High-field magnetic properties of the alternating ferro-antiferromagnetic spin-chain compound Cu$_2$(OH)$_3$Br

TL;DR

This study investigates the high-field magnetic behavior of the Cu$_2$(OH)$_3$Br spin-chain compound, revealing a magnetization plateau, spin-reorientation, complex excitations, and field-dependent magnetic ordering.

Contribution

It provides the first comprehensive high-field analysis of Cu$_2$(OH)$_3$Br, including magnetization, spin dynamics, and phase diagram, highlighting novel magnetic phenomena in ferro-antiferromagnetic chains.

Findings

Observation of a half-saturation magnetization plateau.

Identification of a spin-reorientation transition with angular dependence.

Detection of high-frequency spinon-magnon bound states and antiferromagnetic resonance modes.

Abstract

We present comprehensive high magnetic field studies of the alternating weakly coupled ferro-antiferromagnetic (FM-AFM) spin-$1/2$ chain compound Cu$_2$(OH)$_3$Br, with the structure of the natural mineral botallackite. Our measurements reveal a broad magnetization plateau at about half of the saturation value, strongly suggesting that the FM chain sublattice becomes fully polarized, while the AFM chain sublattice remains barely magnetized, in magnetic fields at least up to $50$ T. We confirm a spin-reorientation transition for magnetic fields applied in the $ac^\ast$-plane, whose angular dependence is described in the framework of the mean-field theory. Employing high-field THz spectroscopy, we reveal a complex pattern of high-frequency spinon-magnon bound-state excitations. On the other hand, at lower frequencies we observe two modes of antiferromagnetic resonance, as a consequence of the long-range magnetic ordering. We demonstrate that applied magnetic field tends to suppress the long-range magnetic ordering; the temperature-field phase diagram of the phase transition is obtained for magnetic fields up to $14$ T for three principal directions ($a$, $b$, $c^\ast$).