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From Cooperative Paramagnet to Néel Order in Y_2Ru_2O_7

Spin correlations in the pyrochlore antiferromagnet Y_2Ru_2O_7 with Curie-Weiss temperature $Θ_{CW}=-1100$ K and critical temperature T_N=77 K were examined through neutron scattering. For $T_N<T<Θ_{CW}/3$ the data show spin relaxation with a rate $\hbarΓ= 1.17(9)k_BT$. For T<T_N spectral weight moves to higher energies with substantial changes up to $4\times k_BT_N$. For T<<T_N there is a $Δ=11(1)$ meV energy gap and a pronounced spectral maximum at 19.7 meV. Throughout the temperature range examined the wave vector dependence of inelastic scattering exhibits a broad peak for $Qd\approx 3.8$ (d is Ru-Ru spacing) consistent with dipolar spin correlations.