Spin dynamics in the magnetic chain arrays of Sr14CU24O41:: A neutron inelastic scattering investigation

Below T approximate to 150 K, the spin arrangement in the chain arrays of Sr14C24O41 is shown to develop in two dimensions. Both the correlations and the dispersion of the observed elementary excitations agree well with a model of interacting antiferromagnetic dimers. Along the chains, the intra- and interdimer distances are equal to 2 and approximate to 3 times the distance (c) between neighboring Cu ions. While the intradimer coupling is J approximate to 10 meV, the interdimer couplings along and between the chains are of comparable strength, J(parallel to) approximate to -1.1 meV and J(perpendicular to) approximate to 1.7 meV, respectively. This remarkable two-dimensional (2D) arrangement satisfies the formal Cu valence of the undoped compound. Our data suggest also that it is associated with a relative sliding of one chain with respect to the next one, which, as T decreases, develops in the chain direction. A qualitative;analysis shows that nearest interdimer spin correlations are ferromagnetic, which, in such a 2D structure, could well result from frustration effects. [S0163-1829(99)12201-X].