  The magnetic domain and bubble structures in the Z-type hexaferrite
Sr$_3$Co$_2$Fe$_{24}$O$_{41}$ were investigated using Lorentz microscopy. This
hexaferrite exhibits a room-temperature magnetoelectric effect that is
attributed to its transverse conical spin structure (TC phase). Upon heating,
the TC phase transforms into a ferrimagnetic phase with magnetic moments in the
hexagonal $ab$ plane between 410 and 480 K (FM2 phase) and into another
ferrimagnetic phase with moments parallel to the $c$ axis between 490 and 680 K
(FM1 phase). Accordingly, in this study, the magnetic domain structures in
Sr$_3$Co$_2$Fe$_{24}$O$_{41}$ were observed to change dramatically with
temperature. In the TC phase, irregular fine magnetic domains were observed
after cooling the specimen from the FM2 to TC phase. In the FM1 phase, striped
magnetic domain walls with pairs of bright and dark contrast were formed
parallel to the $c$ axis. Upon applying an external magnetic field, the striped
magnetic domain walls transformed into magnetic bubbles. The topology of the
magnetic bubbles was dependent on the angle between the external magnetic field
($H$) direction and the easy $c$ axis. Namely, magnetic bubbles with the
topological number $N$ = 1 (type I) were created for $H//c$, whereas magnetic
bubbles with $N$ = 0 (type II) were created when the magnetic field was tilted
from the c axis by 5{\deg}. We attribute the high magnetocrystalline anisotropy
of Sr$_3$Co$_2$Fe$_{24}$O$_{41}$ to the emergence of magnetic bubbles in the
FM1 phase.
