  We study field-driven magnetic domain wall dynamics in garnet strips by
large-scale three-dimensional micromagnetic simulations. The domain wall
propagation velocity as a function of the applied field exhibits a low-field
linear part terminated by a sudden velocity drop at a threshold field
magnitude, related to the onset of excitations of internal degrees of freedom
of the domain wall magnetization. By considering a wide range of strip
thicknesses from 30 nm to 1.89 $\mu$m, we find a non-monotonic thickness
dependence of the threshold field for the onset of this instability, proceeding
via nucleation and propagation of Bloch lines within the domain wall. We
identify a critical strip thickness above which the velocity drop is due to
nucleation of horizontal Bloch lines, while for thinner strips and depending on
the boundary conditions employed, either generation of vertical Bloch lines, or
close-to-uniform precession of the domain wall internal magnetization takes
place. For strips of intermediate thicknesses, the vertical Bloch lines assume
a deformed structure due to demagnetizing fields at the strip surfaces,
breaking the symmetry between the top and bottom faces of the strip, and
resulting in circulating Bloch line dynamics along the perimeter of the domain
wall.
