Field dependence of magnetization reversal by spin transfer

We describe and analyze the effect of an applied field ${(H}_{\mathrm{appl}})$ on the current-driven magnetization reversal in pillar-shaped Co/Cu/Co trilayers. Depending on the magnitude of $H_{\mathrm{appl}},$ we observe two different types of transitions between the parallel (P) and antiparallel (AP) magnetic configurations of the the Co layers. If $H_{\mathrm{appl}}$ is smaller than some threshold value, the transitions between P and AP are relatively sharp and irreversible. For $H_{\mathrm{appl}}$ 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.