臺灣博碩士論文加值系統

本實驗利用濺鍍系統製作具有垂直異向性的人工反鐵磁薄膜，SUB/Ta(10)/MgO(1)/CoFeB(1.4)/W(1)/CoFeB(1.1)/MgO(1)/Ta(3) (單位: nm)，此結構因MgO/CoFeB間的界面效應而具有垂直磁異向性(PMA)。蝕刻成雙十字結構，並量測異常霍爾電阻以及面電阻。量測可以分為:1.固定電流磁場掃描2.固定磁場電流掃描，量測的方式是將電流注入Hall bar元件中，電流方向定為x方向，分別施加垂直膜面(Hz)、平行電流(Hx)以及垂直電流(Hy)的磁場。

本實驗主要呈現結果如下:
1.磁性量測上結果得知上下結構的有效厚度分別為0.649 nm 和0.511 nm，磁化強度為1600 emu/cm3 ，總死磁層厚度為1.34 nm，而異向能場H_k為4000 Oe 與耦合能場H_s為300 Oe。
2.在Rxy(Hz)磁滯曲線與M(H)磁滯曲線比較，明顯的產生了變化呈現出電流效應，原先在M(H)中為3次翻轉分別為正平行態、正反平行態與負平行態，而在Rxy(Hz)中小電流維持與M(H)一樣3次翻轉，但是在大電流時則變為4次轉翻多出負反平行態的翻轉。由於正負電流量測結果相同，因此推測主是受field-like torque 影響，大電流下磁性態變為朝y方向傾斜的反平行態。
3.在Rxy(Hx)中小電流下磁矩明顯受到外加場影響，隨著外加場方向產生x分量上的變化，且同時damping-like torque的加入使磁矩產生z方向的等效場因而產生正負反平行態間的翻轉，隨著電流增加Rxy(Hx)磁性態逐漸變得與Rxy(Hz)相同。而也因為有damping-like torque加入使得磁滯曲線在正負電流走向顛倒。
4.在電流掃描Rxy(I)中由J-H相圖可以看見大小場下斜率明顯不同表示其翻轉機制的不同，而Rxy(Hz)與Rxy(Hx)所看到的臨界電流現象也明顯呈現在上面。小場下由field-like torque主導翻轉電流皆大於臨界電流，磁矩主要偏向在z-y平面上翻轉；大場則主要受外加場影響在z-x平面上翻轉。

A series of magnetic multilayered structures are fabricated by sputtering and the current induced magnetization switching by spin Hall effect is studied in this thesis. The samples include: Si(substrate)/Ta(10)/MgO(1)/CoFeB(x)/W(1)/CoFeB(1.1)/MgO(1)/Ta(3), x= 1.2, 1.3, 1.4 (units are nanometer). All samples are characterized by vibrating sample magnetometer (VSM) and the one with x= 1.4 is also measured by Hall resistance measurements. The anomalous Hall resistance (Rxy) measurements include the current scan Rxy(I) and field scan Rxy(H).

The results of sample x=1.4 nm are divided into two parts:

1.The characterization of VSM: the effective film thickness of the top CoFeB is 0.649 nm and bottom CoFeB is 0.511 nm. The total dead layer is 1.34 nm. The magnetization of the multilayer is 1600 emu/cm3, the anisotropy field is 4000 Oe, and the switching field is 300 Oe.

2.The results of anomalous Hall resistance measurements are further divided in to three parts:

A.Rxy(Hz):
The comparison of the Rxy(Hz) curve with the M(H) curve shows a significant change. The magnetic state varies as current density varies, indicating the current effect due to spin current. This change becomes significant at Current intensity more than ±12 mA, it changed from three magnetic states to four magnetic states. Because of the results of positive and negative current measurements are invariant, this effect must be mainly affected by the field-like torque. The magnetic state varies from the z-direction to the orientation with part of the component in the y-direction. So we can find the critical point (± 12 mA) in the diagram. At the critical point the magnetic state transforms from the a-state into the c-state.

B.Rxy(Hx):
The results of field scan Rxy(Hx) show that the magnetic states change between c-state and inverse c-state by current induced Hz,eff. No obovios spin flop state (v-state) is observed probably because the anisotropy energy is lager than the indicating a strong spin current effect. Basically, the magnetic moments are confined in z-x plane at low current density but the gradually change to be confined z-y plane at I>12mA.

C.Rxy(I):
Rxy(I) measures the anomalous Hall resistance by current scan from 25 mA to – 25 mA with a fixed Hx. The critical current can also be seen in the (J-Hx) phase diagram. We found that the diagram may be divided into a inner region (-100 Oe < Hx < 100 Oe) and outer region elsewhere within the range of 500 Oe. At inner region the switching current is more than the critical current and the slope is larger than outer region. Zero field switching is also observed indicating that the field like torque may initiate process and break the in-plane symmetry.

致謝 i
摘要 ii
圖目錄 vii
第一章 序論 1
第二章 理論背景 5
2.1 人工反鐵磁磁性分析及相圖 5
2.2 電流效應 10
2.3 零場翻轉 14
第三章 實驗方法 17
3.1 樣品製作流程 17
3.2 電性量測 19
第四章 實驗結果與討論 21
4.1 磁性討論 21
4.2 Rxy(Hz) 23
4.3 Rxy(Hx) 28
4.4 Rxy(I)零場翻轉 35
第五章 結論 39
參考資料 41
附錄 42

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