Partially disordered state and spin-lattice coupling in an S=3/2   triangular lattice antiferromagnet Ag2CrO2
  Ag2CrO2 is an S=3/2 frustrated triangular lattice antiferromagnet without orbital degree of freedom. With decreasing temperature, a 4-sublatice spin state develops. However, a long-range partially disordered state with 5 sublattices abruptly appears at TN=24 K, accompanied by a structural distortion, and persists at least down to 2 K. The spin-lattice coupling stabilizes the anomalous state, which is expected to appear only in limited ranges of further-neighbor interactions and temperature. It was found that the spin-lattice coupling is a common feature in triangular lattice antiferromagnets with multiple-sublattice spin states, since the triangular lattice is elastic. 
M. Matsuda, H. Yoshida, M. Isobe, C. de la Cruz, and R. S. Fishman
cond-mat.str-el