We use density-functional band-structure calculations to explore the origin of the up-up-down-down (UUDD) magnetic order in Cu2GeO4 with the frustrated J1−J2 spin chains coupled into layers within the spinel-like crystal structure. In contrast to earlier studies, we find that the nearest-neighbor coupling J1 may be negligibly small, owing to a nearly perfect compensation of the ferromagnetic direct exchange and antiferromagnetic superexchange. Under this condition, weak symmetric anisotropy of the exchange couplings gives rise to the UUDD order observed experimentally and also elucidates the non-trivial ordering pattern between the layers, whereas a small Dzyaloshinsky-Moriya interaction causes a spin canting that may generate local electric polarization. We argue that the buckling of the copper chains plays a crucial role in the suppression of J1 in Cu2GeO4 and sets this compound apart from other J1−J2 chain magnets.
@article{arxiv.1907.10475,
title = {The origin of up-up-down-down magnetic order in Cu$_2$GeO$_4$},
author = {Danis I. Badrtdinov and Vladimir V. Mazurenko and Alexander A. Tsirlin},
journal= {arXiv preprint arXiv:1907.10475},
year = {2019}
}