  We propose an intrinsic spin scattering mechanism in graphene originated by
the interplay of atomic spin-orbit interaction and the local curvature induced
by flexural distortions of the atomic lattice. Starting from a multiorbital
tight-binding Hamiltonian with spin-orbit coupling considered
non-perturbatively, we derive an effective Hamiltonian for the spin scattering
of the Dirac electrons due to flexural distortions. We compute the spin
lifetime due to both flexural phonons and ripples and we find values in the
microsecond range at room temperature. Interestingly, this mechanism is
anisotropic on two counts. First, the relaxation rate is different for
off-plane and in-plane spin quantization axis. Second, the spin relaxation rate
depends on the angle formed by the crystal momentum with the carbon-carbon
bond. In addition, the spin lifetime is also valley dependent. The proposed
mechanism sets an upper limit for spin lifetimes in graphene and will be
relevant when samples of high quality can be fabricated free of extrinsic
sources of spin relaxation.
