Spin injection and spin transport in organic spin valves
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Organic spin valves (OSVs) comprised of a thin organic spacer sandwiched between two ferromagnetic (FM) electrodes have attracted great attention from scientific community in the past decade. Spin injection and spin transport decide the magnetoresistance (MR) response in OSVs. Organic semiconductors (OSECs) possess weak intrinsic hyperfine interaction (HFI) and spin-orbit coupling (SOC) due to their lightweight elements, leading to a much longer spin lifetime than that in inorganic counterparts. Therefore, OSECs have been thought to possess much longer spin diffusion length and hence larger MR in OSVs. In contrast, the interface between organics and FM electrodes and the structural order of the organic interlayer are poorly controlled since epitaxial growth is not possible for OSECs. So the spin transport and spin injection/detection in these devices are complicated, and their complete understanding has remained elusive. In this dissertation, I will address several key issues on spin transport and spin injection in OSECs. For the former, I will show that in addition to intrinsic SOC, geometry-based SOC also has a strong influence on spin transport. Such geometry-based SOC can be uniquely investigated in fullerene where other interactions including HFI and intrinsic SOC are negligibly small. We found that the curvature of C60 and C70 molecules generate considerable SOC and C60 has a larger curvature-based SOC due to its large curve. For the later, to achieve a highly defined organics/FM interface, a thin self-assemble monolayer (SAM) was chemically grown on a bottom FM electrode followed by a thin π-conjugated polymer brush layer, namely poly(3-methylthiophene) (P3MT). The SAM layer helps to solve the resistivity mismatch problem and consequently promotes more efficient spin injection from the FM electrode to the organics. We found that MR in the SAM-based OSVs is nearly an order of magnitude larger than that in the corresponding conventional OSVs at 300 K. Another way to turn the organics/FM interface property is to use organic ferroelectrics. We found that the nature of the spin interface can be largely turned by an external applied electric field. Such strain induced spin interface effect causes up to 1000% spin injection enhancement.
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