Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/10763
標題: 氧含量對鈷鐵/鈷鐵氧化物奈米雙層薄膜結構及磁性質之研究
Effect of Oxygen Contents on Structures and Magnetic Properties of CoFe/(Co,Fe)-oxide Bilayers
作者: 楊侃儒
Yang, Kan-Ju
關鍵字: exchange bias
交換偏壓
ion-beam deposition technique
雙離子束濺鍍技術
出版社: 材料科學與工程學系所
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摘要: 本研究為利用雙離子束濺鍍系統製備鈷鐵/鈷鐵氧化物[CoFe/(Co,Fe)-oxide]雙層薄膜,主要分為四個部份,(i)改變反鐵磁層之氧含量(8~41% O2/Ar),接著將反鐵磁層進行700˚C快速退火,探討初鍍與退火後對於結構與磁性質造成之影響;(ii)以輔助離子束(End-Hall)對41% O2/Ar之鈷鐵氧化物以0~150V不同能量進行轟擊,再鍍上金屬鈷鐵,探討轟擊所造成之改變;(iii)將雙層薄膜鍍於MgO(110)、(100)單晶基板,比較單晶基板與SiO2基板相異之處,並互相比較;(iv)鍍雙層膜時施以一外加磁場(Happ~155 Oe),並改變鐵磁層與反鐵磁層之厚度,探討對磁性質所造成之影響。 結構分析顯示:鈷鐵薄膜為h.c.p.結構,晶格常數a~2.52 Å,c~3.87 Å;初鍍之鈷鐵氧化物為rock-salt結構的(Co,Fe)O,晶格常數a~4.25 Å,晶粒大小約為3~10 nm,而退火後其會相變化成spinel結構的(Co,Fe)3O4,晶格常數a~8.20 Å,晶粒則成長至5~30 nm。 磁性分析顯示:在5K量測時發現,(i)在初鍍不同氧含量可觀察到負(FM coupling)、正(AF coupling)與趨近於零之交換偏壓場,但在反鐵磁層經退火過後之雙層薄膜的交換偏壓場則明顯變小,且皆為典型符號為負之Hex,這可歸因於退火後造成晶格缺陷消除及相變化成磁晶異向性常數較小之(Co,Fe)3O4所致;(ii)而反鐵磁層經離子束轟擊部分,隨轟擊電壓升高,Hex隨之下降並且符號由正轉變為負,這可能是界面轟擊造成磁矩錯位,甚至翻轉磁矩所造成;(iii)單晶基板部分,在使用MgO(100)單晶基板時其可得到最大之Hc (~1550 Oe),且反鐵磁層有經離子束轟擊(VEH=130V)其Hex之符號由負轉變為正,與SiO2基板相反(正轉變為負),但在MgO(110)單晶基板則無符號轉變的現象(皆為正)。而在旋轉試片角度量測部分,MgO(100)與(110)單晶基板在旋轉試片角度(90˚)後,其交換偏壓場之符號皆會轉變(耦合狀態),此現象在SiO2基板並無觀察到,推斷與單晶基板具有方向性所造成;(iv)在增加膜厚部分,其矯頑磁力與交換偏壓場皆明顯下降,這可能是因鐵磁層厚度過厚,反鐵磁層厚度不足以對鐵磁層之磁矩進行pinning作用,因而使交換耦合效應降低。 磁電傳輸性質分析顯示:鈷鐵/鈷鐵氧化物雙層薄膜具有異向性磁電阻(AMR)性質。在77K的低溫下,聲子散射的降低造成總磁阻率的上升,在本系統中以CoFe/(Co,Fe)-oxide 41% O2/Ar (VEH=100V)雙層薄膜具有最大的總磁阻率(~2.45%),可能是由於界面間散射增加所致。
The structural and magnetic properties of CoFe(6 nm)/(Co,Fe)-oxide(10 nm) bilayers were investigated. Transmission electron microscopy results have shown that the top CoFe layer consisted of a h.c.p. CoFe phase (a~ 4.52 Å, c~3.87 Å), whereas the bottom (Co,Fe)-oxide layer consisted of a rock-salt (Co,Fe)O (a~4.25 Å). The grain sizes of these polycrystalline CoFe/(Co,Fe)-oxide bilayers range from 3 nm to 10 nm. For ion beam bombardment effect, CoFe/(Co,Fe)O-41% O2/Ar bilayer deposited on a SiO2 substrate, a smooth interface between a top CoFe layer and a bottom (Co,Fe)O layer was revealed via the cross-sectional TEM. All (Co,Fe)O bottom layers exhibited the same layer rock-salt (Co,Fe)O strucutures with no detectable changes in lattice constants. This indicated that the ion-bombardment only altered the the interface spin structures of the (Co,Fe)O. Magnetometry results at 5 K under FC process have shown that an unusually large positive Hex (~ +480 Oe) was observed in a CoFe/(Co,Fe)O (41% O2/Ar)/SiO2 (VEH =0 V, i.e., unbombarded) bilayer. However, the positive Hex decreases with incresing VEH and switches to a conventional negative Hex with VEH greater than 100 V. The changes in Hex are attributed to uncompensated and misaliged AF (Co,Fe)O spins created due to the Ar ion-beam bombardment. In addition, very different exchange bias effects were observed when the (Co,Fe)O bilayer was deposited on a MgO (100) and (110) substrate. The CoFe/(Co,Fe)O (VEH =0 V) bilayer exhibited a Hex polarity switch (from positive to negative) by changing substrates from MgO(110) to (100). In contrast, positive Hex was preserved with a CoFe/(Co,Fe)O (VEH =130 V) bilayer deposited on either a MgO (110) or (100) substrate. The magnetotransport studies have shown that these CoFe/(Co,Fe)-oxide bilayers exhibit the anisotropic magnetoresistance (AMR) behavior. The total MR ratio measured at 77 K is larger than at room temperature, ascribed to the reduced interfacial scattering between FM and AF layer. The CoFe/(Co,Fe)-oxide 41% O2/Ar (VEH=100V) bilayer has the largest total MR ratio(~2.45%) among all samples at 77K. It is due to the strong anisotropic scattering at the interface.
URI: http://hdl.handle.net/11455/10763
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