High-energy Cu spin excitations in PrBa ₂Cu ₃O _6+x

This paper describes high-energy neutron inelastic scattering measurements of propagating magnetic excitations in PrBa ₂Cu ₃O _6+x (x≈0.2 and 0.93). The measurements probe the acoustic and optic modes of the antiferromagnetically ordered copper-oxygen bilayers in the energy range 50-150 meV. The observed magnon dispersion can be described satisfactorily in this energy range by a spin wave model including intra- and inter-layer nearest-neighbor exchange constants J _∥ and J _⊥. We find J _∥=127±10 meV and J _⊥=5.5±0.9 meV. The value of J _∥ is virtually the same as that found in YBa ₂Cu ₃O _6.2, but J _⊥ is a factor of 2 smaller. To within experimental error the values of J _∥ and J _⊥ for PrBa ₂Cu ₃O _6+x do not vary with oxygen doping.