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標題: 鎳鐵/鎳鐵氧化物雙層薄膜之結構及磁性研究
Structural and Magnetic Properties of NiFe/NixFe1-xO Bilayers
作者: 曾譯民
Min, Tzeng Yi
關鍵字: exchange bias;交換偶合;AMR;NiFe;磁電傳輸;鎳鐵
出版社: 材料工程學研究所
X光繞射(XRD)研究顯示:鎳鐵(Ni80Fe20)薄膜(輔助離子束氧含量0%,以下簡稱0% O2)為面心立方(f.c.c.)結構,其晶格常數為a=3.54Å。33% O2之試片由岩鹽(rock-salt)結構組成,其晶格常數為a=4.24Å。17%O2之試片由雙相之f.c.c. NiFe(a = 3.53 Å)及rock-salt NixFe1-xO(a = 4.26 Å)固溶體所組成。電子顯微鏡(TEM)分析顯示:鎳鐵及鎳鐵氧化物薄膜,其晶粒尺寸約5 nm ~ 15 nm。TEM截面試片(X-TEM)研究結果顯示:鎳鐵(~ 25 nm)/鎳鐵氧化物(~40 nm)雙層薄膜之生長模式為垂直於基材表面的柱狀結構。試片經真空熱處理後,產生如下之相變化:annealed 7%O2試片之結構主要包含Ni(a = 3.53 Å)、Fe(a = 2.90 Å)和NiO(a = 4.17 Å)等相;annealed 33%O2試片之結構由Ni(a = 3.53 Å)、NiO(a = 4.17 Å)、Fe3O4(a = 7.98 Å)和NiFe2O4(a = 8.33 Å)等相組成。經真空熱處理後之試片,產生晶粒成長,其晶粒尺寸介於20 nm ~ 85 nm之間。
震動試樣磁力計(VSM)研究結果顯示:鎳鐵薄膜室溫之磁性質為飽和磁化強度Ms = 720 emu/cm3,矯頑磁力Hc = 2 Oe。當氧含量增加至33%O2,試片轉變為反鐵磁之鎳鐵氧化物固溶體。此試片經真空熱處理後具有磁滯曲線,其Hc 300 Oe,主要由於較硬相之NiFe2O4及Fe3O4等相生成所致。
鎳鐵/鎳鐵氧化物(Ni80Fe20/as-deposited 33%O2)雙層薄膜之矯頑磁力、交換偏壓場(He)與溫度間存在一強烈的相依性。
輔助離子束電壓(End-Hall Voltage,VEH = 80 V ~ 150 V)對鎳鐵/鎳鐵氧化物雙層薄膜研究顯示:T = 150 K時,上述試片除VEH = 80 V外,試片之磁滯曲線由兩個minor loop所組成。此顯示高電壓(即高氧含量)時,鎳鐵氧化物之結構由多相構成。此外,當VEH = 120 V時,鎳鐵/鎳鐵氧化物雙層薄膜其He隨溫度降低而增加,且其值均大於零(positive exchange field)。其餘試片之He均小於零(traditional negative exchange field)。上述試片之磁電傳輸性質研究顯示:鎳鐵/鎳鐵氧化物雙層薄膜均具有異向性磁電阻(anisotropic magnetoresistance,AMR)性質。試片室溫之磁電阻值均小於4%。當VEH = 80 V時,試片在T = 77K時具有最大之磁電阻值(total MR ratio 8.04 %)。

The structural and magnetic properties of polycrystalline Ni80Fe20 (~25 nm)/NixFe1-xO (~40 nm) bilayers prepared by a dual ion-beam deposition technique were studied to gain an understanding of exchange interaction between ferromagnetic (FM) and antiferromagnetic (AFM) materials. X-ray diffractometry has shown that the Ni80Fe20 top film layer has a fcc structure (a= 3.54 ) while the bottom NixFe1-xO layer has a rock-salt structure. Different films were examined with a NixFe1-xO bottom layer that was prepared with assist-beam %O2 ratios from 25% to 36% with corresponding End-Hall voltages of 80 V to 150 V. The grain sizes of these Ni80Fe20 and NixFe1-xO range from 5 nm to 15 nm, as revealed by transmission electron microscopy (TEM). The bottom oxide layers undergo complex phase transformation after annealing.
These bilayers exhibit an enhanced coercivity (~ 30 Oe) at room temperature, compared to that of a reference Ni80Fe20 single layer. A strong temperature dependence of exchange field Hex was observed in these Ni80Fe20/NixFe1-xO bilayers whose magnitude increases linearly with decreasing temperature. A double hysteresis loop was observed at T= 150 K for samples with %O2 greater than 30% O2, indicating the existence of multiple magnetic phases in the bottom NixFe1-xO layer for samples with this and higher oxygen content in the assist beam during deposition. All the samples exhibit a typical negative exchange bias as a function of temperature except the Ni80Fe20/NixFe1-xO (34%O2) bilayer, where a positive exchange field Hex~ 60 Oe was observed at T= 150 K. The appearance of positive exchange bias at low temperature is likely due to temperature dependent changes in the interfacial pinning responsible for exchange coupling that are due to a complex NixFe1-xO structure from ion-beam bombardment during film deposition.
The typical anisotropic magnetoresistance (AMR) behavior has been observed in these Ni80Fe20/NixFe1-xO bilayers. The maximum total MR ratio (~ 8.04%) was obtained at T= 77 K in the Ni80Fe20/NixFe1-xO (25%O2) bilayer.
Appears in Collections:材料科學與工程學系

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