Topological stability of the double vertical Bloch line in a canted antiferromagnetic system

Topological defects in the magnetic system have been attracting a great attention, and the skyrmion, in particular, has been considered as an important candidate for a nanoscale digital information carrier in the spintronic applications. In this work, we investigate the topological defects in canted antiferromagnetic materials with the orthoferrite as a model system, and demonstrate that the vertical Bloch line (VBL) can couple to the other VBL to form a stable antibonding state. Whereas such a double VBL is a topologically non-trivial object with a skyrmion number equivalent to that of the skyrmion itself, we demonstrate that a transition to other topological states is possible only when a strong enough magnetic field is applied. As long as the double VBL is in a stable condition, it is found that that the magnetic field can drive it to move straight with a high speed, 3000 m/s without experiencing either a walker-breakdown or a skyrmion Hall effect.