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

TL;DR

This paper maps the magnetic phase diagram of rouaite, Cu$_2$(OH)$_3$NO$_3$, revealing complex magnetic orders and phase transitions, and suggesting its potential for studying spinon-magnon mixing phenomena.

Contribution

It provides the first detailed magnetic phase diagram of rouaite, identifying its low-field phases and magnetic structures, and highlights its potential for exploring spinon-magnon mixing.

Findings

Low-temperature magnetic state with alternating ferro- and antiferromagnetic chains and cycloidal order.

Higher-temperature phase exhibits helical spin modulation across Cu planes.

Presence of field-induced phase transitions at low temperatures.

Abstract

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.