Field dependence of magnetization reversal by spin transfer

We describe and analyze the effect of an applied field (formula presented) on the current-driven magnetization reversal in pillar-shaped Co/Cu/Co trilayers. Depending on the magnitude of (formula presented) we observe two different types of transitions between the parallel (P) and antiparallel (AP) magnetic configurations of the the Co layers. If (formula presented) is smaller than some threshold value, the transitions between P and AP are relatively sharp and irreversible. For (formula presented) exceeding this threshold value, the transitions are progressive and reversible. We show that this behavior can be precisely accounted for by introducing the current-induced torque of the spin transfer models into a Landau-Lifshitz-Gilbert equation to determine the stability or instability of the P and AP states. This analysis also provides a good description for the field dependence of the critical currents.