Triangular lattice antiferromagnet RbFe(MoO4)2 in an applied magnetic field

RbFe(MoO4)2 is a rare example of a nearly two-dimensional Heisenberg antiferromagnet on a triangular lattice. We have studied its low-temperature magnetic properties by means of single crystal ESR spectroscopy and magnetization measurements. On the basis of the obtained results an H-T phase diagram, containing at least five different magnetic phases is constructed. In zero field, RbFe(MoO4)2 undergoes a phase transition at TN=3.8 K into a noncollinear spin structure with all the spins confined in the hexagonal plane. The application of an external magnetic field in the plane induces a collinear spin state (Hc1=47 kOe, Hc2=71 kOe) and produces magnetization plateaux at one-third of the saturation moment. Both the ESR and 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 unclear