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The magnetic domain and bubble structures in the Z-type hexaferriteSr$_3$Co$_2$Fe$_{24}$O$_{41}$ were investigated using Lorentz microscopy. Thishexaferrite exhibits a room-temperature magnetoelectric effect that isattributed to its transverse conical spin structure (TC phase). Upon heating,the TC phase transforms into a ferrimagnetic phase with magnetic moments in thehexagonal $ab$ plane between 410 and 480 K (FM2 phase) and into anotherferrimagnetic phase with moments parallel to the $c$ axis between 490 and 680 K(FM1 phase). Accordingly, in this study, the magnetic domain structures inSr$_3$Co$_2$Fe$_{24}$O$_{41}$ were observed to change dramatically withtemperature. In the TC phase, irregular fine magnetic domains were observedafter cooling the specimen from the FM2 to TC phase. In the FM1 phase, stripedmagnetic domain walls with pairs of bright and dark contrast were formedparallel to the $c$ axis. Upon applying an external magnetic field, the stripedmagnetic domain walls transformed into magnetic bubbles. The topology of themagnetic bubbles was dependent on the angle between the external magnetic field($H$) direction and the easy $c$ axis. Namely, magnetic bubbles with thetopological number $N$ = 1 (type I) were created for $H//c$, whereas magneticbubbles with $N$ = 0 (type II) were created when the magnetic field was tiltedfrom the c axis by 5{\deg}. We attribute the high magnetocrystalline anisotropyof Sr$_3$Co$_2$Fe$_{24}$O$_{41}$ to the emergence of magnetic bubbles in theFM1 phase.

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