  The generation of short spin-current pulses is essential for fast spintronic
devices. So far, spin current pulses are generated by femtosecond laser pulses
which drive spins from a ferromagnetic metal layer. However, the need for
miniaturization, simplicity and energy efficiency favour electric-field control
of spintronic devices over optic or thermal control. Here, we combine ab initio
calculations of electronic density of states at MgO/Fe interface with
continuous model for charge transport to investigate the dynamics of the
spin-dependent potential. We demonstrate that the voltage-driven instability of
the electronic band structure due to the electronic resonant states at the
Fe/MgO interface results in the generation of the femtosecond spin-polarized
current pulse with the spin polarization up to P=700% that propagates from the
interface to the bulk. The dynamics of the current pulses driven by the Stoner
instability depends neither on the dielectric relaxation time nor on the
details of how the instability is achieved by changing the voltage, i.e. as
long as the voltage changes are slow (quasi-static) with respect to the time
determined by the spin diffusion constant, being of the order of fs.The
presence of the instability may be detected by transport or magnetic
measurements.
