Ultralong 100 ns Spin Relaxation Time in Graphite at Room Temperature*

The emergence of spintronics technology is the driving motivation for understanding spin dynamics in 2D materials. Graphite, which has recently relaunched the interest for 2D materials, has been intensively studied, yet the electron spins dynamics remains an unresolved problem even 70 years after the first experiments. The central quantities, the longitudinal T₁ and transverse T₂ relaxation times were postulated as equals, which is the case for standard metals, but T₁ has never been measured for graphite. Here, based on a detailed band structure calculation included spin-orbit coupling (SOC), we give a full description of the relaxation times. We have measured the T₁ in graphite by saturation ESR measurements and find it markedly different from T₂. We observe that spins injected with perpendicular polarization with respect to the graphene plane have an extraordinarily long lifetime of 100 ns at room temperature. The spin diffusion length across graphite planes is thus expected to be ultralong, on the scale of 70 um, suggesting that thin films of graphite --- or multilayer AB graphene stacks --- can be excellent platforms for spintronics applications compatible with 2D van der Waals technologies. Finally, we provide a qualitative account of the observed spin relaxation based on the anisotropic spin admixture of the Bloch states in graphite obtained from DFT calculations.