  We investigated the aspect ratio (thickness/width) dependence of the
threshold current density required for the current-driven domain wall (DW)
motion for the Ni81Fe19 nanowires. It has been shown theoretically that the
threshold current density is proportional to the product of the hard-axis
magnetic anisotropy Kperp and the DW width lamda. (Phys. Rev. Lett. 92, 086601
(2004).) We show experimentally that Kperp can be controlled by the magnetic
shape anisotropy in the case of the Ni81Fe19 nanowires, and that the threshold
current density increases with an increase of Kperp*l. We succeeded to reduce
the threshold current density by half by the shape control.
