  To optimize the design of STT-MRAM (spin-transfer torque magnetic random
access memory), it is necessary to be able to predict switching (error) rates.
For small elements, this can be done using a single-macrospin theory since the
element will switch quasi-uniformly. Experimental results on switching rates
suggest that elements large enough to be thermally stable switch by some
mechanism with a lower energy barrier. It has been suggested that this
mechanism is local nucleation, but we have also previously reported a global
magnetostatic instability, which is consistent with the lower experimental
energy barriers. In this paper, we try to determine which of these mechanisms
is most important by visualizing the switching in a "U-NU" (uniform -
nonuniform) phase diagram. We find that switching trajectories follow the
horizontal U axis (i.e., quasi-uniform precession) until they reach a critical
amplitude, at which the magnetostatic instability grows exponentially and a
domain wall forms at the center, whose motion completes the switching. We have
tried unsuccessfully to induce local nucleation (a domain wall at the edge). We
conclude that the dominant switching mechanism is not edge nucleation, but the
magnetostatic instability.
