  We simulate the spin torque-induced reversal of the magnetization in thin
disks with perpendicular anisotropy at zero temperature. Disks typically
smaller than 20 nm in diameter exhibit coherent reversal. A domain wall is
involved in larger disks. We derive the critical diameter of this transition.
Using a proper definition of the critical voltage, a macrospin model can
account perfectly for the reversal dynamics when the reversal is coherent. The
same critical voltage appears to match with the micromagnetics switching
voltage regardless of the switching path.
