Andrew Boothroyd

Abstract:

I give a short review of neutron and X-ray scattering experiments undertaken to study the magnetic properties of non-superconducting PrBa2Cu3O6+x. The measurements have confirmed the magnetic ordering of the Pr sublattice at temperatures below T-Pr approximate to 20 K, and have revealed an incommensurate magnetic structure with a long period (similar to 600 Angstrom) involving both the Pr and Cu spins. The Cu spins in this Pr ordered phase are found to be non-collinear. Taken as a whole, the results provide strong evidence for a substantial magnetic coupling of the pseudodipolar type between the Pr and Cu ions. The effect of this coupling is also evident in the spectrum of low energy magnetic excitations determined by neutron inelastic scattering. (C) 2000 Elsevier Science S.A. All rights reserved.

Abstract:

Measurements of the low-energy magnetic excitations in single-crystal PrBa2Cu3O6.2, and in YBa2Cu3O6.2 for comparison, have been performed using inelastic neutron scattering. An excitation with weak dispersion is seen, which is compared to a spin-wave model based on the lowest lying crystal field levels of the Pr ion, and including the effect of an anisotropic coupling to the Cu magnetic moments, as well as a Pr-Pr exchange interaction. (C) 2000 Elsevier Science B.V. All rights reserved.

Abstract:

We report the discovery of incommensurate magnetic order in non-superconducting single crystals PrBa2Cu3O6.92. Resonant X-ray magnetic scattering at the Pr L-II and L-III edges and high resolution neutron diffraction were used to characterise the magnetic order on the different magnetic sublattices. The data reveal that the Pr sublattice orders at a temperature of 19 K in an incommensurate structure with ordering wavevector (0.5 +/- delta, 0.5, 0) or (0.5, 0.5 +/- delta, 0), delta approximate to 0.006-0.008, A spin reorientation in the Pr sublattice to a commensurate antiferromagnetic structure with ordering wavevector (0.5, 0.5, 0.5) takes place at a temperature close to 9 K. (C) 1999 Elsevier Science B.V. All rights reserved.

Abstract:

Inelastic neutron scattering measurements have been performed, for the first time, on a large, high quality single-crystal of PrBa2Cu3O6+x. Two relatively broad peaks are observed, the one at lower energy showing significant dispersion in the basal plane. Neither feature is observable above the Pr ordering temperature. The pressure dependence of the Cu and Pr ordering temperatures up to 1.75 GPa has also been determined. (C) 1999 Elsevier Science B.V. All rights reserved.