Frustrating interactions and broadened magnetic excitations in the edge-sharing CuO₂ chains in La₅Ca₉Cu₂₄O₄₁

Inelastic neutron scattering measurements have been performed on La₅Ca₉Cu₂₄O₄₁, which consists of edge-sharing CuO₂ chains and shows an antiferromagnetic long-range order below T_N=10.5 K with ferromagnetic arrangement within the chain. Although the intrachain coupling is expected to be ferromagnetic from the geometrical structure, it is found that the interactions are weakly antiferromagnetic both parallel and perpendicular to the chain and there exist frustrating interactions between them. It is also found that magnetic excitations are broadened significantly. The broadening is probably enhanced by the frustration and a disorder originating from a structural distortion and doped holes although the broadening at finite Q is expected even in the pure system without frustration and disorder.