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 S r14C24O41 is shown to develop in two dimensions. Both the correlations and the dispersion of the observed elementary excitations agree well with a mod el of interacting antiferromagnetic dimers. Along the chains, the intra- an d interdimer distances are equal to 2 and approximate to 3 times the distan ce (c) between neighboring Cu ions. While the intradimer coupling is J appr oximate to 10 meV, the interdimer couplings along and between the chains ar e of comparable strength, J(parallel to) approximate to -1.1 meV and J(perp endicular to) approximate to 1.7 meV, respectively. This remarkable two-dim ensional (2D) arrangement satisfies the formal Cu valence of the undoped co mpound. 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 interdim er spin correlations are ferromagnetic, which, in such a 2D structure, coul d well result from frustration effects. [S0163-1829(99)12201-X].