Abstract
The recently discovered material Cs3Fe2Br9 contains Fe2Br9 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 Fe2Br9 bi-octahedra host semiclassical S = 5/2 Fe(3+) spins with a pronounced easy-axis anisotropy along c, and interestingly, the intradimer spins are ordered ferromagnetically. The high degree of magnetic frustration due to (various) competing intradimer and interdimer couplings leads to a surprisingly rich magnetic phase diagram.
The zero-field ground state is already reached via an intermediate phase, and the high-field magnetization and thermal expansion data for H parallel to c identify 10 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.