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

Abstract

Transport properties along the out-of-plane direction in layered materials providedfascinating physical phenomena, such as giant magnetoresistance in magnetic multilayers andintrinsic Josephson effect in high-temperature superconductors. Studying the mechanism for theemergence of giant magnetoresistance and high-temperature superconductivity has a highimportance in modern solid-state physics studies and industrial applications.This thesis consists of two main parts

one is out-of-plane transport properties ofgraphene-based spin-valve junctions and another is out-of-plane transport properties onSmFeAsO0.85 single crystal, one of iron-based superconductors.In Part I, magnetoresistance of graphene-based spin-valve junctions was investigatedwith current-perpendicular-to-plane geometry. Potential applications to spintronics weretriggered after the discovery of giant magnetoresistance in epitaxially grown magneticmultilayers. Adopting crystalline barrier (metallic or insulating) between the ferromagnets hasled the progress of studies in magnetoresistance. The tunnelling magnetoresistance, in particular,provided almost 1000%-magnetoresistance with crystalline MgO insulating barrier, which haslattice mismatch of 3.8% from crystalline Fe. Hence, the measurements of magnetoresistance inferromagnet/graphene/ferromagnet (FGF) junction has attracted much interest due to small spinorbitcoupling in graphene and the consequently predicted long spin relaxation length. Graphenegrown by chemical-vapor-deposition was first adopted as barrier in FGF junctions. Wide rangeof junction resistance and different transport regimes (metallic and insulating properties),however, were observed with small magnetoresistance. In these experiments, the externalfactors, defects in synthesis and transfer procedure of graphene, may have affected the spindependenttransport properties in FGF junctions.In our study, sandwiching naturally stacked graphene of varied number of layers byevaporating ferromagnetic electrodes for both sides of graphene layers, we investigatedtransparency dependence of magnetoresistance in FGF junctions. Thus-achieved transparentinterface condition offers the relatively high magnetoresistance (maximum magnetoresistance of4.6 % at T = 4.2 K in a FGF junction with the four-layer graphene insertion with the resistanceareaproduct of 0.2 Ω·μm2). The junction resistance plays a main role in FGF junction sincelarge discrepancy of magnetoresistance depending on the junction resistance was observed evenfor inserting graphene consisting of the same number of atomic layers. Furthermore, ourtransparent junctions exhibiting non-metallic R−T curves represent the intrinsic spin-dependenttransport properties for FGF junctions by minimizing the interfacial scattering since theresistance-temperature curves of FGF junctions resemble one of out-of-plane resistance ofgraphite as the junction resistance is reduced to highly transparent regime. Finally, inverseproportionality between the resistance-area product and the magnetoresistance implies that thespin-flip at the interfaces in FGF junctions reduces the efficiency of the spin-injection along theout-of-plane direction. Improving the interface properties by growing crystalline ferromagnetwith minor lattice mismatch directly on graphene is essential for enhancing the spin-injectionefficiency in graphene/ferromagnet interface.In Part II, the properties of out-of-plane critical currents in SmFeAsO0.85 single crystalare mainly discussed. Similarities and differences between iron-based superconductors andcuprate superconductors provided a fresh insight into the investigation of high-temperaturesuperconductivity. The presence of ferromagnetic iron atoms in iron-based superconductorssuggests spin-fluctuation as the possible glue for superconducting pairing in high-temperaturesuperconductors, while layered structures and dome-shaped superconducting phase diagramupon charge doping were observed in both superconductors. On the other hand, utilizing theintrinsic Josephson coupling, which is established between adjacent superconducting layersseparated by insulating layers in highly anisotropic cuprate superconductors, is very helpful toinvestigate the gap symmetry by means of measuring the density of states from interlayertunneling spectroscopy. Thus, the examination of the presence of intrinsic Josephson coupling iniron-based superconductors is of high interest as a stepping stone to obtain information on thedensity 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 ofupper critical field (γH~5) among iron-based superconductors. SmFeAsO0.85 single crystals wereobtained by self-flux method under high temperature and pressure. The electron-beamlithography, Ar-ion etching and SiN stencil masks were adopted to fabricate the mesa structurefor out-of-plane transport of material. In this study, comprehensive electrical transportmeasurements on SmFeAsO0.85 single crystals were made by adopting three- and four-terminalmeasurement configurations to obtain more conclusive insights into the out-of-plane transportof 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 revealweakly anisotropic bulk transport properties of the material rather than formation of Josephsonweak 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 electronicstructures and Fermi surfaces of iron-based superconductors distinguished from twodimensionalityof the cuprate superconductors.