鈮鎳多層膜邊際效應與介面電阻之研究

Abstract

我們主要在研究超導與鐵磁物質的介面性質，我們選用的鐵磁材料是具有磁性的鎳金屬(Ni)，而超導材料是選用元素鈮(Nb)，鈮是元素超導中，超導轉變臨界溫度(TC)最高的，而且也是元素超導中少數為二類超導體的材料。這也給了我們許多空間去研究超導與鐵磁間的介面現象。 電流平行膜面(CIP)量測可以得到樣品的超導轉變臨界溫度與上臨界磁場(HC2)，在本文討論中是用Ni(50nm)/Nb(x)/Ni(50nm)三層膜的系統來看臨界溫度隨著鈮厚度的變化，數據中得到當鈮的厚度減少時，超導臨界溫度會降低的比純鈮系統快，我們可以根據介面效應的機制來擬合並得到一個超導臨界厚度(dSC~45nm)且分析其介面效應的強度，因為旁邊鐵磁層的磁交換場對庫柏電子對有拆散作用，所以當鈮的厚度在臨界厚度以下，即使溫度降至我們量測系統的最低溫度(1.7K)，超導特性也不會出現。 HC2的樣品是[Ni(50nm)/Nb(x)]6/Ni(50nm)多層膜的結構，在不同磁場下量測電阻與溫度的關係，定義出超導轉變臨界磁場，量測上分為磁場垂直膜面與平行膜面，但主要要看平行膜面時HC2隨著鈮厚度轉變，HC2與溫度的關係會有二維到三維的特性改變，維度轉換大概的範圍坐落在100nm~130nm間，這也是界面效應的影響；而磁場垂直膜面的量測可讓我們定義出超導的相干長度。 最後一個部份是電流垂直膜面(CPP)量測，可以直接給我們介面電阻的相關訊息，因為幾何因素電阻值會是CIP量測的百萬分之一倍，所以我們利用超導量子干涉儀(SQUID)作為皮伏特計(pico-volt meter)。我們有四組樣品，利用基本的線性分析可以得到四個物理量，分別是：超導態鈮與鎳的介面電阻(ARS/F)、正常態鈮與鎳的介面電阻(ARN/F)、鎳的電阻率(ρNi)以及正常態鈮的電阻率(ρNb)。由我們的分析得知，介面效應的強度除了磁性層的交換場大小，也與超導/鐵磁間電子波函數的傳輸透明度有關，而此能障(barrier)是由介面電阻來主導，因此我們可藉以上量測來定性及定量的分析鈮/鎳系統的介面效應。

We have studied the proximity effect in superconduct Nb / ferromagnetic Ni (S/F) multilayer system. The superconducting critical temperature (Tc) variation of F/S/F trilayer samples with fixed Ni thickness (dNi) and varied Nb thickness (dNb) were measured by conventional current-in-plan (CIP) measurement. As the dNb was reduced below 100 nm, the Tc rapidly decreased monotonically. The critical thickness, below which superconductivity vanishes, with fixed dNi was derived by fitting the data in terms of proximity theory. We got a critical thickness of Nb about 40nm when sandwiched in 50nm of Ni, below this thickness there is no superconductivity even if our samples were cooled down to 0.3K. According to the Ginzburg-Landau (G-L) equation, the upper critical magnetic field (HC2) is a function of temperature (T). We changed the dNb to see how the dimension of Nb thin film affected the HC2(T) behavior. From our results, the 2-dimensional to 3-dimensional crossover was about 100~130nm for Nb/Ni multilayers. The superconductivity was strongly suppressed due to the pair-breaking effect from the Ni film. Even though the thickness of Nb was larger than perpendicular coherence length, the HC2 had square-root behavior dependence on T. Current-perpendicular-to-plan (CPP) measurements provide us quantitative information about the interfacial resistance between Ni and Nb with either superconducting or normal (N) states in the S/F heterostructure. To fit the data simultaneously we had used a global fit technique and the results showed that the interface resistance of Ni and Nb, which was in superconducting and normal state, were about 4.2 fΩ-m2 and 1.5 fΩ-m2 respectively. The interface transparency parameter γB was related to the normal state interface resistance and we calculated γB ~2.1.