  Electric current exerts torques-so-called spin transfer torques (STTs)-on
magnetic domain walls (DWs), resulting in DW motion. At low current densities,
the STTs should compete against disorders in ferromagnetic nanowires but the
nature of the competition remains poorly understood. By achieving
two-dimensional contour maps of DW speed with respect to current density and
magnetic field, here we visualize unambiguously distinct roles of the two
STTs-adiabatic and nonadiabatic-in scaling behaviour of DW dynamics arising
from the competition. The contour maps are in excellent agreement with
predictions of a generalized scaling theory, and all experimental data collapse
onto a single curve. This result indicates that the adiabatic STT becomes
dominant for large current densities, whereas the nonadiabatic STT-playing the
same role as a magnetic field-subsists at low current densities required to
make emerging magnetic nanodevices practical.
