  We determine the spin-lifetime anisotropy of spin-polarized carriers in
graphene. In contrast to prior approaches, our method does not require large
out-of-plane magnetic fields and thus it is reliable for both low- and
high-carrier densities. We first determine the in-plane spin lifetime by
conventional spin precession measurements with magnetic fields perpendicular to
the graphene plane. Then, to evaluate the out-of-plane spin lifetime, we
implement spin precession measurements under oblique magnetic fields that
generate an out-of-plane spin population. We find that the spin-lifetime
anisotropy of graphene on silicon oxide is independent of carrier density and
temperature down to 150 K, and much weaker than previously reported. Indeed,
within the experimental uncertainty, the spin relaxation is isotropic.
Altogether with the gate dependence of the spin lifetime, this indicates that
the spin relaxation is driven by magnetic impurities or random spin-orbit or
gauge fields.
