We demonstrate spin transfer torque (STT) switching in multi-nanocontact STT devices fabricated
using hole mask colloidal lithography. We also study the STT device resistance and switching
properties as a function of applied magnetic field and nanocontact current. At low nanocontact
current, magnetoresistance measurements show sharp, single-step switching at low switching
fields. When the current is increased, the switching becomes multistep, and the switching field
increases dramatically. We explain these results as arising from a transition from a predominantly
single domain like switching to switching involving a vortex state. Micromagnetic simulations
corroborate this picture, indicating that a single magnetic vortex nucleates in between the
nanocontacts through the influence from the total Oersted field generated by the nanocontact
ensemble.