鈷/鉍/鈷擬自旋閥系統的磁電阻研究

Abstract

摘要 本實驗中，我們研究以半金屬(鉍)為中間層的磁電阻效應。由於費米面的高度異向性、小的載子等效質量和極長的載子平均自由徑使得準金屬鉍擁有極為特殊的電性。在4.2 凱氏溫度下，鉍金屬的平均自由徑長達釐米等級。。另外鉍金屬的電阻率(1.15×10-6 歐姆．米)遠大於一般金屬(10-8 歐姆．米)。因此，以鉍金屬為中間層的樣品會有較大的背景電阻，就有機會使用電流方向垂直膜面的量測方式。這種結構的樣品可以用較簡單的公式來表示也比較容易與理論分析結果相比較。 為得到明確三層接面磁平行與反平行對比態，可以因此有明顯磁阻行為，我利用自旋閥結構，即在三層接面下鍍上反鐵磁性(鐵錳)物質。為確定鐵磁物質/反鐵磁物質介面具有偏倚耦合現象。我們首先研究鎳鐵/鐵錳介面並尋求最佳條件下製造出最大偏倚耦合場。我們發現偏倚耦合場受到在鎳鐵(5 奈米)/鐵猛(20 奈米)的雙層膜偏倚耦合系統中橫向晶粒尺寸的影響最為明顯。利用穿透式電子顯微鏡的截面影像可以顯示出鍍在較厚的鉭底層上之鎳鐵/鐵錳雙層膜的橫向晶粒尺寸較大。我們觀察到在較厚的鉭底層可以得到較大的偏倚耦合場；其原因來自於較厚的鉭底層可以提供較大的橫向晶粒面積增強了鐵磁/反鐵磁介面的偏倚耦合現象。 利用得到的最佳製造條件來製作自旋閥，用以研究三層接面的磁電阻現象。但在本實驗中，受到製程條件過於複雜的原因，產生樣品電阻不穩定，造成擁有自旋閥結構的樣品並沒有得到預期好的磁電阻訊號。 最後本實驗中，採用在鈷(2.5 奈米)/鉍(5 奈米)/鈷(40 奈米)的擬自旋閥樣品，利用奈米接點技術來縮小樣品的有效截面積(100×300 奈米2)以增加背景電阻。為了降低電路中的雜訊，我們採用交流四點量測來決定樣品電性。我們得到鈷/鉍/鈷樣品的背景電阻為2.3 歐姆。這個樣品的電阻率約為0.03%，遠小於理論計算的數值40%。接點電阻和不均勻的電流密度可能是造成低電阻率的原因。

Abstract In this work the magnetoresistance (MR) effect by using semimetal (Bi) as a spacer was investigated. Bismuth is a semimetal with unusual electronic properties related to its highly anisotropic Fermi surface, small carrier effective masses, and long carrier mean free path. The carrier mean free path in bismuth can be as long as a millimeter at 4.2 K. The resistivity of bismuth (1.15×10-6 Ω．m) is much larger than normal metal (10-8 Ω．m). Therefore, a sample with bismuth as a spacer has larger base resistance and the current-perpendicular-to plane (CPP) measurement could be applied. The CPP-MR is usually described by simpler equations and easily compares to theoretical result. In order to obtain more definite parallel or antiparallel state of the trilayer junction, the spin-valve structure could be adopted. In the CPP-MR spin-valve junction the magnetoresistance effect is much obvious. The spin-valve structure is constructed by trilayer junction deposited on an antiferromagnetic material. To make sure of the exchange bias effect happened at the ferromagnetic/antiferromagnetic interface. The best fabricating condition could be obtained by the detail study of the ferromagnetic/antiferromagnetic interface. The exchange field in the NiFe(5 nm)/FeMn(20 nm) bilayer films was influenced significantly by the lateral grain size on the exchange bias. The cross-section TEM investigations reveal that NiFe(5 vi nm)/FeMn(20 nm) bilayers deposited on thicker Ta underlayers have larger lateral grain diameter. A large increase in exchange bias field was observed with increasing thickness of Ta underlayer, which is attributed to the enhanced FM/AFM coupling originated from larger interface lateral grain size. The spin valve structure was fabricated by the best fabricating condition and the magnetoresisitance investigation of the trilayer junction with spin valve structure was carried out. However, the signal of the resistance in spin valve junction is not stable due to more complicated fabricating procedure. Therefore, the measurement in spin valve junction is not successful. In this work, the current-perpendicular-to-plane (CPP) pseudo spin valve junction with Co(2.5 nm)/Bi(5 nm)/Co(40 nm) was investigated. The nano-contact technique is utilized to reduce the effective area (100×300 nm2) of the sample and increase the base resistance. In order to reduce the noise of the circuit, the ac four-probe method was adopted to determine the electric properties of the sample. The base resistance of the Co/Bi/Co junction is about 2.3 Ω. The MR ratio of this CPP junction is about 0.03%, which is much smaller than theoretically estimated value for this structure (40%). The low MR ratio might be caused by the large contact resistance and non-uniform current densities. The spin valve structure was fabricated by the best fabricating condition and the magnetoresisitance investigation of the trilayer junction with spin valve structure was carried out. However, in this work fabrication of the CPP spin valve junction is not successful due to more complicated fabricating procedure.