Interaction between topological defects in Cu₂OSeO₃: skyrmion lattice annihilation by 3D point defects

Materials with topologically nontrivial spin textures promise technological advances and have attracted tremendous interest in recent years. Recently, an intriguing discovery of skyrmion phases at low temperatures and in the presence of an applied magnetic field has been made in an insulating multiferroic Cu₂OSeO₃ [1,2].

Its phase diagram shows a variety of noncollinear spin states. At low magnetic fields, the ground state is an incommensurate helical spiral with a long period of about 60nm. The magnetic field-temperature phase diagram was similar to those reported for the metallic alloys that show skyrmions. Moreover, the system can become polar when the hexagonal skyrmion lattice is formed in Cu2OSeO3 with a small applied magnetic field [3].

Our previous study [4] with the effective Hamiltonian scheme has reproduced the magnetic field-temperature phase diagram. We use the same approach to reveal the hidden connection between various topological defects (3D point defects and skyrmions) in Cu2OSeO3. We show (i) that Cu2OSeO3 hosts different topological defects with a non-zero local topological charge at different regions of the phase diagram (H, T). (ii) Point defects exerting external pressure on skyrmions, leading to their gradual annihilation as temperature increases. (iii) The annihilation of skyrmions is done by the melting of the lattice into a glassy-like state of spins that connects with the paramagnetic phase [5].