Spin Injection And Transport Mechanisms In Organic Molecule Thin Films

In the past several decades organic electronics have aroused considerable interest because of unique advantages of organic semiconductoers (OSs). Remarkable progress has been made in electronics and optics. Several devices, such as organic light emitting diodes (OLEDs), organic thin-film transistor (OTFT) and organic photovoltaic cell, have been demonstrated based on organic semiconductor materials. OLEDs are already in commercial production. On the other hand, spintronics is another rapid developing area which utilizes the spin degree of freedom of electrons to transfer and manipulate information, offering low power and high speed electronic devices.Organic spintronics is a new field stands at the crossroads between organic electronics and conventional inorganic spintronics. OSs are suitable for spin transport because of the much longer spin relaxation time in comparison with inorganic materials, which stems from the weak spin-orbit coupling and hyperfine interaction. Although a great deal of work has been done since the first demonstration of magnetoresistance in organic devices in 2002, many fundamental physical issues have yet to be understood in organic spintronics.During my Ph.D study, I have focused on exploring the spin injection, transport and relaxation mechanisms in OSs. The detailed contents of my thesis are as follows:1. To understand the origin of MR sign in organic spin valve, the spin transport properties wre studied in prototypical La0.7Sr0.3MnO3 (LSMO)/Alq3/Co organic spin valve (OSV) with SrTiO3 or Al2O3 tunnel barrier between LSMO and Alq3. A highly asymmetric magnetoresistance (MR) bias dependence was observed, with the inverse MR peaking at a negative bias and a sign reversal occurring at a positive bias. The MR bias voltage dependence is independent on the type of the tunnel barrier. Together with first-principle calculations, we demonstrate that the strongly hybridized Co d-states with Alq3 molecules at the interface are responsible for the efficient d-states spin injection and the observed MR bias dependence is originated from the energy dependent density of states of Co d-states.2. The spin transport in LSMO/Alq3/Al/Co organic spin valve (OSV) devices has been systematically investigated. The resistance and current-voltage curve symmetry are dramatically altered as increasing Al thickness, indicating that an electron-unipolar OSV is obtained. Moreover, the magnetoresistance sign depends on the voltage polarity for certain Al thickness. We attribute this phenomenon to the Fermi and the lowest unoccupied molecular orbits energies of the two electrodes responding to the spin injection and detection, respectively.3. Pure spin current injection from Y3FesOi2 (YIG) into Alq3 molecule in YIG/Alq3/Pd sandwich structure is studied by spin pumping. An unusual angular dependence of inverse spin Hall effect voltage is observed as the magnetic field swept out-of-plane, similar as that Watanabe et al. reported [Nat. Phys.10,308 (2014)]. We provide solid experimental evidences that this behavior is originated from the non-uniformity of the microwave magnetic field of the coplanar waveguide rather than the Hanle effect. Furthermore, quantitative temperature dependent measurements show that the spin diffusion length is almost independent on the temperature from 8 K to 300 K, which is only expected for the exchange-induced spin transport. The absence of the Hanle effect and temperature independence of spin diffusion length strongly support that the pure spin transport in Alq3 is dominant by the exchange coupling between the carriers.