with our experiment results.
The present thesis shows that Fe3O4 Mn3O4 mental-oxide film can form high quality single crystal thin film. The single layer can be as thin as 4 or 1 nm. The most important result is that the coupling between two ferromagnetic layers is associated with spiral configuration (twisted phase) in perpendicular direction. When the thickness of Mn3O4 is less than 4 nm, the twisted phase becomes unstable.

目錄

第一章 簡介............................................................................................1
第一節 前言......................................................................................1
第二節 研究動機..............................................................................5
第二章 相關理論......................................................................................7
第一節 交換耦合（exchange coupling）............................................7
第二節 磁性物質............................................................................10
第三節 Fe3O4、Mn3O4、MgO 結構介紹..........................................12
第四節 Mn3O4/F3O4 超晶格...........................................................18
第五節 Spin flopping...................................................................19
第三章 實驗儀器與實驗步驟................................................................22
第一節 實驗儀器............................................................................22
第二節 實驗流程與步驟................................................................31
第四章 實驗結果與討論........................................................................35
第一節 Mn3O4/Fe3O4 超晶格之 M-T 曲線..................................35
第二節 Mn3O4/Fe3O4 超晶格之 M-H...........................................39
第五章 結論............................................................................................56
References................................................................................................58

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