  We present spin transport studies on bi- and trilayer graphene non-local
spin-valves which have been fabricated by a bottom-up fabrication method. By
this technique, spin injection electrodes are first deposited onto
Si$^{++}$/SiO$_2$ substrates with subsequent mechanical transfer of a
graphene/hBN heterostructure. We showed previously that this technique allows
for nanosecond spin lifetimes at room temperature combined with carrier
mobilities which exceed 20,000 cm$^2$/(Vs). Despite strongly enhanced spin and
charge transport properties, the MgO injection barriers in these devices
exhibit conducting pinholes which still limit the measured spin lifetimes. We
demonstrate that these pinholes can be partially diminished by an oxygen
treatment of a trilayer graphene device which is seen by a strong increase of
the contact resistance area products of the Co/MgO electrodes. At the same
time, the spin lifetime increases from 1 ns to 2 ns. We believe that the
pinholes partially result from the directional growth in molecular beam
epitaxy. For a second set of devices, we therefore used atomic layer deposition
of Al$_2$O$_3$ which offers the possibility to isotropically deposit more
homogeneous barriers. While the contacts of the as-fabricated bilayer graphene
devices are non-conductive, we can partially break the oxide barriers by
voltage pulses. Thereafter, the devices also exhibit nanosecond spin lifetimes.
