  We examined the formation mechanisms of magnetic bubbles in an M-type
hexaferrite via Lorentz microscopy. When magnetic fields were perpendicularly
applied to a thin sample of BaFe$_{12-x-0.05}$Sc$_x$Mg$_{0.05}$O$_{19}$ ($x =
1.6$), Bloch lines, which were identified as reversals of domain-wall
chirality, appeared, and magnetic bubbles were formed when the magnetic stripes
were pinched off at these Bloch lines. The number of Bloch lines increased with
the amount of Sc in BaFe$_{12-x-0.05}$Sc$_x$Mg$_{0.05}$O$_{19}$ probably
because of the reduction in magnetic anisotropy. A Lorentz microscopic
observation revealed that Bloch lines with high magnetostatic energy may play
an important role in the formation of magnetic bubbles.
