NEUTRON-SCATTERING STUDY OF ANTIFERROMAGNETISM IN YBA2CU3O6.15

Both the static and dynamic properties of the antiferromagnetic state in YBa2Cu3O6.15 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(x2-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 YBa2Cu3O6+x. A search for the optical spin-wave modes at excitation energies up to 60 meV was unsuccessful. Analysis of acoustic spin-wave measurements yields an intralayer superexchange energy J(parallel-to) of 120 +/- 20 meV (without correction for quantum renormalization) with an anisotropy alpha(xy) = (7 +/- 1) X 10(-4), and an effective next-nearest-layer exchange J(perpendicular-to 2) of 0.04 +/- 0.01 meV. A lower limit for the intra-bilayer exchange, J(perpendicular-to 1), 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 fluctuations in superconducting YBa2Cu3O6.6 on an absolute scale. The results for the superconductor are shown to be consistent with the relaxation rates determined by nuclear magnetic resonance.