Partial Disordered Phase of Ising Spin System in Distorted Triangular Lattice RbCoBr3

Powder and single-crystal neutron diffraction measurements were performed on RbCoBr 3 . The magnetic phase transitions of this compound are well explained by the Ising spin model for a distorted triangular lattice. Magnetic peaks of powder diffraction patterns at 34 and 20 K closely fit a partial disordered phase and a ferrimagnetic phase, respectively. The temperature dependences of the ( h , h ,1) scan profile and magnetic Bragg peak heights at (1,1,1), (1/3,1/3,1), and (2/3,2/3,1) were measured for a single-crystal sample. We conclude that the ferrimagnetic phase only exists below 30.5 K. The partial disordered phase is identified between 30.5 (= T N2 ) and 37.2 K (= T N1 ). We found a characteristic linear increase in the (1,1,1) peak height below T N2 with decreasing temperature. These results show that a lattice distortion couples to the spin system. Numerical simulations are also performed to estimate magnetic interaction parameters.