  We consider a dilute fluorinated graphene nanoribbon as a spin-active
element. The fluorine adatoms introduce a local spin-orbit Rashba interaction
that induces spin-precession for electron passing by. In the absence of the
external magnetic field the transport is dominated by multiple scattering by
adatoms which cancels the spin precession effects, since the direction of the
spin precession depends on the electron momentum. Accumulation of the spin
precession effects is possible provided that the Fermi level electron passes
many times near the same adatom with the same momentum. In order to arrange for
these conditions a circular n-p junction can be introduced to the ribbon by
e.g. potential of the tip of an atomic force microscope. In the quantum Hall
conditions the electron current gets confined along the junction. The electron
spin interaction with the local Rashba field changes with the lifetime of the
quasi-bound states that is controlled with the coupling of the junction to the
edge of the ribbon. We demonstrate that the spin-flip probability can be
increased in this manner by as much as three orders of magnitude.
