Oblique triangular antiferromagnetic phase inCsCu1−xCoxCl

Abstract

The spin-1 2 stacked triangular antiferromagnet CsCu1−xCoxCl3 with 0.015 < x < 0.032 undergoes two phase transitions at zero field. The low-temperature phase is produced by the small amount of Co 2+ doping. In order to investigate the magnetic structures of the two ordered phases, the neutron elastic scattering experiments have been carried out for the sample with x ≈ 0.03. It is found that the intermediate phase is identical to the ordered phase of CsCuCl3, and that the low-temperature phase is an oblique triangular antiferromagnetic phase in which the spins form a triangular structure in a plane tilted from the basal plane. The tilting angle which is 42 • at T = 1.6 K decreases with increasing temperature, and becomes zero at TN2 = 7.2 K. An off-diagonal exchange term is proposed as the origin of the oblique phase. §1. Introduction Among the group of antiferromagnets which have a hexagonal ABX 3 triangular structure, many experimental and theoretical investigations for CsCuCl 3 have been done by several authors , 1-4) due to the remarkable features of this compound. Since the Cu 2+ ion is JahnTeller active, CsCuCl 3 undergoes a structural phase transition at T t = 423 K, where Jahn-Teller distorted octahedra CuCl 6 order so that their longest axes form a helix along the c-axis with a repeatlength of six . 5-7) The low-symmetric crystal structure produces the antisymmetric interaction of the Dzyaloshinsky-Moriya (D-M) type between the adjacent spins in the chain with the Dvector parallel to the c-direction. The exchange interactions along and between the chains are ferromagnetic and antiferromagnetic, respectively, and their values have been evaluated as J 0 /k B = 28 K and J 1 /k B = −4.9 K 8, 9) in the definition of H = − <i,j> 2J ij S i · S j . The magnetic phase transition occurs at T N = 10.5 K . 10, 11) In the ordered state, spins lie in the basal plane and form the 120 • -structure, while along the c-direction, a long period (about 71 triangular layers) helical incommensurate arrangement is realized, due to the competition between the ferromagnetic interaction and the D-M interaction along the chain.Since the magnitude of spin on Cu 2+ is S = 1 2 , quantum fluctuations are important when one describes the magnetic behavior in the magnetic field . 12) Particularly, the magnetization process with a strong field often reveals quantum effects as macroscopic phenomena. Using quantum Monte Carlo calculations or spin wave theory, for the 2D triangular antiferromagnet (TAF) in the isotropic limit, it is predicted that the magnetization plateau appears at around one third of the saturation value M s . 13, 14) According to these predictions, when * Present address: Faculty of Education, Chiba University, 263-8522 Chiba, Japan.the external field H is applied parallel to the c-axis, 2D TAF undergoes a successive phase transition as shown in Fig. 1 (b) low-field coplanar, (c) collinear and (d) highfield coplanar structure in that order. In the field region where the magnetization plateau appears, the spin structure c...

Development of multi-frequency ESR system for high-pressure measurements up to 2.5 GPa

“…CsCuCl 3 undergoes the antiferromagnetic transition at T N = 10 K and forms the 120 • structure in the c-plane. It is known that the quantum-fluctuation-induced phase transition occurs under a high magnetic field and numerous studies have tried to control this transition by changing the magnetic anisotropy by applying the chemical [26] and physical pressures [27,28]. Another reason for the interest in this material is the structural phase transition induced by the pressure.…”

Pressure-Induced Successive Magnetic Phase Transitions in the Spin Gap System TlCuCl3

“…25) However, the successive phase transitions in the present system seem not to be explained by this case because the present system undergoes two-sublattice collinear magnetic ordering, as shown in Fig. 1 29) In these mixed systems, the inclinations of ordered spin moments have been discussed in terms of the competing random anisotropies or the off-diagonal exchange interaction. However, the successive phase transitions in the present system without disorder may not be explained by competing anisotropies of the quadratic form because in pure system they cannot produce the gradual change of spin direction, but produce sudden change.…”

Electron Spin Resonance in Triangular Antiferromagnets

“…By means of elastic neutron scattering experiments, it was found that the intermediate phase is identical to the ordered phase of CsCuCl 3 , and that the low-temperature phase is an oblique triangular antiferromagnetic phase in which the spins form a triangular structure in a plane tilted from the basal plane. 79,80) The ESR modes in CsCu 1−x Co x Cl 3 were investigated by Ono et al…”