異常能斯特效應在磁性材料中厚度的相依性

Abstract

近年來，由於在效率、應用及能源上具有極大的潛力，熱激發自旋電子越來越被重視，其中一個重要的效應: 異常能斯特效應是在磁性材料中將熱能轉換為具有自旋相依性的電能訊號，在此效應中電子、熱能以及自旋，皆參與其中。雖然利用此效應的元件、多層膜結構、熱電偶近幾年來皆被大家關注且廣泛的研究，但是異常能斯特效應在磁性材料中對厚度的相關性如此重要議題卻從來沒有被人重視並研究過。在這篇論文中，我們使用垂直方向的熱傳輸研究異常能斯特效在鐵磁性材料中如: 鎳鐵合金、鈷以及鐵的厚度相依性，在研究中我們發現鐵的訊號和鈷還有鎳鐵合金是相反的，並且無論是哪種鐵磁性材料，在厚度大於十奈米時皆是常數，在小於十奈米的薄膜時，鈷以及鎳鐵的異常能斯特訊號隨著厚度減少而減少直至兩奈米時訊號測量不到，即使在此厚度下仍存在著磁性，另外我們發現了鐵的特殊現象，在小於六奈米時，異常能斯特效應的訊號和塊材的鐵訊號相反，更重要的是訊號隨著厚度減少而增加，我們推測這種現象是與厚度強烈相關的鐵的自身特性，在我們的研究中，我們深入探討這些厚度相依性的現象並且展示異常能斯特的厚度相依性在現今多層膜的研究中扮演的重要角色。

The connection between thermoelectricity and spintronics has recently attracted much attention because of its potential application for the high-efficiency electric devices and the iste heat recycling. In particular, anomalous Nernst effect (ANE), the conversion of thermal energy into the spin-dependent electric signal in ferromagnetic metals(FMs), is one of the most important mechanism to study the coupling between charge, spin, and heat. Although, the spintronic devices based on the ANE, including the multilayer structure and ferromagnetic thermal thermopile, have been extensively investigate, the issue for the thickness dependence of ANE has never been addressed. In this work, by using a vertical temperature gradient, we systematically study the thickness dependence of the ANE in several ferromagnetic metals (FMs), including permalloy (Py), iron (Fe), and cobalt (Co) at room temperature. The sign of the ANE can immediately tell that the sign of the Nernst angle in Fe is opposite to that of Py and Co. In the thickness-dependent measurement, we report that the ANE signal in all FMs samples are constant in thick films region while the signal decreases with decreasing thicknesses (less than 10 nm, except for ultra-thin Fe). Importantly, there is no measurable ANE signal when thickness is less than 2 nm, even the ferromagnetic ordering with Curie temperatures still above room temperature. After taking the thickness dependence of the resistivity and magnetization into account, we are able to determine the intrinsic ANE coefficient in ferromagnetic metals for the first time. More surprisingly, we observed the sign change of ANE signal between thick and thin Fe samples. In addition, unlike other FMs, the ANE voltage showed increased instead of decreased signal, when the thickness of Fe sample is less than 6 nm. We suggest this enhancement signal is origin from the intrinsic property of ultra-thin Fe film. Our results provide an important insight on how the role of thickness effect has a strong influence on the development of multilayer structure or ferromagnetic thermal thermopile devices.