Ca ₂Y ₂Cu ₅O ₁₀: The first frustrated quasi-1d ferromagnet close to criticality

Ca ₂Y ₂Cu ₅O ₁₀ is built up from edge-shared CuO ₄ plaquettes forming spin chains. From inelastic neutron scattering data we extract an in-chain nearest-neighbor exchange J ₁-170K and the frustrating next-neighbor J ₂32K interactions, both significantly larger than previous estimates. The ratio α=|J ₂/J ₁|=0.19±0.01 places the system close to the critical point α _c=0.25 of the J ₁-J ₂ chain but in the 1D ferromagnetic regime. We establish that the vicinity to criticality only marginally affects the dispersion and coherence of the spin-wave-like magnetic excitations but instead results in a dramatic T dependence of high-energy Zhang-Rice singlet excitation intensities.