Spin diffusion in fullerene-based devices: Morphology effect

Abstract

The buckminsterfullerene C${}_{60}$ molecules are composed of \ensuremath{\sim}99$%$ naturally abundant ${}^{12}$C carbon atoms having spinless nucleus and thus zero hyperfine interaction. Therefore it has been assumed that the spin diffusion length in C${}_{60}$-based spin-valves is large. We fabricated spin-valves based on C${}_{60}$ and studied the magnitude of the obtained giant magnetoresistance (GMR) as a function of bias voltage, temperature, and C${}_{60}$ layer thickness. Surprisingly, we found that GMR first increases as the C${}_{60}$ layer thickness increases, reaching a maximum at \ensuremath{\sim}35 nm, then exponentially decreases with thickness from which we extracted a small spin diffusion length of \ensuremath{\sim}12 nm at 10 K. From our data, we obtain two important conclusions. First, the morphology related disorder that originates from the C${}_{60}$ nano-crystalline grains embedded into an amorphous phase of C${}_{60}$ is responsible for an unusual spin diffusion process that results in short spin diffusion length. Second, we identify the main spin relaxation dynamics in the fullerene to be the grain boundaries in which spin-orbit coupling is enhanced by the local electric field.