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National Chung Hsing University Institutional Repository - NCHUIR > 工學院 > 材料科學與工程學系 > 依資料類型分類 > 碩博士論文 >  離子束轟擊及反鐵磁層厚度對磁性薄膜之結構及磁性質研究

Please use this identifier to cite or link to this item: http://nchuir.lib.nchu.edu.tw/handle/309270000/152971

標題: 離子束轟擊及反鐵磁層厚度對磁性薄膜之結構及磁性質研究
The influence of ion-beam bombardment and anti-ferromagnetic layer thicknesses Of magnetic thin films
作者: 陳姵希
Chen, Pei-Shi
Contributors: 林克偉
Ko-Wei Lin
材料科學與工程學系所
關鍵字: 雙離子束濺鍍;交換偏壓;矯頑磁力
Ion Beam Assisted Deposition;exchange bias;coercivity
日期: 2012
Issue Date: 2013-11-18 11:39:22 (UTC+8)
Publisher: 材料科學與工程學系所
摘要: 本研究主要分成兩大部分:利用雙離子束濺鍍系統製備(i)鈷鐵/鈷鐵氧化物雙層薄膜,其中經輔助離子束(End-Hall)以0~70V不同能量轟擊鈷鐵氧化物表面,探討轟擊過後之影響;(ii)氧化鈷/鈷雙層薄膜,藉由改變氧化鈷的厚度,來探討不同厚度下對雙層薄膜之結構及磁性質效應。

從TEM結果顯示鈷鐵/鈷鐵氧化物雙層薄膜為h.c.p.結構之鈷鐵,晶格常數約為a=2.4 A,c=4.0 A,以及fcc岩鹽(rock-salt)結構,晶格常數約為a=4.4 A之鈷鐵氧化物所組成,其晶粒大小皆介於3~14 nm之間。磁性質分析結果顯示,在場冷過程中(T=50K),離子束轟擊能量最高之雙層薄膜(VEH=70V)具有最大的矯頑磁力(Hc~ 356 Oe)及交換偏壓(Hex~ -259 Oe),而未經離子束轟擊之雙層薄膜其交換偏壓為最小(Hex~ -155 Oe),顯示經離子束轟擊後會改變反鐵磁層之自旋結構,且有助於提升交換偏壓(Hex)。 然而在不同溫度(T=50~400 K)之零場冷及場冷下(Happ= 100 Oe)結果顯示,經離子束轟擊後會造成blocking temperature(TB)的增加,在轟擊能量為60V時,有最大值230K,可能為轟擊過後,形成非補償狀態(uncompensated state),導致分子間的交互作用力增加,故TB較高。

根據XRD分析氧化鈷/鈷顯示,底層鐵磁Co薄膜為h.c.p.結構,晶格常數為a=2.5 A,c=3.9 A,而上層反鐵磁CoO(15% O2/Ar)薄膜為fcc岩鹽(rock-salt)結構,晶格常數為a=4.3 A。磁性質分析方面,經場冷後(T=200K),在外加場+100Oe下,皆呈現負交換偏壓(-Hex),在CoO(117nm) 有最大值負交換偏壓(-Hex)約為-200 Oe;在外加場-100 Oe下,交換偏壓(Hex)正負號皆有出現,在CoO (170nm)有最大值正交換偏壓(+Hex)約為+140Oe。然而隨著不同反鐵磁層厚度下,其交換偏壓有些微改變,其可能是反鐵磁磁區的形成所導致,因此不同反鐵磁厚度會影響鐵磁/反鐵磁界面間交換耦合作用以及反鐵磁層之自旋結構,因而造成交換偏壓效應的改變。然而在NiFe/CoO雙層膜則呈現不同的現象,對於不同反鐵磁層厚度則沒有太大的效應,推測其因鐵磁層NiFe異向性小於Co,故在鐵磁/反鐵磁層中,交換耦合作用較弱。
A series of (i)CoFe(15 nm)/(Co,Fe)O(20 nm) and (ii)CoO(x nm)/Co(65 nm) bilayers were fabricated using a dual ion-beam deposition technique.
(i) CoFe/(Co,Fe)O bilayers enables different interfacial conditions to be
created using Ar-ion bombardment voltages. Microstructural characterization has shown that the top CoFe layer consisted of a h.c.p. structure (a~ 2.4 A, c~ 3.9 A) whereas the bottom (Co,Fe)O layer consisted of a rock-salt f.c.c. solid solution (a~ 4.3 A). Magnetometry results at 50 K after a 12 kOe field-cooling process, the CoFe/(Co,Fe)O VEH= 0 V bilayer exhibited an enhanced coercivity (Hc~ 300 Oe). However, a CoFe/(Co,Fe)O where the (Co,Fe)O surface was VEH=50 V Ar-ion bombarded resulted in an enhanced Hex ~ -250 Oe at 50 K, like from more uncompensated AF spins from the roughed interface.Increasing the VEH to 70 V resulted a decrease in Hex (~ -230 Oe) that is attributed to an increase in AF (Co,Fe)O spin misalignment at the AF/FM interface.
(ii) CoO/Co with different CoO thicknesses (20 to 160nm) were studied the competition between interface and bulk AF spin structures responsible for the exchange bias. Microstructural characterization has shown that the CoO/Co bilayer consisted of a rock-salt structure (a~ 4.25 A) and a h.c.p. structure (a~ 2.53 A, c~ 4.26 A).Magnetic properties have shown that under a positive field (HFC= +100 Oe), all CoO/Co bilayers exhibited typical hysteresis loop shift with negative exchange bias field. However compared to the CoO/Co bilayers, the NiFe/CoO bilayers exhibited the conventional exchange bias effects where the positive HFC resulted in negative Hex whereas the negative HFC resulted in positive Hex. In addition, the larger exchange bias strengths in CoO/Co bilayers are attributed to larger magnetocrystalline anisotropy of Co over NiFe. Further, the variations of Hex in CoO/Co bilayers are likely due to the formation of domains throughout the AF CoO volume. Whether the exchange bias effects in CoO/Co bilayers affected by coupling at FM/AF interfaces or by AF CoO spin structures depends on AF CoO thicknesses.
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