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Out-of-plane transport properties of graphene-based spin-valve junction and iron-based superconductor

Out-of-plane transport properties of graphene-based spin-valve junction and iron-based superconductor
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Transport properties along the out-of-plane direction in layered materials provided fascinating physical phenomena, such as giant magnetoresistance in magnetic multilayers and intrinsic Josephson effect in high-temperature superconductors. Studying the mechanism for the emergence of giant magnetoresistance and high-temperature superconductivity has a high importance in modern solid-state physics studies and industrial applications. This thesis consists of two main parts
one is out-of-plane transport properties of graphene-based spin-valve junctions and another is out-of-plane transport properties on SmFeAsO0.85 single crystal, one of iron-based superconductors. In Part I, magnetoresistance of graphene-based spin-valve junctions was investigated with current-perpendicular-to-plane geometry. Potential applications to spintronics were triggered after the discovery of giant magnetoresistance in epitaxially grown magnetic multilayers. Adopting crystalline barrier (metallic or insulating) between the ferromagnets has led the progress of studies in magnetoresistance. The tunnelling magnetoresistance, in particular, provided almost 1000%-magnetoresistance with crystalline MgO insulating barrier, which has lattice mismatch of 3.8% from crystalline Fe. Hence, the measurements of magnetoresistance in ferromagnet/graphene/ferromagnet (FGF) junction has attracted much interest due to small spinorbit coupling in graphene and the consequently predicted long spin relaxation length. Graphene grown by chemical-vapor-deposition was first adopted as barrier in FGF junctions. Wide range of junction resistance and different transport regimes (metallic and insulating properties), however, were observed with small magnetoresistance. In these experiments, the external factors, defects in synthesis and transfer procedure of graphene, may have affected the spindependent transport properties in FGF junctions. In our study, sandwiching naturally stacked graphene of varied number of layers by evaporating ferromagnetic electrodes for both sides of graphene layers, we investigated transparency dependence of magnetoresistance in FGF junctions. Thus-achieved transparent interface condition offers the relatively high magnetoresistance (maximum magnetoresistance of 4.6 % at T = 4.2 K in a FGF junction with the four-layer graphene insertion with the resistancearea product of 0.2 Ω·μm2). The junction resistance plays a main role in FGF junction since large discrepancy of magnetoresistance depending on the junction resistance was observed even for inserting graphene consisting of the same number of atomic layers. Furthermore, our transparent junctions exhibiting non-metallic R−T curves represent the intrinsic spin-dependent transport properties for FGF junctions by minimizing the interfacial scattering since the resistance-temperature curves of FGF junctions resemble one of out-of-plane resistance of graphite as the junction resistance is reduced to highly transparent regime. Finally, inverse proportionality between the resistance-area product and the magnetoresistance implies that the spin-flip at the interfaces in FGF junctions reduces the efficiency of the spin-injection along the out-of-plane direction. Improving the interface properties by growing crystalline ferromagnet with minor lattice mismatch directly on graphene is essential for enhancing the spin-injection efficiency in graphene/ferromagnet interface. In Part II, the properties of out-of-plane critical currents in SmFeAsO0.85 single crystal are mainly discussed. Similarities and differences between iron-based superconductors and cuprate superconductors provided a fresh insight into the investigation of high-temperature superconductivity. The presence of ferromagnetic iron atoms in iron-based superconductors suggests spin-fluctuation as the possible glue for superconducting pairing in high-temperature superconductors, while layered structures and dome-shaped superconducting phase diagram upon charge doping were observed in both superconductors. On the other hand, utilizing the intrinsic Josephson coupling, which is established between adjacent superconducting layers separated by insulating layers in highly anisotropic cuprate superconductors, is very helpful to investigate the gap symmetry by means of measuring the density of states from interlayer tunneling spectroscopy. Thus, the examination of the presence of intrinsic Josephson coupling in iron-based superconductors is of high interest as a stepping stone to obtain information on the density of states for quasi particles in superconducting and pseudogap states. In our study, we investigated the out-of-plane transport properties of SmFeAsO0.85, which has relatively large superconducting transition temperature (Tc~56K) and anisotropy of upper critical field (γH~5) among iron-based superconductors. SmFeAsO0.85 single crystals were obtained by self-flux method under high temperature and pressure. The electron-beam lithography, Ar-ion etching and SiN stencil masks were adopted to fabricate the mesa structure for out-of-plane transport of material. In this study, comprehensive electrical transport measurements on SmFeAsO0.85 single crystals were made by adopting three- and four-terminal measurement configurations to obtain more conclusive insights into the out-of-plane transport of the material. The temperature dependence of the resistance, the current-voltage characteristics, and the magnetic field dependence of the critical currents along the out-of-plane direction reveal weakly anisotropic bulk transport properties of the material rather than formation of Josephson weak links because of the strong induction of superconductivity in the non-superconducting Sm- O layers. From the experimental results, one can identify the three-dimensional electronic structures and Fermi surfaces of iron-based superconductors distinguished from twodimensionality of the cuprate superconductors.
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