Frustrated magnetism of S=5/2 moments on a coupled triangular lattice in Cs${}_3$Fe${}_2$Br${}_9$

Cs${}_3$Fe${}_2$Br${}_9$ is a hexagonal material consisting of Fe${}^{3+}$ ions with S=5/2 in face-sharing Fe${}_2$Br${}_9$ bi-octahedra, which form hexagonal double layers of the Fe ions. The triangular arrangement of the magnetic ions in the individual layers causes magnetic frustration. The type of magnetic ground state depends on the ratio between the magnetic exchange couplings: the intradimer coupling J, the intralayer (or in-plane) coupling J${}_p$, and the interlayer coupling J${}_c$. The magnetic ground state of Cs${}_3$Fe${}_2$Br${}_9$ is not a singlet-dimer state as in isostructural Cs${}_3$Cr${}_2$Br${}_9$ and Cs${}_3$Cr${}_2$Cl${}_9$, but there is evidence for antiferromagnetic order with T${}_N$ = 13.5 K. However, our measurements up to 17 T on large single crystals reveal a very unusual magnetic field vs. temperature phase diagram. For an in-plane field, we find linear M(H) curves, whereas a field H$\parallel$c causes multiple phase transitions including a magnetization plateau of 1/3 M${}_{sat}$ = 10 ${\mu }_B$. Neutron diffraction resolved the magnetic structure of two phases which indicate an increasing intralayer coupling. Additionally, we present pulsed-field magnetization measurements revealing further transitions, before reaching saturation around 40 T for H$\parallel$c.