  The influence of the tantalum buffer layer on the magnetic anisotropy of
perpendicular Co-Fe-B/MgO based magnetic tunnel junctions is studied using
magneto-optical Kerr-spectroscopy. Samples without a tantalum buffer are found
to exhibit no perpendicular magnetization. The transport of Boron into the
tantalum buffer is considered to play an important role on the switching
currents of those devices. With the optimized layer stack of a perpendicular
tunnel junction, a minimal critical switching current density of only 9.2
kA/cm$^2$ is observed. As of today, this value is the lowest reported value for
current-induced magnetization reversal.
