X-ray and neutron scattering

Abstract:

We describe neutron-scattering experiments performed to investigate the magnetic order and dynamics of half-doped La1.5Sr0.5CoO4. This layered perovskite exhibits a near-ideal checkerboard pattern of Co2+/Co3+ charge order at temperatures below ~800 K. Magnetic correlations are observed at temperatures below ~60 K but the magnetic order only becomes established at 31 K, a temperature at which a kink is observed in the susceptibility. On warming above 31 K we observed a change in the magnetic correlations which we attribute either to a spin canting or to a change in the proportion of inequivalent magnetic domains. The magnetic excitation spectrum is dominated by an intense band extending above a gap of approximately 3 meV up to a maximum energy of 16 meV. A weaker band exists in the energy range of 20–30 meV. We show that the excitation spectrum is in excellent quantitative agreement with the predictions of a spin-wave theory generalized to include the full magnetic degrees of freedom of high-spin Co2+ ions in an axially distorted crystal field, coupled by Heisenberg exchange interactions. The magnetic order is found to be stabilized by dominant antiferromagnetic Co2+–Co2+ interactions acting in a straight line through Co3+. No evidence is found for magnetic scattering from the Co3+ ions, supporting the view that Co3+ is in the S=0 state in this material.

Abstract:

We present the magnetic phase diagram of multiferroic TbMnO3 for H parallel to b determined using X-ray resonant scattering at the Tb L-3-edge. Investigations of two different magnetic domains, C-type and F-type, demonstrate very similar behaviour in high applied magnetic fields, with a low temperature incommensurate to commensurate transition at H similar or equal to 4.5T, concurrent with the polarisation flop observed in bulk magnetoelectric measurements. (C) 2009 Elsevier B.V. All rights reserved.