Observation of magnetic domain and bubble structures in magnetoelectric Sr$_3$Co$_2$Fe$_{24}$O$_{41}$

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.