  We study the band structure and transport properties of ferromagnetic
tetragonal silicene nanoribbons by using the non-equilibrium Green's function
method. The band structure and spin-dependent conductance are discussed under
the combined effect of the external electric field, potential energy, exchange
field and the spin-orbit coupling. One can easily realize a phase transition
from a semimetallic to a semiconducting state by changing the transverse width
of the nanoribbon. Separation of spin-dependent conductances arises from the
effect of exchange field and the spin-orbit coupling, while zero-conductance
behaviors exhibit spin-dependent band gaps induced by the electric field. We
propose a device configuration of four-terminal tetragonal silicene nanoribbon
with two central channels. It is found that spin current can be controlled by
utilizing two switches. The switch with a high potential barrier can block
electrons flowing from the central scattering region into other terminals.
Interestingly, applying only one switch can realize spin-dependent zero
conductance and large spin polarization. Two switches can provide multiple
operations for controlling spin-dependent transport properties. The two-channel
ferromagnetic tetragonal silicene nanoribbon can realize an effective
separation of spin current, which may be a potential candidate for spintronic
devices.
