Field-induced successive phase transitions in the J1−J2 buckled honeycomb antiferromagnet Cs3Fe2Cl9

The magnetic properties of ${\mathrm{Cs}}_{3}{\mathrm{Fe}}_{2}{\mathrm{Cl}}_{9}$ single crystals were investigated by magnetic and thermal measurements. The crystal structure of ${\mathrm{Cs}}_{3}{\mathrm{Fe}}_{2}{\mathrm{Cl}}_{9}$ consists of a bilayer triangular lattice of ${\mathrm{Fe}}^{3+}$ ions. The analysis of magnetic susceptibility clarified that the intra- and interdimer interactions are comparable with each other, and ${\mathrm{Cs}}_{3}{\mathrm{Fe}}_{2}{\mathrm{Cl}}_{9}$ is regarded as a spin-5/2 ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ buckled honeycomb antiferromagnet with the ferromagnetic interlayer interaction ${J}_{3}$. The susceptibility for the magnetic field $H||c$ axis exhibits a sudden drop at the N\'eel temperature ${T}_{\mathrm{N}}=5.4\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, indicating a first-order magnetic phase transition. One of the characteristic features is the linear temperature dependence below ${T}_{\mathrm{N}}$. Furthermore, ${\mathrm{Cs}}_{3}{\mathrm{Fe}}_{2}{\mathrm{Cl}}_{9}$ undergoes successive magnetic phase transitions in high magnetic fields along the $c$ axis. We obtained a rich H-T phase diagram in which the ${M}_{\mathrm{s}}/2$ magnetization-plateau phase is included. These unique magnetic behaviors probably originate from the competition of magnetic interactions and the easy-axis anisotropy in the antiferromagnetic ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ buckled honeycomb network.