Polarized spin and valley transport across ferromagnetic silicene junctions

We study ballistic transport of Dirac fermions through silicene barriers, of width $d$, with an exchange field $M$ and metallic gates above them that provide tunable potentials of height $U$. Away from the Dirac point (DP) the spin- and valley-resolved conductances, as functions of $U$, exhibit resonances and close to it a pronounced dip that becomes a transport gap when an appropriate electric field $E_z$ is applied. The charge conductance $g_c$ of such a junction changes from oscillatory to a monotonically decreasing function of $d$ beyond a critical $E_z$. This tuning of $g_c$ can be used to realize electric-field-controlled switching. Further, the field $M$ splits each resonance of $g_c$ in two spin-resolved peaks. The spin polarization $p_s$ of the current near the DP increases with $E_z$ or $M$ and becomes nearly perfect above certain of their values. We also show that $p_s$ can be inverted either by varying $U$ or by reversing the direction of $M$. For two barriers there is no splitting in $g_c$ when the fields $M$ are in opposite directions. Most of these phenomena have no analogs in graphene.