The Magnetic Ground State of Atacamite Cu$_2$Cl(OH)$_3$: The Crucial Role of Frustrated Zigzag Chains Revealed by Inelastic Neutron Scattering

TL;DR

This study uses inelastic neutron scattering to explore the magnetic properties of atacamite, revealing the importance of frustrated zigzag chains and identifying two dispersive spin-wave modes with a significant energy gap.

Contribution

It demonstrates the crucial role of zigzag-chain models in understanding atacamite's low-energy magnetism, supported by experimental and first-principles calculations.

Findings

Observation of two dispersive spin-wave modes

Identification of a minimum 0.75 meV energy gap

Validation of the zigzag-chain model for magnetic behavior

Abstract

We report inelastic neutron scattering (INS) measurements on the magnetically frustrated $S=\frac12$ sawtooth-chain compound atacamite Cu$_2$Cl(OH)$_3$ featuring inequivalent Cu(1) and Cu(2) sites. Transverse to the sawtooth chains, INS reveals two dispersive spin-wave modes and a gap of at least 0.75 meV. This behavior is rationalized within a zigzag-chain model of Cu(2) spins in an effective magnetic field of Cu(1) spins. The model is compatible with first-principles calculations and accounts for INS dispersions within linear spin-wave theory calculations. Our results reveal a unique case of an effective separation of energy scales between two differently oriented one-dimensional chains, with the zigzag-chain model being essential to fully characterize atacamite's low-energy magnetism.