Spin Seebeck Effect In Ferromagnetic Transition Metals

Since the giant magnetoresistance devices have been widely used,the great potential of electron spin has aroused wide interest of researchers.It's a wonderful expectation to use spin as a carrier to achieve high-performance,low-power consumption equipment for information storage and computing.The generation,transport,and detection of spin currents are still hot topics in spintronics.In 2008,Uchida et al.realized spin transport using thermal spin injection method,which attracted much attention,and it started the research of long-range spin transport and detection.In the magnet/heavy metal bilayer systems,the longitudinal heat-driven spin current is injected from the magnetic layer into the heavy metal layer.Then the injected spin current is converted into a detectable charge current due to strong spin-orbit coupling in heavy metal.This process is called the longitudinal spin Seebeck effect(LSSE).It's challenge to carry out the LSSE of ferromagnetic metals(FM)because of the parasitic thermoelectric signal and the inaccurate temperature difference measurement method.In this paper,we analyzed all interference signals,and calibrated the temperature difference on the film to get the SSE coefficient of FM.We studied the thermal spin transport characteristics in different FM.This paper is organized by the following four chapters:Chapter 1:Introduce the development of spintronics and the experimental progress of thermal-driven spin transport phenomena.We summarized the experimental difficulties of LSSE and the motivation of our works.Chapter 2:Introduce the sample preparation and test method,and mainly compared the transverse and longitudinal heating configurations.Chapter 3:In this chapter,we quantitatively studied the LSSE in the FM/Pt system,where FM is Fe,Co,Ni and Ni80Fe20,respectively.According to the parallel current source model,the anomalous Nernst effect(ANE)is deducted by comparing the thermoelectric current of the FM single layers and FM/Pt bilayers.According to the thermal-driven spin transport model,the self-induced inverse spin Hall effect(ISHE)in FM was separated,which is comparable to ISHE in Pt.The heat flux method was used to estimate the temperature difference on the film,and the reliability of the temperature difference on the film was calibrated by measured ANE in transverse configuration.Thus,we measured the spin Seebeck coefficients of Fe,Co,Ni and Ni₈₀Fe₂₀ in the temperature range of 90 K to 300 K.To our surprise,we found that the LSSE of Ni has opposite sign with the other three metals.Further,experimentally,we combined the conventional Seebeck and SSE coefficients to obtain the spin-dependent Seebeck coefficients_?and_?of each channel,which are consistent with the calculated result based on the DOS of metals.This revealed the thermal spin transport characteristics of ferromagnetic metals are closely related to their electronic structures.Chapter 4:The thermal spin transport characteristics in ferromagnetic metals are closely related to their electronic structure.By doping Cr into Ni,we distorted the density of states at the Fermi level to regulate the SSE coefficients continuously or enhance the SSE coefficients.Although the obtained SSE signal is contaminated by the self-induced ISHE in FM,it still reflects trend of SSE of NiCr.It was confirmed that when the doping content of Cr is around 1.6%,the SSE related coefficient of NiCr alloy will be enhanced,and with the increase of the doping content,the SSE of NiCr gradually decreasing,which are consistent with the density of states of NiCr.