超導鐵磁薄膜系統的界面電阻與臨近效應

Abstract

鐵磁與超導這兩種材料都是自旋有序的相轉變系統，然而鐵磁有序驅使電子自旋同向排列，超導體的庫珀電子對則是傾向自旋電子以相反方向來互相配對，因此彼此之間交互作用引發出許多有趣的物理現象。而最直接和最有系統的研究方式來探討這互相競爭的有序參數，是利用製作鐵磁與超導層狀結構(F/S)並進一步量測其傳輸性質例如超導臨界溫度,臨界磁場，和臨界電流，在層狀結構中超導的波函數會滲透過鐵磁層，並受到鐵磁物質中多數自旋與少數自旋能量不同的影響，而作空間上的修正，因此鐵磁與超導界面的臨近效應會誘導庫珀電子對波函數在進入鐵磁性物質時產生振盪的行為。此量子效應吻合實驗上所量測到的統計行為，這些因為鐵磁與超導的交互作用所引發的物理現象其實都是發生在 F/S 的邊界。由於在實驗上層狀結構的界面相當的複雜甚至會影響所觀察到的物理特性，因此在本論文中，我們主要以電流垂直膜面的量測技術定量的分析出塊材與界面的個別貢獻，這個重要且基礎的傳輸參數，在了解與設定鐵磁超導臨近效應的邊界條件中扮演舉足輕重的角色。 我們將所有系列的樣品區分三組，這三組樣品的臨界溫度與臨界磁場對超導膜厚的行為，都經由電流平行膜面的四點探針分式來量測得知，根據鐵磁超導臨近效應的理論分析，我們得出每一系列樣品的臨界厚度，低於此厚度，超導特性會不存在。臨界磁場與溫度行為的量測中，可以得知樣品維度的轉換行為與磁力線的釘扎機制。藉由電流垂直膜面的量測搭配電阻串聯模型，我們可以經由超導膜厚的改變，定量分析出鐵磁超導在正常態與超導態的界面電阻。在第一組樣品中我們量測與分析鐵磁物質鈷Co和不同比例的鈮鈦合金(NbxTi1-x, with x = 1, 0.6, and 0.4)之界面電阻，並將界面傳輸透明度的物理量，以界面電阻和其它物理參數來表達並作系統性的分析與討論，我們的結果發現超導態的界面電阻會受到超導體的散射中心和滲透到超導體逐漸消失的電子波函數的影響。第二組樣品，我們藉由製備鐵/鈮，鈷/鈮，鎳/鈮[Fe/Nb (bcc/bcc), Co/Nb (hex/bcc), Ni/Nb (fcc/bcc)]層狀結構，利用不同結構和材料的強鐵磁性物質，來研究其與超導的臨近效應，從結果發現鐵磁與超導的晶格常數匹配程度會影響其界面電阻。第三組樣品則是研究弱磁性銅鎳合金與超導鈮的臨界效應，藉由分析超導臨界溫度對鐵磁與超導厚度的行為，我們發現其界面擁有高的傳輸透明度，這樣好的界面會造成弱的鐵磁性擁有強的拆散效應，並影響臨界磁場對溫度的空間維度轉換厚度，進一步在電流垂直膜面量測中，我們證明界面電阻和傳輸透明度有高度的相依性。

The interplay between superconductivity and ferromagnetism results in many interesting physical phenomena. Both materials are phases of matter with ordered electronic spins. While ferromagnetic order forces the spins to align in parallel, the Cooper pairs in singlet superconductivity prefer an antiparallel spin orientation with total spin zero. The most straightforward way to study the competition of the two order parameters is to fabricate ferromagnet/superconductor (F/S) layered structure and to measure the transport properties such as critical temperature, critical field, and critical current in the superconducting state. The superconducting wavefunction penetrating inside the F is modulated by the energy difference between the minority and majority spin bands. Thus, the proximity effect at F/S interface would induce damped oscillatory behavior of the Cooper pair wavefunction within the ferromagnetic material. These physical phenomena of the proximity effect are related to the interplay between superconductivity and magnetism and occur at the boundary of F/S structures. However the character of the real interface in the F/S systems complicates the physical situation considerably. In this dissertation, we use current perpendicular measurement technique to quantitatively separate the interface and bulk contribution. The fundamental information of the transport properties given by this useful tool plays a dominant role in the boundary condition of the microscopic model within the proximity effect. We divide a series of samples into three groups. The behaviors of superconducting transition temperature Tc and upper critical field Hc2 as a function of different superconductor thicknesses have been investigated in all groups with current flowing in the plane by a standard four-probe technique. We deduce superconductor critical thicknesses, below which superconductivity vanishes, by analyzing the data in terms of the proximity effect theory. The temperature dependence of Hc2 measurement reveals the spatial dimensional crossover and the flux pinning mechanism in the superconductor. Using the current perpendicular to plane measurements (CPP) with a series resistor model, we can, by varying the thickness of S, extract the unit area resistance for one pair of F/S interface when S is in the superconducting and normal states. In Group 1, the quantitative interface resistance between polycrystalline ferromagnetic Co and NbxTi1-x, with x = 1, 0.6, and 0.4, are measured and analyzed. The interface transparencies in terms of the ratio between interface resistance and various physical quantities are discussed. Our results show that the superconducting state interface resistance is influenced by the scattering centers and the penetration depths of the electron evanescent wave into the superconductors according to the Pippard model. In Group 2, we study the proximity in Fe/Nb (bcc/bcc), Co/Nb (hex/bcc), and Ni/Nb (fcc/bcc) with a sputtered layered system. The influence of lattice mismatch on interface resistance is found to be important. In Group 3, we report the proximity effect between a weak ferromagnet Cu0.5Ni0.5 and a superconductor Nb. High interfacial transparency is derived from the behavior of the superconducting critical temperature as a function of the S and F layer thicknesses. A strong pair-breaking effect as a result of the high interface quality influences the spatial dimensional crossover in the temperature dependence of the upper critical magnetic field. Here, by using the CPP measurement with a series resistor model, a close correlation between the interfacial transparency and the interface resistance is demonstrated.