Both the static and dynamic properties of the antiferromagnetic state in YBa2Cu306. &5 have been reinvestigated by neutron scattering. The crystal studied exhibited a Neel temperature of 410 + 3 K, with a continuous transition below 15 K to a magnetic structure with a doubled unit cell along the c axis. The Cu magnetic form factor has been extracted from magnetic Bragg peak intensities measured at 15 K, and it is shown to have the large anisotropy expected for a Cu 3d 2 y2 state. The form-factor anisotropy can explain much of the Q dependence of the inelastic magnetic cross section that has been observed in superconducting YBaqCu306+ . A search for the optical spinwave modes at excitation energies up to 60 meV was unsuccessful. Analysis of acoustic spin-wave measurements yields an intralayer superexchange energy J~~o f 120 + 20 meV (without correction for quantum renormalization) with an anisotropy o. "=(7+ 1) x 10, and an effective next-nearestlayer exchange J&2 of 0.04 6 0.01 meV. A lower limit for the intra-bilayer exchange, Jzz, of 8 meV is established. Use of theoretical and experimental results for Q-integrated spin-wave intensities in the antiferromagnet, together with a crystal volume normalization based on phonon measurements, allows us to put previous measurements of spin Quctuations in superconducting YBaqCu306 6 on an absolute scale. The results for the superconductor are shown to be consistent with the relaxation rates determined by nuclear magnetic resonance.

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