  We numerically study ultra fast resonant spin torque (ST) magnetization
reversal in magnetic tunnelling junctions (MTJ) driven by current pulses having
a direct current (DC) and a resonant alternating current (AC) component. The
precessional ST dynamics of the single domain MTJ free layer cell are modelled
in the macro spin approximation. The energy efficiency, reversal time, and
reversal reliability are investigated under variation of pulse parameters like
direct and AC current amplitude, AC frequency and AC phase. We find a range of
AC and direct current amplitudes where robust resonant ST reversal is obtained
with faster switching time and reduced energy consumption per pulse compared to
purely direct current ST reversal. However for a certain range of AC and direct
current amplitudes a strong dependence of the reversal properties on AC
frequency and phase is found. Such regions of unreliable reversal must be
avoided for ST memory applications.
