New vortex phase in antiferromagnetic incommensurate magnet Ba₂CuGe₂O₇

Ba₂CuGe₂O₇ is a quasi-2D insulator characterised by a tetragonal, noncentrosymmetric space group (P-4₂1m) with Dzyaloshinskii–Moriya interactions (DMI). Because of the DMI, at ground state below T_N = 3.05 K, it exhibits an almost AF cycloidal spin structure with a pitch of 220 Å. The magnetic structures of Ba₂CuGe₂O₇ for different temperature and magnetic field values have been the topic of numerous experiments [1] indicating a rich phase diagram with a multitude of incommensurate (IC) phases. Following a recent theoretical prediction, the existence of a vortex phase [2] with non-trivial topological properties in Ba₂CuGe₂O₇ has been verified experimentally, by means of neutron scattering and bulk measurements of specific heat and AC susceptibility. Particularly, hints towards the presence of a vortex phase were found in the neutron scattering data. There significant measured intensity is seen in the region of interest, noticeably higher than in the paramagnetic phase. This intensity is evenly distributed among four IC satellites on a square plane in reciprocal space, even in the presence of a small in-plane magnetic field component. However, it remains unclear whether the observed phase is due to static long range order or is a fluctuating dynamic texture that is on the verge of ordering.



[1] Mühlbauer S, et al., Phys. Rev. B, 86(2), 1–12. https://doi.org/10.1103/PhysRevB.86.024417 (2012)

[2] Wolba B, PhD thesis. Karlsruher Instituts für Technologie, (2021)