⁸⁹Y NMR of the spin-chain system Ca₂Y ₂Cu₅O₁₀

We report measurements of the ⁸⁹Y NMR lineshape, spin-lattice relaxation time T₁, and spin-spin relaxation time T₂ for the copper-oxide spin-chain compound Ca₂Y₂Cu ₅O₁₀. The NMR data presented here, obtained at 17 MHz (8.1 T), show broad, asymmetric, two-peaked lineshapes with width Δν ≈ 90 kHz. The appreciable shift of the line, ΔH/H ≈ 0.7%, decreases with increasing temperature. T₂ increases with increasing temperature at lower temperatures, but plateaus at T₂ = 13 ± 1 ms for temperatures above 325 K. For the spin-lattice relaxation, we observe a T ₁ minimum around 275 K that is 100 times more shallow than expected for random ⁸⁹Y-Cu²⁺ dipolar interactions. Such behavior indicates unusually strong spin correlations, and is surprising for the apparently small intrachain exchange coupling, J_nn/k_B ≈ 40 K. The long Cu²⁺ electronic timescale (τ_e ∼ 10 ^-8 s) evident at temperatures below the T₁ minimum indicates a slowing down of spin correlations far above T_N.