Sprecher
Beschreibung
Incommensurate spiral magnets have raised tremendous interest in recent years, mainly motivated by their wealth of spin structures with potential non-trivial topology. A second field of interest is multiferroicity: Helical spin structures are in general ferroelectric[1]. Both fields present enormous potential for future devices, where spin and charge degrees of freedom are coupled. Antiferromagnetic $Ba_{2}CuGe_{2}O_{7}$, characterized by a quasi-2D structure with Dzyaloshinskii-Moriya interactions (DMI) combines these regards with a third one: a variety of unconventional magnetic phase transitions. $Ba_{2}CuGe_{2}O_{7}$ is an insulator characterized by a tetragonal, non-centrosymmetric space group (P-421m) with lattice parameters a = 8.466 Å and c = 5.445 Å. The magnetic structure is mainly due to the square arrangement of Cu2+ ions in the tetragonal (a,b) plane with dominant nearest-neighbor AF exchange along the diagonal in the plane and much weaker FM exchange between planes. Below the Néel temperature $T_{N}$ = 3.05K, the DMI term leads to a long-range incommensurate, almost AF spin cycloid with the spins (almost) confined in the (1,-1,0) plane [2,3].
At zero external field, neutron diffraction is used for a careful examination of the distribution of critical fluctuations in reciprocal space, associated with the paramagnetic to helimagnetic transition of $Ba_{2}CuGe_{2}O_{7}$. Caused by the reduced dimensionality of $Ba_{2}CuGe_{2}O_{7}$, a crossover from incommensurate antiferromagnetic fluctuations to 2D antiferromagnetic Heisenberg fluctuations is observed, highlighting the rich cornucopia of magnetic phase transitions in spiral magnetic textures.
Recently, a new phase with a vortex-antivortex magnetic structure has been theoretically described[4]. It has been experimentally confirmed in a phase diagram pocket at around 2.4K and an external field along the crystalline c-axis of around 2.2T. A lack of evidence for a thermodynamic phase transition towards the paramagnet in high resolution specific heat measurements and a finite linewidth in energy and momentum of the incommensurate peaks in neutron scattering, as opposed to the cycloidal ground state, seem to mark the vortex phase as a slowly fluctuating structure at the verge of ordering. Polarization measurements and neutron experiments including electrical field to investigate its interplay with an external magnetic field are planned and will allow for further pinning down multiferroic properties of $Ba_{2}CuGe_{2}O_{7}$ [5].
[1] M. Mostovoy. Phys. Rev. Lett., 96:1–4, 2006.
[2] S. Mühlbauer et al, Rev. Mod. Phys, 91, 015004 (2019)
[3] A. Zheludev, et al. Phys. Rev. B, 54 (21):15163- 15170, (1996).
[4] B. Wolba. PhD thesis, KIT, 2021.
[5] H. Murakawa et al. Phys. Rev. Lett., 103(14):2–5,2009.
