A number of models for highhyphen;temperature superconductivity in the layered cuprate perovskites require strong antiferromagnetic superexchange interactions between Cu atoms. The existence of such interactions in La2CuO4minus;yhas previously been demonstrated. In this paper a review is given of recent neutron scattering work performed at Brookhaven studying antiferromagnetism in RBaCu3O6+xwith R=Y, Nd. Measurements on ceramic samples with a range of oxygen concentrations have shown that threehyphen;dimensional antiferromagnetic ordering of the Cu spins occurs throughout the nonsuperconducting tetragonal phase, corresponding to 0le;xlsim;0.4. Nearxsim;0 the Cu2+atoms in the CuO2layers form a simple Neacute;el lattice, and there is no ordering of the nonmagnetic Cu1+atoms in the lsquo;lsquo;chainrsquo;rsquo; sites. With increasing oxygen content some of the Cu1+atoms are converted to Cu2+, which leads to frustration of the interplanar coupling and eventually to a second type of ordering. Very recently, large single crystals of YBa2Cu3O6+xhave been grown, making possible inelastic scattering studies of magnetic excitations. Spin waves within the CuO2planes are observed to have a velocity gsim;0.5 eVthinsp;Aring;.
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