Magnetic Phase Diagram of Rouaite, Cu\(_2\)(OH)\(_3\)NO\(_3\)

Spinon-magnon mixing was recently reported in botallackite Cu\(_2\)(OH)\(_3\)Br with a uniaxially compressed triangular lattice of Cu\(^{2+}\) quantum spins [Zhang et al., Phys. Rev. Lett. 125, 037204 (2020)]. Its nitrate counterpart rouaite, Cu\(_2\)(OH)\(_3\)NO\(_3\), has a highly analogous structure and might be expected to exhibit similar physics. To lay a foundation for research on this material, we clarify rouaite's magnetic phase diagram and identify both low-field phases. The low-temperature magnetic state consists of alternating ferro- and antiferromagnetic chains, as in botallackite, but with additional canting, leading to net moments on all chains which rotate from one chain to another to form a 90\(^\circ\) cycloidal pattern. The higher-temperature phase is a helical modulation of this order, wherein the spins rotate from one Cu plane to the next. This extends to zero temperature for fields perpendicular to the chains, leading to a set of low-temperature field-induced phase transitions. Rouaite may offer another platform for spinon-magnon mixing, while our results suggest a delicate balance of interactions and high tunability of the magnetism.