  Spin dynamics of field-driven domain walls (DWs) guided by Permalloy
nanowires are studied by high-speed magneto-optic polarimetry and numerical
simulations. DW velocities and spin configurations are determined as functions
of longitudinal drive field, transverse bias field, and nanowire width.
Nanowires having cross-sectional dimensions large enough to support vortex wall
structures exhibit regions of drive-field strength (at zero bias field) that
have enhanced DW velocity resulting from coupled vortex structures that
suppress oscillatory motion. Factor of ten enhancements of the DW velocity are
observed above the critical longitudinal drive-field (that marks the onset of
oscillatory DW motion) when a transverse bias field is applied. Nanowires
having smaller cross-sectional dimensions that support transverse wall
structures also exhibit a region of higher mobility above the critical field,
and similar transverse-field induced velocity enhancement but with a smaller
enhancement factor. The bias-field enhancement of DW velocity is explained by
numerical simulations of the spin distribution and dynamics within the
propagating DW that reveal dynamic stabilization of coupled vortex structures
and suppression of oscillatory motion in the nanowire conduit resulting in
uniform DW motion at high speed.
