Low Spin Polarization in Heavy-Metal–Ferromagnet Structures Detected Through Domain-Wall Motion by Synchronized Magnetic Field and Current

$\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ alloy 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 $\mathrm{Ta}/Co\text{−}Fe\text{−}\mathrm{B}/Mg\mathrm{O}$ 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 perform experiments of DW velocity measurement by applying magnetic field and current simultaneously. Working in the precessional regime, we are able to observe the effect of spin-polarized current, which allows the polarization rate of the charge carriers to be evaluated. An unexpected quite low spin-polarization rate down to 0.26 is obtained, which can explain the low efficiency of DW motion induced by the spin-polarized current. Possible reasons for this low rate, such as spin relaxation in the $\mathrm{Ta}$ layer, are analyzed.