  We demonstrate the reduction of critical spin-transfer torque (STT) switching
currents in Co-Fe-B/MgO based magnetic tunnel junctions (MTJ) with
perpendicular magnetization anisotropy (PMA). The junctions yield tunnel
magnetoresistance (TMR) ratios of up to 64% at 4 monolayer (ML) tunnel barrier
thickness. In this paper, the reduction of the critical switching current
density is studied. By optimizing the applied bias field during DC-STT
measurements, ultra low critical switching current densities of less than 20
kA/cm$^2$, even down to 9 kA/cm$^2$, are found. With the reduced switching
currents, our samples are ideal candidates for further experimental studies
such as the theoretical predicted thermally driven spin-transfer torque effect.
