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

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.

11 pages, 12 figures (minor corrections and 3 additional have been included in V3)