  We report here the charge transport behavior in graphene nanojunctions in
which graphene nanodots, with relatively long relaxation time, are interfaced
with ferromagnetic electrodes. Subsequently we explore the effect of
substitutional doping of transition metal atoms in zigzag graphene nanodots
(z-GNDs) on the charge transport under non-collinear magnetization. Only
substitutional doping of transition metal atoms in z-GNDs at certain sites
demonstrates the spin filtering effect with a large tunnelling
magnetoresistance as high as 700%, making it actually suitable for spintronic
applications. From the electrical field simulation around the junction area
within the electrostatic physics model, we find that the value of electric
field strength increases especially with doped graphene nanodots, as the gap
between the gate electrode and tip axis is reduced from 3 nm to 1 nm. Our
detailed analysis further suggests the onset of asymmetric Coulomb oscillations
with varying amplitudes in graphene nanodots, on being doped with magnetic
ions. Such kind of tunability in the electronic conductance can potentially be
exploited in designing spintronic logic gates at nanoscale.
