Elastic antiferromagnets on a triangular lattice

Elastic Ising antiferromagnets are studied on a triangular lattice. Magnetic 'frustration' is removed at low temperatures by a lattice distortion, and the resulting ground state has alternating rows of up and down spins. The authors discuss the onset of this magnetoelastic instability, and the symmetry breakings involved. The effective Poisson's ratio approaches a universal value sigma =+1 close to the transition. The behaviour of XY spins (relevant to the study of oxygen layers) is compared and contrasted to Ising spins.