Multiple field-induced phases in the frustrated triangular magnet Cs$_3$Fe$_2$Br$_9$

TL;DR

This study reveals a complex magnetic phase diagram in Cs$_3$Fe$_2$Br$_9$, featuring multiple field-induced phases, including fractional magnetization states, driven by magnetic frustration and strong magnetoelastic coupling.

Contribution

It uncovers ten distinct magnetic phases in Cs$_3$Fe$_2$Br$_9$, highlighting the effects of frustration and anisotropy in a triangular lattice with high-spin Fe$^{3+}$ ions.

Findings

Ten different ordered magnetic phases identified.

Presence of fractional magnetization plateaus at 1/3 and 1/2 saturation.

Strong hysteresis and magnetoelastic effects observed.

Abstract

The recently discovered material Cs$_3$Fe$_2$Br$_9$ contains Fe$_2$Br$_9$ bi-octahedra forming triangular layers with hexagonal stacking along the $c$ axis. In contrast to isostructural Cr-based compounds, the zero-field ground state is not a nonmagnetic $S=0$ singlet-dimer state. Instead, the Fe$_2$Br$_9$ bi-octahedra host semiclassical $S=5/2$ Fe$^{3+}$ spins with a pronounced easy-axis anisotropy along $c$ and interestingly, the intra-dimer spins are ordered ferromagnetically. The high degree of magnetic frustration due to (various) competing intra- and inter-dimer couplings leads to a surprisingly rich magnetic phase diagram. Already the zero-field ground state is reached via an intermediate phase, and the high-field magnetization and thermal expansion data for $H\parallel c$ identify ten different ordered phases. Among them are phases with constant magnetization of 1/3, respectively 1/2 of the saturation value, and several transitions are strongly hysteretic with pronounced length changes reflecting strong magnetoelastic coupling.