橫向鐵磁(鎳鐵)/超導(鈮)接面系統與逆鄰近效應之研究

Study on Lateral Ferromagnet (NiFe)/Superconductor (Nb) Junction System and Inverse Proximity Effect

林呂圭(Lu-Kuei Lin)

指導教授 ： 黃金花 ； 李尚凡

國立清華大學/工學院/材料科學工程學系/博士(2014年)

https://doi.org/10.6843/NTHU.2014.00049

摘要

鐵磁與超導的交互作用引發出許多有趣的物理現象，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，超導體的庫柏電子對則是傾向自旋電子以相反方向來互相配對。因此這兩種不同的自旋電子有序是互相抵抗的，而利用製作鐵磁與超導的異質接面結構是最直接的方式來研究這互相競爭的有序參數。在此鐵磁/超導的異質結構中，鄰近效應在過去已經被廣為研究，然而相對於鄰近效應，鐵磁對超導的逆鄰近效應則在這近十年中被提出並引起廣大的興趣。從過去的文獻中可以發現，橫向鐵磁/超導異質接面的研究仍然較缺乏，尤其是鐵磁對超導的逆鄰近效應在橫向鐵磁/超導異質接面中的影響更是少見。因此，提供了我們對鐵磁對超導的逆鄰近效應在橫向鐵磁超導異質接面中研究的動機。 在本論文中，我們利用電性量測的方式對這橫向鐵磁/超導異質接面做了一系列的研究。我們主要製造了兩種不同的橫向鐵磁/超導異質接面，一個是利用兩鐵磁材料鎳鐵長條跨接在兩個超導電極鈮，所製造的橫向雙鐵磁/超導接面，另一個則是利用鎳鐵長條與超導長條交叉跨接所形成弱連結的橫向約瑟芬接面。 在兩鐵磁跨接的橫向鐵磁/超導異質接面中，我們改變了兩鐵磁長條之間的間距，從150奈米到700奈米，來研究自旋電子在此區域間的弛豫長度。在小間距(150和250奈米)的樣品中，我們發現了由於逆鄰近效應而造成電荷堆積在接面形成的兩鐵磁長條的自旋閥效應。另外在大間距(500和700奈米)的樣品中，我們發現了自旋閥效應消失，取而代之的是兩鐵磁長條的異向性磁阻現象。而我們另外製造更多數目的鐵磁長條跨接在超導電極，自旋閥效應以及異向性磁阻現象出現與兩鐵磁長條間距的關係能然可以觀察的到。利用改變兩鐵磁長條之間的間距，觀察自旋閥效應以及異向性磁阻現象的變化，我們可以估計出自旋電子在超導鈮中橫向的弛豫長度。而在自旋閥效應出現的樣品中，我們還發現了超導的臨界溫度隨著兩鐵磁長條的磁區排列有所改變，兩鐵磁長條磁區反平行的臨界溫度比平行的臨界溫度最多提高了58.3 mK。除此之外，在電阻對溫度的量測中，我們還發現了奇異的電阻陡降與陡升，在此我們推測可能成因來自於鐵磁與超導間的交互作用。 在鎳鐵長條與超導長條交叉跨接所形成弱連結的橫向約瑟芬接面中，我們探討此橫向約瑟芬接面的弱連結是由鐵磁對超導的逆鄰近效應所形成的，也就是說鐵磁藉由逆鄰近效應在超導材料中誘發出磁性而形成橫向約瑟芬接面中的弱連結區域。我們利用觀察在此橫向約瑟芬接面中直流以及交流的超導約瑟芬效應來證明此弱連結來是來自於鐵磁對超導的逆鄰近效應所形成的。這是利用電性量測方式對逆鄰近效應的直接證明。另外，我們也估算了此橫向約瑟芬接面的Thouless能量大小。我們亦探討了此超導約瑟芬效應與超導渦旋電流移動所造成的效應不同。在此橫向約瑟芬接面中，超導電流能流經由鐵磁誘發的弱連結區域長達2.1微米，因此我們推測，此約瑟芬接面中具有如理論預測的自旋三重態的電子。此研究提供了一種全新的方式來製作超導約瑟芬接面，未來應用在製作超導量子干涉儀上有取代現今複雜製作方式的優勢。

關鍵字

超導 ； 鐵磁 ； 鄰近效應 ； 約瑟芬接面

並列摘要

The heterogeneous properties around the interface between ferromagnet (F) and conventional s-wave superconductor (S) hybrid structures exhibit several remarkable phenomena. The superconductivity and ferromagnetism are two competing orders. The ferromagnetic order forces the spins to align in parallel, and Cooper pairs in singlet superconductivity prefer an antiparallel spin orientation with total spin zero. Due to the incompatible nature of S and F, singlet superconductivity and ferromagnetic order should not coexist in bulk materials. Thus, fabricating S/F heterostructure is the immediate way to study the proximity phenomena in the competition of the two order parameters. However, the S/F proximity effect has attracted much attention over the past years. Aspect of the S/F proximity effect, the “inverse proximity” effect which is the ferromagnetic order-parameter penetrates into the superconductor, has gained renewed interest only in the last decade. Following the literatures, the characterizations of the S/F lateral junction have been studied rear, especially the inverse proximity. Therefore, it gives us the motivations to study the inverse proximity effect in S/F lateral junction.
In this dissertation, we systematically investigate the lateral S/F junctions by electron transport measurement. We fabricated two types of S/F lateral junction. One is the junctions between two superconductor Nb leads were connected by two bars of the ferromagnetic NiFe, another is lateral Josephson junction with a weak link by fabricating a ferromagnetic NiFe strip across a superconducting Nb strip.
In the lateral two NiFe bars junctions, the separation distance l between NiFe bars was varied from 150 to 700 nm to study the spin relaxation effect in Nb. In small l (150 and 250 nm) samples, a spin-valve like behavior related to the inverse proximity effect was observed, which manifest itself as charge accumulation due to spin imbalance near the interface. However, for samples with large l (500 and 700 nm), the spin-valve behavior vanishes and the anisotropic magnetoresistance effect dominates. The spin-valve and anisotropic magnetoresistance (AMR) behaviors were also observed on multiple bars samples. From the two different behaviors observation, the spin relaxation length of Nb lateral device can be estimated. The transition temperature enhancement of Nb was observed on the spin-valve behavior samples. The transition temperature enhancement of the l = 250 and 300 nm samples are 35.5 and 58.3 mK, respectively. In addition, there are some striking resistance peaks observed in the R-T measurements. It is attributed to the interaction between S and F.
In the lateral Josephson junctions with a weak link by fabricating a ferromagnetic NiFe strip across superconducting Nb strips. We argue that the effective weak link of the Josephson junction was induced by the inverse proximity effect around the Nb interface in contact with the NiFe. The argument has been proved by the observation of the dc and ac Josephson effect. We also estimated the strength of the Josephson junctions by Thouless energy analysis. In our observations, the supercurrents can go through such long distance up to 2.1μm of the junctions, implying that a spin triplet component possibly exists as the theoretical prediction, the spin triplet component induced by inverse proximity effect. In addition, we also discussed the correlation of the vortex dynamics and the critical current modulations, and ruled out the possibility of the vortex dynamics in our data.

並列關鍵字

superconductivity ； ferromagnetism ； proximity effect ； Josephson junction

參考文獻

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