  In ferroelectrics, complex interactions among various degrees of freedom
enable the condensation of topologically protected polarization textures. Known
as ferroelectric solitons, these particle-like structures represent a new class
of materials with promise for beyond CMOS technologies due to their ultrafine
size and sensitivity to external stimuli. Such polarization textures have
scarcely been reported in multiferroics. Here, we report a range of soliton
topologies in bismuth ferrite strontium titanate superlattices. High-resolution
piezoresponse force microscopy and Cs-corrected high-angle annular dark-field
scanning transmission electron microscopy reveal a zoo of topologies, and
polarization displacement mapping of planar specimens reveals center-convergent
and divergent topological defects as small as 3 nm. Phase field simulations
verify that some of these topologies can be classed as bimerons, with a
topological charge of plus and minus one, and first-principles-based effective
Hamiltonian computations show that the co-existence of such structures can lead
to non-integer topological charges, a first observation in a BiFeO3-based
system. Our results open new opportunities in multiferroic topotronics.
