  The spin transfer torque generated by a spin-polarized current can induce the
shift of the magnetic domain-wall position. In this work, we study
theoretically the current-induced domain-wall motion by using the collective
coordinate approach [Gen Tatara and Hiroshi Kohno, Phys. Rev. Lett. 92, 86601
(2004)]. The approach is extended to include not only the domain-wall position
and the polarization angle changes but also the domain-wall width variation. It
is demonstrated that the width variation affects the critical current.
