Spin pumping, graphene, gilbert damping, spin charge interconversion, inverse spin hall effect, ferromagnetic resonance
Within the exciting current trend to explore novel low-dimensional systems, the possibility to inject pure spin currents in graphene and other two-dimensional crystals has attracted considerable attention in the past few years. The theoretical prediction of large spin relaxation times and experimentally observed mesoscopic-scale spin diffusion lengths places graphene as a promising base system for future spintronics devices. This is due to the unique characteristics intrinsic to the two-dimensional lattice of carbon atoms forming graphene, such as the lack of nuclear spins and weak spin-orbit coupling of the charge carriers. Interestingly for some spintronic applications, the latter can be chemically and physically engineered, with large induced spin-orbit couplings found in functionalized graphene sheets. Understanding spin injection, spin current and spin dynamics in graphene is of a great interest, both from the fundamental and applied points of view. This thesis presents an experimental study of dynamical generation of spin currents in macroscopic graphene sheets by means of spin pumping from the precessing magnetization of an adjacent ferromagnet. The spin pumping characteristics are studied by means of ferromagnetic resonance (FMR) measurements in Permalloy/graphene (Py/Gr) bilayers. Changes in the FMR linewidth induced by the presence of graphene (when compared to studies with only Py films) correspond to an increase in the Gilbert damping in the ferromagnetic layer (proportional to the FMR linewidth) and interpreted as a consequence of spin pumping at the Py/Gr interface driven by the Py magnetization dynamics (i.e., magnetic induced by the microwave stimulus). FMR experiments are performed on different FM/Gr interfaces, completing a set of studies designed to systematically identify and eliminate damping enhancement arising from processes other than spin pumping. Remarkably, a substantial enhancement of the Gilbert damping observed in Py/Gr strips with graphene protruding a few micrometers from the strip sides is univocally associated to spin pumping at the quasi-onedimensional interface between the Py strip edges and graphene. This increase in the FMR linewidth compares with observations in other bilayer systems, in where thick (thicker than the spin diffusion length) layers of heavy metals with strong spin-orbit coupling are employed as the non-magnetic layer, indicating that spin relaxation in chemically grown graphene must be greatly enhanced in order to account for the losses of angular momentum lost by the ferromagnet. The fundamental implications of the results presented in this thesis point to a non-trivial nature of the spin pumping mechanism owing to the two-dimensionality of the non-magnetic layer (i.e., graphene). In addition, a spintronics device designed to interconvert charge and spin currents has been designed. A high-frequency microwave irradiation lock-in modulation technique is employed to detect the small electrical voltages generated by the inverse spin Hall effect (ISHE). As a proof of principle, a successful spin-charge interconversion in Py/Pt-based devices is experimentally demonstrated in this thesis. The challenges associated with the spin-charge interconversion in twodimensional devices are discussed and systematically addressed, and a potential device geometry for measuring the ISHE in Py/Gr-based systems is provided.
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Del Barco, Enrique
Doctor of Philosophy (Ph.D.)
College of Sciences
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Dissertations, Academic -- Sciences; Sciences -- Dissertations, Academic
Singh, Simranjeet, "Dynamical spin injection in graphene" (2014). Electronic Theses and Dissertations. 4606.