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研究生:林美祝
論文名稱:分子束磊晶Mn3O4/Fe3O4超晶格反平行態中界面效應導致的補償點及相圖
指導教授:陳恭陳恭引用關係
學位類別:碩士
校院名稱:國立中正大學
系所名稱:物理系
學門:自然科學學門
學類:物理學類
論文種類:學術論文
論文出版年:2000
畢業學年度:88
語文別:中文
論文頁數:56
中文關鍵詞:補償點部分補償表面效應界面效應磁壁扭轉相
外文關鍵詞:compensation pointpartial compensationsurface effectinterface effectdomain wallZeeman EnergyExchange Couplingtwisted phase
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我們曾在MgO(001)基板上成長Mn3O4/Fe3O4超晶格,並且在許多單層厚度較薄(<100Å)的樣品中觀察到Fe3O4和Mn3O4呈現反平行態,在改變溫度及外加磁場時觀察到豐富的相圖,其中至少有Mn3O4 Aligned phase、Fe3O4 Aligned phase、twisted phase、純鐵態及鐵磁飽和態。同時在特定溫度有補償點的產生(此時Mn3O4和Fe3O4磁矩大小相等、方向相反) 。
本論文中討論將單層厚度推展到較厚的系統:[Mn3O4 (300Å)/ Fe3O4 (300Å)]x2各層為300Å,總厚度控制在約1200Å,主要的目的在探討界面的影響。在磁性的量測上是使用超導量子干涉儀(SQUID Magnetomen) 包括量測磁場從0到5T,溫度範圍為5K~300K,而方向分別是沿著膜面平行及垂直的方向。在磁場平行於膜面的量測結果中,我們發現在溫度高於約55K時,磁滯曲線呈現如Fe3O4的典型鐵磁性反應,其Ms»320emu/cm3;但在55K以下,磁滯曲線則呈現明顯的溫度效應。最重要的特點在於外加磁場超過2000Oe(或小於-2000Oe)時產生新的迴路,使得磁化強度增加到約470emu/cm3,這些迴路到外加磁場40000Oe時達到飽和。此外,我們也觀察到這些磁滯曲線的Hc(»200Oe)及Mr(»50emu/cm3)在T=25K時都呈現不為零的極小值,與單層較薄的Mn3O4/Fe3O4超晶格相比,目前的系統明顯也具有補償點的作用,表示在界面也有反平行態的產生。
與單層較薄的系統比較,目前系統中的界面效應並不包括整個薄膜部分,因此補償現象只能視為局部的反應,在各單層的中心仍保持原有的鐵磁狀態。我們估計界面呈現如磁壁(成螺旋態)的結構而磁壁的厚度大約100 Å左右,這些磁壁的估計純粹由磁滯曲線中的磁化量的多少而獲得。
此外部分補償點T=25K,附近磁鐵的Hc»200Oe,小於一般Fe3O4物質,可能表示目前的系統不僅類似單磁區(single domain),同時界面的束縛較一般的鐵磁反應要自由的多。

Previous studies have examined the magnetic coupling in a series of Mn3O4/Fe3O4 superlattices on MgO(001) with constituent layer thickness <100Å and found that Mn3O4 and Fe3O4 are antiparallel. The magnetic response of those samples have shown a variety of phases as a function of temperature and external field. Total 5 phases are identified, namely, Mn3O4 Aligned, Fe3O4 Aligned, twisted, ferromagnetic saturated, and pure Fe3O4 phase. In addition, a compensation point are identified at temperature below the Tc of Mn3O4, where magnetic moments of Mn3O4 and Fe3O4 are equal but opposite to each other.
In the thesis, we extend the study to a [Mn3O4 (300Å)/ Fe3O4 (300Å)]x2 multilayer, which has much thick layer thickness so that the interface effect can be compared with previous results. We used a SQUID magnetometer to measure the magnetic property including the magnetization vs. external field(from0-5T) and magnetization vs. temperature (5K-300K), and the directions of magnetic field, either parallel or normal to the surface of the film. In the results of the field parallel to the surface, we found that when the temperature above 55K, the magnetic behavior of the superlattice is similar to the result of a pure Fe3O4 film, and the Ms»320 emu/cm3. However, at temperature below 55K the magnetic response exhibits strong temperature effect. An important observation is that the M-H curves open other loops at high external fields (greater than 2 KOe and smaller than —2 KOe), and the magnetization reaches~470 emu/cm3, (saturated at external field>40KOe) which is close to the expected sum of magnetization of Fe3O4 and Mn3O4 pure layers. Besides, we also found the Hc(»200Oe)and Mr (»50emu/cm3) of the loop at T=25K become minima but without vanishing. Compare with the previous Mn3O4/Fe3O4 superlattice with much thinner layer thickness, present system seems also exhibit a compensate effect. But this system only has the antiparallel state of Fe3O4 and Mn3O4 limited around the interface area and, more precisely, with a partial compensation point at ~ 25K.
A domain wall structure, originally suggested by Camely and coworker, supposes to exist at the interface of Fe3O4 and Mn3O4 layers. The domain wall thickness is related to the Exchange Coupling and Zeeman Energy. We estimate that domain wall thickness in the present system is ~100Å. This length scale is smaller than the layer thickness ~ 300 A and this is why the interface effect is always accompanied by a ferromagnetic base.
Moreover, the Hc ~ 200 Oe at the partial compensation point T=25K, its Hc»200Oe is smaller than Fe3O4, maybe show that this system not only like single domain but also the chain in the interface is more “free” than the normal ferromagnetic effect.

目 錄
第一章 簡介.................................................1
第一節 前言.................................................1
第二節 氧化物多層膜及研究動機................................7
第二章 理論基礎.............................................10
第一節 Fe3O4、Mn3O4結構介紹................................10
第二節 表面態(surfacemode).................................15
第三節 磁相關的能量........................................18
第三章 實驗儀器與實驗步驟....................................23
第一節 實驗儀器............................................23
第二節 實驗步驟............................................31
第四章 實驗結果與討論.......................................35
第五章 結論.......... ......................................52
Reference.....................................................54

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