Magnetic order and spin dynamics in natural mineral brochantite Cu₄SO₄(OH)₆

Main structural components of mineral brochantite (BRO), Cu₄SO₄(OH)₆, are zigzag Cu²⁺ chains running along the c-axis and turning this material into a low-dimensional quantum magnet. Despite this, BRO orders below 6 K with a commensurate antiferromagnetic (CAFM) structure. Inelastic neutron scattering reveals gapped magnon-like excitations with considerable dispersion along the c*-axis and almost flat branches in other principal directions. The most striking observation is that the strong modes along are centered at the incommensurate positions, suggesting spiral spin dynamics in spite of experimentally observed CAFM. Such a behaviour can be explained by the effect of the uniform Dzyaloshinskii-Moriya interaction, which elevates the degeneracy of the spin-wave modes, shifting them in opposite directions in reciprocal space.

Response of BRO to external magnetic fields is very anisotropic with 5 and 14 T metamagnetic transitions for H//a and b, respectively. Theoretical modelling of the high-field properties of brochantite is currently in progress.