Multiferroicity in the generic easy-plane triangular lattice antiferromagnet RbFe(MoO4)2

Abstract

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...

Magnetic phase diagram ofBa3CoSb2O9as determined by ultrasound velocity measurements

“…8 The subject of this work, Ba 3 CoSb 2 O 9 , represents the first truly quantum (effective spin-1/2) antiferrromagnet with a perfect triangular lattice and as expected it exhibits a clear magnetization plateau in the range of 10-15 T.…”

High-field magnetization and ESR in the triangular-lattice antiferromagnets Ba3MnSb2O9 and Ba3TNb2O9 (T=Ni, Co)

“…The zero-field ground state is highly degenerate with a 120° spin structure [5,6]. Application of external magnetic field modifies the 120° ordered phase and induces an up-up-down state in a finite range of magnetic field [7,8]. As a result, a magnetization plateau in which the magnetization is 1/3 of the saturation magnetization M s , emerges in the magnetization process [9][10][11].…”

Order by Quenched Disorder in the Model Triangular Antiferromagnet RbFe(MoO4)2

“…The nearest-neighbor exchange constant J ij is assumed to have a weak random variation between the bonds: J ij ¼ hJ ij i þ δJ ij , hδJ ij δJ kl i ¼ δJ 2 δ ij;kl , and δJ ≪ J ¼ hJ ij i. Inelastic neutron scattering experiments yield J 0 ¼ 0.086 meV and D 0 ¼ 0.027 meV for RbFeðMoO 4 Þ 2 [27]. The effect of bond disorder and/or thermal and quantum fluctuations can be semiquantitatively represented by adding a biquadratic term…”