Magnetic phases of the S =5/2 triangular-lattice antiferromagnet RbFe(MoO_4)_2 as determined by ultrasound velocity measurements

摘要

RbFe(MoO_4)_2 is one of the best physical examples of a quasi-two-dimensional easy-plane triangular-lattice antiferromagnet. Although the magnetic phase diagram of this material has been well-studied, there are discrepancies in the literature concerning the number of magnetic phases, as well as the nature of the phase transitions. By performing ultrasound velocity measurements, we have obtained a detailed phase diagram with magnetic fields parallel and perpendicular to the triangular layers. The order of the phase transitions was found to be consistent with previous theoretical studies of generic two-dimensional triangular antiferromagnets. To support our experimental findings, we present a phenomenological model that includes biquadratic and next-nearest-neighbor interplane interactions. This model is sufficient to reproduce the phase diagram at low temperatures, and the next-nearest-neighbor interactions are shown to stabilize the incommensurate phases (Y and V). Furthermore, the model provides insight regarding the magnetic ordering along the r-axis in the magnetic plateau phase, suggesting that period 3 is established whenever the next-nearest-neighbor superexchange interactions are unequal. In the systems in which these interactions are expected to be equal, the model predicts degeneracy between period 2 and period 3 structures, at the classical level.