Incommensurate Magnetic Phase Transitions in the Triangular XY-Like Antiferromagnet RbFeCl3

RbFeCl 3 is composed of one-dimensional chains in which the spins interact ferromagnetically with XY-like anisotropy. These chains are coupled antiferromagnetically in the hexagonal basal plane. It undergoes three transitions at T N1 =2.5 K, T N2 =2.35 K and T N3 =1.95 K. In order to understand the nature of these transitions, we performed a neutron scattering study. Besides the previously determined 120°-structure observed at T < T N 3 , we found two different incommensurate structures in the basal plane at T N3 < T < T N2 and T N2 < T < T N1 . Only, the transition at T N3 is the first order, but the other two are the second order. The effect of the field applied parallel to the easy plane has also been studied.