Origin of up-up-down-down magnetic order in Cu2GeO4

Abstract

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 should be negligibly small, due 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 nontrivial ordering pattern between the layers, whereas a small Dzyaloshinskii-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. © 2019 American Physical Society.