On the Role of Spin-Orbit Coupling in the Spin Response of C$_{60}$-based Spintronics Devices*

We report comprehensive studies of the spin response in C$_{60}$-based spintronics devices such as spin valves and diodes. The buckeyball C$_{60}$ molecules are composed of $\sim $99{\%} $^{12}$C carbon atoms having spinless nuclei with zero hyperfine interaction. Therefore it was believed that the spin diffusion length in C$_{60}$-based spin-valves is large, and the magnetoresistance (MR) in C$_{60}$ diodes is negligible small. Surprisingly, we obtained a small spin diffusion length which we believe to be due to a relatively strong spin-orbit (SO) coupling in the material. We also found that the MR in C$_{60}$ diodes is relatively small, with characteristic magnetic field response dominated by the SO coupling with strength, $\xi \approx \quad \raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} \quad \mu $eV, more than ten times larger than the HFI constant\textbf{. }This was verified by measuring the response of $^{13}$C-rich C$_{60}$ diodes.