The static and dynamic magnetic properties of an S = 1/2 stacked triangular antiferromagnet RbCuCl3 with a helical spin structure due to lattice distortion were investigated by magnetic susceptibility, high-field magnetization process and ESR measurements. The magnetic susceptibilities were analyzed by high-temperature expansion approximation in terms of ferromagnetic intrachain coupling and the antiferromagnetic interchain coupling. The magnetization saturates at Hs ≈ 66.8 T at 1.5 K, the value of which is twice that of CsCuCl3. A small magnetization jump indicative of a phase transition was observed at Hc = 21.2 T ≈ (1/3)Hs for H⊥c. ESR modes observed for H c are well described by the calculation based on the helical spin structure. From the present measurements, the ferromagnetic intrachain and two kinds of antiferromagnetic interchain exchange interactions, and the planar anisotropy energy were determined as J0/kB = 25.7 K, J1/kB = −10.6 K, J ′ 1 /kB = −17.4 K, and ∆J0/kB = −0.45 K, respectively. KEYWORDS: RbCuCl 3 , distorted triangular antiferromagnet, helical spin structure, susceptibility, high-field magnetization process, field-induced phase transition, ESR §1. Introduction Triangular antiferromagnets of the hexagonal ABX 3 type with the CsNiCl 3 structure exhibit a rich variety of magnetic phase transitions due to spin frustration and quantum fluctuation.1) Recently, the magnetic properties of CsCuCl 3 have been actively investigated. CsCuCl 3 exhibits field-induced phase transition for H c, which is caused by the competition between planar anisotropy and quantum fluctuation.2-4) A small amount of Co 2+ doped in CsCuCl 3 produces successive magnetic phase transitions, 5) and the low-temperature phase was found to be an oblique triangular antiferromagnetic phase in which spins form triangular structure in a plane tilted from the basal plane.
6)The magnetic properties of the closely related RbCuCl 3 have been less extensively studied. At high temperatures, RbCuCl 3 exhibits the highly symmetric CsNiCl 3 structure (space group P 6 3 /mmc). With decreasing temperature, RbCuCl 3 transforms into an orthorhombic structure (P bcn) at T t1 = 340 K and further into a monoclinic structure (C2) at T t2 = 260 K due to the Jahn-Teller effect.7, 8) RbCuCl 3 undergoes magnetic phase transition at T N ≈ 19 K.9) The magnetic susceptibility of RbCuCl 3 was first investigated by Tazuke et al.9) They analyzed their susceptibility data in terms of * E-mail: syuji@lee.phys.titech.ac.jp. * * E-mail: tanaka@lee.phys.titech.ac.jp.antiferromagnetic intrachain coupling and ferromagnetic interchain coupling. Recent neutron powder diffraction by Reehuis et al. 10) revealed that an incommensurate helical spin structure which is close to the 120• spin structure is realized in the basal plane as shown in Fig. 1, and that spins are arranged ferromagnetically along the c-axis. The spin structure is expressed by the ordering vector Q 0 = (0, 0.5985, 0) for the monoclinic representation and Q 0 = (0.2993, 0.2993, 0) for the hexagonal re...
