Zig-zag ladders with staggered magnetic chirality in S = 3/2 compound beta-CaCr2O4

The crystal and magnetic structures of the S = 3/2 chain antiferromagnet beta-CaCr2O4 have been investigated by means of specific heat, magnetization, muon relaxation and neutron powder diffraction between 300K and 1.5K. Owing to the original topology of the Cr3+ magnetic lattice, which can be described as a network of triangular ladders, equivalent to chains with nearest and next-nearest neighbors interactions, evolution of the magnetic scattering intensity in this compound evidences two magnetic regimes : for 21K < T < 270K, a low-dimensionality magnetic ordering of the Cr3+ spins is observed, simultaneously with a strong contraction of the ladder legs, parallel to c. Below TN = 21K, a complex antiferromagnetic ordering is evidenced, with an incommensurate propagation vector k = (0, 0, q) (q ~ 0.477 at 1.5K), as exchange interactions between ladders become significant. This complex magnetic ordering can be described as a honeycomb-like arrangement of cycloids, running along c, with staggered chiralities. The experimental observation of this staggered chirality can be understood by taking into account antisymmetric Dzyaloshinskii-Moriya exchange terms.