Anomalous Nernst Valve achieved by FerromagneticThermopile and FeRh Alloy

• Main Authors: Ting-Wei Weng, 翁廷瑋
• Other Authors: Ssu-Yen Huang
• Format: Others
• Language: en_US
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/ndcqc4

碩士 === 國立臺灣大學 === 應用物理研究所 === 107 === Most of the proposed spintronic devices involve spin-polarized transport. The control, manipulation, and detection of spin polarization play a central role in the emerging field of spintronics. Recently, it has been shown that the anomalous Nernst effect (ANE) can be readily used to excite spin-polarized current by only a small amount of thermal energy even without charge current. Related effects and phenomena has attracted a great deal of attention. Here, we demonstrate anomalous Nernst valve realized in the ferromagnetic thermopile and FeRh alloy. We first show that ferromagnetic-materials-based thermopile device, whose laterally connected structure can not only enhance its own thermoelectric signal but also turn itself into a four-state anomalous Nernst valve. By utilizing the opposite sign of the ANE voltage between iron (Fe) in thicker films and permalloy (Py), a thermopile made of the two materials connected electrically in series is realized. More importantly, the voltage of the ferromagnetic thermopile can be significantly increased even if only one material Fe is used, since the ANE voltage of Fe can be reversed and dramatically increased by decreasing thickness. The relatively high thermoelectric efficiency is introduced and obtained from our device. Furthermore, the ANE voltage in thermopile resulting from different magnetization configurations between two ferromagnetic stripes can be clearly distinguished, that is, an anomalous Nernst valve with multiple states is demonstrated. Second, we demonstrate that, endowed by the first-order antiferromagnetic-ferromagnetic (AFM-FM) phase transition nature, epitaxial growth FeRh can be an anomalous Nernst valve with its thermally excited spin current controllable between zero and one by external magnetic field. Besides, because of the existence of AFM-FM phase transition in FeRh near room temperature, the direction of the Néel vector can be readily manipulated via a moderate field at room temperature during cooling process. Interestingly, the relative difference of magnetoresistance for the resultant Néel vector with different orientations can be significantly enhanced at low temperature with reversed sign. We even observe giant-magnetoresistance-like spin-dependent transport behavior in both epitaxial and polycrystalline FeRh. Our results introduce using two different approaches to manipulate spin polarized current in FM and AFM systems that are important for low power non-volatile spintronic devices.