Spin Jahn-Teller antiferromagnetism in CoTi2O5

We have used neutron powder diffraction to solve the magnetic structure of orthorhombic CoTi2O5, showing that the long-range ordered state below 26 K identified in our muon-spin rotation experiments is antiferro-magnetic with propagation vector k = (+/- 1/2, 1/2, 0) and moment of 2.72(1)mu(B) per Co2+ ion. This long-range magnetic order is incompatible with the experimentally determined crystal structure because the imposed symmetry completely frustrates the exchange coupling. We conclude that the magnetic transition must therefore be accompanied by a spin Jahn-Teller effect which lowers the structural symmetry and thereby relieves the frustration. These results show that CoTi2O5 is a highly unusual low-symmetry material exhibiting a purely spin-driven lattice distortion critical to the establishment of an ordered magnetic ground state.