  We report the observation and micromagnetic analysis of current-driven
magnetization switching in nanoscale ring-shaped magnetic tunnel junctions.
When the electric current density exceeds a critical value of the order of
$6\times 10^{6}$A/cm$^2$, the magnetization of the two magnetic rings can be
switched back and forth between parallel and antiparallel onion states.
Theoretical analysis and micromagnetic simulation show that the dominant
mechanism for the observed current-driven switching is the spin torque rather
than the current-induced circular Oersted field.
