Antiferromagnetism in YBa2Cu3O6+x (invited)

Abstract

A number of models for high‐temperature superconductivity in the layered cuprate perovskites require strong antiferromagnetic superexchange interactions between Cu atoms. The existence of such interactions in La₂CuO_4−y has previously been demonstrated. In this paper a review is given of recent neutron scattering work performed at Brookhaven studying antiferromagnetism in RBaCu₃O_6+x with R=Y, Nd. Measurements on ceramic samples with a range of oxygen concentrations have shown that three‐dimensional antiferromagnetic ordering of the Cu spins occurs throughout the nonsuperconducting tetragonal phase, corresponding to 0≤x≲0.4. Near x∼0 the Cu²⁺ atoms in the CuO₂ layers form a simple Néel lattice, and there is no ordering of the nonmagnetic Cu¹⁺ atoms in the ‘‘chain’’ 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 YBa₂Cu₃O_6+x have been grown, making possible inelastic scattering studies of magnetic excitations. Spin waves within the CuO₂ planes are observed to have a velocity ≳0.5 eV Å.