Magnetic field induced polar phase in the chiral magnetCsCuCl3

Abstract

Magnetoelectric effects in the chiral magnet CsCuCl3 have been investigated through the magnetization and electric polarization measurements in pulsed high magnetic fields. If the magnetic field is applied normal to the spin chiral c-axis, a plateau and a jump in magnetization are observed. Between the plateau and the jump in magnetization, a small but significant electric polarization of about 0.25 µC/m 2 along the a-axis is observed with the field applied almost perpendicular to the ac-plane. In addition, the paramagnetoelectric effect expected from the point group symmetry is confirmed. The emergence of the electric polarization is explained by the cooperation of the local electric polarization on the chiral solitonic spin arrangement and the paramagnetoelectric effect. Hence, we interpret this novel multiferroic phase in CsCuCl3 as a polar solitonic phase.PACS numbers: 75.30.Kz, 75.85.+t Various triangular lattice antiferromagnets (TAFM) have long been studied with the focus on their geometric frustration from the antiferromagnetic coupling of the neighboring spins on a triangular lattice [1][2][3]. Among the TAFM systems, a hexagonal CsCuCl 3 has received sustained interest because of its distinctive chiral crystal structure and its resultant magnetism under applied magnetic fields. The S = 1/2 spins of the Cu 2+ ions form ferromagnetically coupled chains along the c-axis with J 0 = 28 K [4]. The chains are antiferromagnetically coupled and form a triangular lattice in the ab-plane, leading to a 120• spin structure below T N = 10.7 K at zero field [5]: the coupling strength is estimated to J 1 = -4.9 K [7]. The Curie-Weiss temperature obtained by the high-temperature magnetic susceptibility is comparable to T N for the magnetic fields along and perpendicular to the triangular lattice [1,6]. One of the most striking characteristics in CsCuCl 3 is that the Cu-chains are displaced helically in the triangular plane because of the cooperative Jahn-Teller distortion below 423 K [8]: the Cu-chains form helices along the c-axis with a six-ion periodicity [see Fig. 4(b)] [9]. Depending on the chirality, CsCuCl 3 has space group of P 6 1 22 or P 6 5 22 (i.e., point group 622) [10]. The absence of inversion symmetry makes the Dzyaloshinskii-Moriya (DM) interaction active: the strength is estimated to D = 5 K [5], which is comparable with J 1 . The ferromagnetic (FM) interaction tends to align spin moments, whereas the DM interaction favors perpendicular arrangements of adjacent spins. The resultant spin configuration for CsCuCl 3 is helical arising from the competition of these interactions with a period of 11.8c [5].

Continuous control of classical-quantum crossover by external high pressure in the coupled chain compound CsCuCl3

“…Therefore, the effective spinS should be determined in such a way that it reflects the Theoretical magnetization curves obtained by the 1/S expansion method 15 with the "cutting-at-1/3" procedure 18 using the model parameters J 0 (P), Δ 0 (P), and J 1 (P) at temperature T = 0 for P = 0, 0.75, 0.83, 0.9 GPa, together with the corresponding experimental data of the present measurements at T = 1.8 K, Miyake et al at T = 1.4 K (for increasing fields) 41 strength of quantum correlation effects. The stabilization of Y/UUD/V orders against the classical umbrella order is the most significant role of quantum correlations in TLAFs 14 .…”

“…It is well known 14 that the magnetization curve of TLAFs with strong quantum correlations exhibits a plateau structure at onethird of the saturation magnetization M s in a certain field range, H c1 < H < H c2 . The previous high-field experiments for CsCuCl 3 had reported only the existence of a first-order phase transition with no plateau for H∥c at low temperatures 35,39,41,45 , which has been interpreted as the transition from the "umbrella" to "Vcoplanar" state 15,45 (Fig. 3a).…”

“…where the intrachain and interchain exchange couplings are assumed to be ferromagnetic and antiferromagnetic, respectively (J 0 , J 1 > 0). Here, we took into account the possible existence of easy-plane anisotropy perpendicular to the c axis (Δ 0 > 0) in the intrachain coupling, which is the case for CsCuCl 3 [35][36][37][38][39][40][41]45 .…”

“…We describe the magnetic properties of the coupled-chain TLAFs under magnetic fields parallel to the c axis by the following Hamiltonian with spin- S operators on site i of the n -th triangular layer: where the intrachain and interchain exchange couplings are assumed to be ferromagnetic and antiferromagnetic, respectively ( J 0 , J 1 > 0). Here, we took into account the possible existence of easy-plane anisotropy perpendicular to the c axis (Δ 0 > 0) in the intrachain coupling, which is the case for CsCuCl 3 35 – 41 , 45 .

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“…Introducing a "squash" mapping, we show that the magnetic properties of coupled spin chains can be phenomenologically described by a single-layer TLAF model with effective spinS. The crucial experimental step is a series of precise magnetic measurements conducted under high pressure up to P = 1.21 GPa on a CsCuCl 3 single crystal [35][36][37][38][39][40][41] , which allows us to determine the exchange couplings to great accuracy and, consequently, extract the parameters of the effective model. We thus demonstrate that the value of effective spinS can be actually controlled by external pressure through the change in the material parameters.…”