Curvature-enhanced Spin-orbit Coupling and Spinterface Effect in Fullerene-based Spin Valves

We investigated curvature-enhanced spin-orbit coupling (SOC) and spinterface effect in carbon-based organic spin valves (OSVs) using buckyball C(60) and C(70) molecules. Since the naturally abundant (12)C has spinless nuclear, the materials have negligible hyperfine interaction (HFI) and the same intrinsic SOC, but different curvature SOC due to their distinct curvatures. We fitted the thickness dependence of magnetoresistance (MR) in OSVs at various temperatures using the modified Jullière equation. We found that the spin diffusion length in the C(70) film is above 120 nm, clearly longer than that in C(60) film at all temperatures. The effective SOC ratio of the C(70) film to the C(60) film was estimated to be about 0.8. This was confirmed by the magneto-electroluminescence (MEL) measurement in fullerene-based light emitting diodes (LED). Next, the effective spin polarization in C(70)-based OSVs is smaller than that in C(60)-based OSVs implying that they have different spinterface effect. First principle calculation study shows that the spin polarization of the dz(2) orbital electrons of Co atoms contacted with C(60) is larger causing better effective spin polarization at the interface.