Phase diagram and optimal switching induced by spin Hall effect in a perpendicular magnetic layer

In a ferromagnet/heavy-metal bilayer device with strong spin Hall effect an in-plane current excites magnetic dynamics through spin torque. We analyze bilayers with perpendicular magnetization and calculate three-dimensional phase diagrams describing switching by external magnetic field at a fixed current. We then concentrate on the case of a field applied in the plane formed by the film normal and the current direction. Here we analytically study the evolution of both the conventional up/down magnetic equilibria and the additional equilibria created by the spin torque. Expressions for the stability regions of all equilibria are derived, and the nature of switching at each critical boundary is discussed. The qualitative picture obtained this way predicts complex hysteresis patterns that should occur in bilayers. Analyzing the phase portraits of the system we find regimes where switching between the up and down states proceeds through the current-induced state as an intermediate. The first step of such two-step process is fast and resembles ballistic switching for the reasons discussed in the paper. Using numeric simulations we analyze the switching time and compare it to that of a conventional spin torque device with collinear magnetizations of the polarizer and the free layer.