Quasi-two-dimensional antiferromagnet on a triangular lattice $\mathrm{RbFe}({\mathrm{MoO}}_4{)}_2$

$\mathrm{RbFe}({\mathrm{MoO}}_4{)}_2$ is a rare example of a nearly two-dimensional Heisenberg antiferromagnet on a triangular lattice. Magnetic resonance spectra and magnetization curves reveal that the system has a layered spin structure with six magnetic sublattices. The sublattices within a layer are arranged in a triangular manner with the magnetization vectors $120°$ apart. The $H-T$ phase diagram, containing at least five different magnetic phases is constructed. In zero field, $\mathrm{RbFe}({\mathrm{MoO}}_4{)}_2$ undergoes a phase transition at $T_N=3.8\mathrm{}\mathrm{K}$ into a noncollinear triangular spin structure with all the spins confined in the basal plane. The application of an in-plane magnetic field induces a collinear spin state between the fields $H_{c1\mathrm{}}=47\mathrm{}\mathrm{kOe}$ and $H_{c2\mathrm{}}=71\mathrm{}\mathrm{kOe}$ and produces a magnetization plateau at one-third of the saturation moment. Both the ESR and the magnetization measurements also clearly indicate an additional first-order phase transition in a field of 35 kOe. The exact nature of this phase transition is uncertain.