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Spin-1/2 Heisenberg antiferromagnets Cs2CuCl4 and Cs2CuBr4 with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their exchange parameters. The viability of the proposed method was proven by applying it to Cs2CuCl4, yielding J/kB=4.7(2)  K, J'/kB=1.42(7)  K, [J'/J≃0.30] and revealing good agreement with inelastic neutron-scattering results. For the isostructural Cs2CuBr4, we obtain J/kB=14.9(7)  K, J'/kB=6.1(3)  K, [J'/J≃0.41], providing exact and conclusive information on the exchange couplings in this frustrated spin system.

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RbFe(MoO4)2 is a quasi-two-dimensional (quasi-2D) triangular lattice antiferromagnet (TLA) that displays a zero-field magnetically-driven multiferroic phase with a chiral spin structure. By inelastic neutron scattering, we determine quantitatively the spin Hamiltonian. We show that the easy-plane anisotropy is nearly 1/3 of the dominant spin exchange, making RbFe(MoO4)2 an excellent system for studying the physics of the model 2D easy-plane TLA. Our measurements demonstrate magnetic-field induced fluctuations in this material to stabilize the generic finite-field phases of the 2D XY TLA. We further explain how Dzyaloshinskii-Moriya interactions can generate ferroelectricity only in the zero field phase. Our conclusion is that multiferroicity in RbFe(MoO4)2, and its absence at high fields, results from the generic properties of the 2D XY TLA. PACS numbers: 75.85.+t, The two-dimensional (2D) triangular lattice antiferromagnet (TLA) is a prototypical model in which to study frustrated magnetic interactions. For easy-plane magnetic anisotropy, the 120 • structure forms the zero-field groundstate. How the system evolves under an in-plane magnetic field has long been a subject of investigation since the equilibrium spin structures are expected to depend sensitively on both thermal 1-3 and quantum fluctuations 4 . Consequently, similar magnetic structure phase diagrams are expected within both XY and Heisenberg models 1-9 . Furthermore, a number of TLAs have multiferroic ground states, but the role of the triangular magnetic topology in the emergence of ferroelectricity is not well understood.While experimental realizations with which to test the predictions of the 2D TLA models are rare, RbFe(MoO 4 ) 2 (RFMO) stands out as an excellent example of a quasi-2D easy-plane TLA 10-14 . In addition, the zero field magnetically-ordered phase of RFMO is ferroelectric 14,15 , so the material provides a unique opportunity to study how multiferroicity is related to the generic fluctuations of the easy-plane TLA.In RFMO, magnetic Fe 3+ ions (S=5/2) form equilateral triangular lattice planes stacked along the c-axis [ Fig. 1 (a)]. In zero field, for T < T N ∼ 3.8 K the system displays incommensurate (IC) proper screw order with a 120 • structure in plane. The corresponding wave vector Q = (1/3,1/3,q z ), where q z ∼ 0.46 14 . The 120 • structure is chiral, since for any spin triangle there are two equivalent, yet distinct, ways to arrange the spins [see Figs. 1 (b) and (c)]. These chiral 120 • structures break the crystal inversion symme-( ) a Rb + Fe 3+ ( ) MoO 4 2-J J' J'' J''' a a c ( ) b ( ) c s (1) s (2) ( ) e 0 T 2 K 0 T 2 K ( ) d s (1) s (2) + 6 T 2.8 K 10 T 0.1 K a a H || [1-10] m 0 H || [1-10] m 0 s (1) s (2) + FIG. 1. (Color online) (a) The low temperature P 3 structure of RbFe(MoO4)2. O 2− mediated superexchange interaction paths are indicated, and the exchange hierarchy is J ≫ J ′ > J ′′ ∼ J ′′′ . (b)-(e) Magnetic structures within a single triangular lattice layer for fields up to 10 T 14 . (b) and (c)show degenerate ze...