Spin structure of CsCu 1-x Co x Cl 3 in magnetic fields

Elastic neutron scattering experiments were performed in order to determine the magnetic structure of the triangular antiferromagnetic system CsCu1-xCoxCl3 (x≈0.03) in magnetic fields. CsCu0.97Co0.03Cl3 undergoes two phase transitions, one at TN1=10.4 K and the other at TN2=7.4 K, in zero field. The low-temperature phase is a new ordered phase produced by the effect of the Co2+ dopant. It is found that the spin structure, in the new ordered phase, is an oblique triangular antiferromagnetic phase, in which spins form a triangular structure in a spin plane tilted away from the basal plane. The magnetic field, at which the new phase becomes unstable, is a minimum for H∥c and a maximum for H⊥c. The field variation of the tilting angle φ of the spin plane depends on the field direction. With increasing field, φ decreases gradually for H⊥c. On the other hand, for H∥c, φ is almost constant up to H≈3 T and then abruptly becomes zero.