  We investigate the dependence of switching current density on the junction
sizes in the spin transfer torque nanopillar structures by using micromagnetic
simulations. While the macro spin model predicts weak dependence of switching
current density on the junction sizes, we find that the switching current
density is a sensitive function of the junction sizes. It can be explained with
the complicated spin configurations and dynamics during the switching process.
The detail spin configurations and dynamics are determined by spin wave
excitation with the finite wave vector, which is related with the exchange
coupling energy and junction shape.
