  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.
