  CoFeB is a very soft material, in which Domain Wall (DW) can be moved easily
under a weak magnetic field. However, it is very difficult to move DWs in
Ta\CoFeB\MgO nanowires with interfacial perpendicular magnetic anisotropy
through a spin-polarized current, and this limits the perspectives of racetrack
memory driven by the current-in-plane mechanism. To investigate this
phenomenon, we performed experiments of DW velocity measurement by applying a
magnetic field and a current simultaneously. Working in the precessional
regime, we have been able to see a very important effect of the spin-polarized
current, which allows evaluating the polarization rate of the charge carriers.
An unexpected quite low spin polarization rate down to 0.26 have been obtained,
which can explain the low efficiency of DW motion induced by the spin-polarized
current. Possible reasons for this low rate are analyzed, such as the spin
relaxation in the Ta layer.
