  Current-driven magnetization switching in low-resistance
Co40Fe40B20/MgO/Co40Fe40B20 magnetic tunnel junctions (MTJs) is reported. The
critical-current densities Jc required for current-driven switching in samples
annealed at 270C and 300C are found to be as low as 7.8 x 10^5 A/cm^2 and 8.8 x
10^5 A/cm^2 with accompanying tunnel magnetoresistance (TMR) ratios of 49% and
73 %, respectively. Further annealing of the samples at 350C increases TMR
ratio to 160 %, while accompanying Jc increases to 2.5 x 10^6 A/cm^2. We
attribute the low Jc to the high spin-polarization of tunnel current and small
MsV product of the CoFeB single free layer, where Ms is the saturation
magnetization and V the volume of the free layer.
