  We investigate a novel way to manipulate the spin polarized transmission in a
two terminal zigzag graphene nanoribbon in presence of Rashba spin-orbit (SO)
interaction with circular shaped cavity engraved into it. A usual technique to
control the spin polarized transport behaviour of a nanoribbon can be achieved
by tuning the strength of the SO coupling, while we show that an efficient
engineering of the spin polarized transport properties can also be done via
cavities of different radii engraved in the nanoribbon. Simplicity of the
technique in creating such cavities in the experiments renders an additional
handle to explore transport properties as a function of the location of the
cavity in the nanoribbon. Further, a systematic assessment of the interplay of
the Rashba interaction and the dimensions of the nanoribbon is presented. These
results should provide useful input to the spintronic behaviour of such
devices. In addition to the spin polarization, we have also included an
interesting discussion on the charge transmission properties of the nanoribbon,
where, in absence of any SO interaction a metal-insulator transition induced by
the presence of a cavity is observed.
