Figure 3
From: Domain wall of a ferromagnet on a three-dimensional topological insulator
The energy of the domain wall EDW as a function of ϕ for (a) J3 = 0, (b) J3 = J0.
When J3 = 0, the lowest energy domain wall structure is at ϕ = 0 for μ in the 2D valence/conduction bands or inside the gap. It turns into ϕ = π when μ is in the 3D valence/conduction bands. When J3 = J0, on the other hand, ϕ = 0 is most stable when μ is in the 2D conduction band and nearly Bloch wall 
is stable for μ inside the gap. ϕ = π is the most stable for other cases. This behavior can be understood by considering the DM derived from the 2D surface states.
