Anisotropy stabilized magnetic phases of the triangular antiferromagnet RbFe(MoO$_4$)$_2$

The magnetic H - T phase diagram of a quasi-two-dimensional easy plane antiferromagnet RbFe(MoO$_4$)$_2$ (S = 5/2) with an equilateral triangular lattice structure is studied with $^{87}$Rb NMR technique for field directed along hard axis C3. The studies confirm the two step transition from the low field umbrella-like incommensurate magnetic phase to the paramagnetic state observed recently (Mitamura et al. 2016). The transitions were identified as a lambda anomaly in the spin-lattice relaxation rate and a step increase of magnetic susceptibility at intermediate transition. The $^{87}$Rb NMR study precludes the possibility of either V or fan spin textures in the X phase. The additional transition is presumably associated with loss of inter plane magnetic order before the transition to paramagnetic state of individual triangular planes.