Professor Paolo G. Radaelli OSI

Abstract:

The commensurate and incommensurate magnetic structures of the magnetoelectric system YMn2O5, as determined from neutron diffraction, were found to be spin-density waves lacking a global center of symmetry. We propose a model, based on a simple magnetoelastic coupling to the lattice, which enables us to predict the polarization based entirely on the observed magnetic structure. Our data accurately reproduce the temperature dependence of the spontaneous polarization, particularly its sign reversal at the commensurate-incommensurate transition.

Abstract:

A technique that allows the spatial distribution of crystallographic phases in the interior of an object to be reconstructed from neutron time-of-flight (TOF) diffraction is described. To this end, the shift of the Bragg peaks due to the so-called 'geometrical aberration' is exploited. A collimated incident white beam is used to perform a translational or rotational scan of the object whilst collecting a TOF data set for each sample position or orientation. Depending on the location of any scattering material along the line of the incident beam path through the object, the measured d-spacings of the corresponding Bragg peaks are shifted with respect to their nominal values, which are attained only at the geometrical centre of the instrument. Using a formula that is usually employed to correct for sample offset, the phase distribution along the incident beamline can be directly reconstructed, without the need to perform a tomographic reconstruction. Results are shown from a demonstration experiment carried out on a cylindrical Al container enclosing an arrangement of Cu and Fe rods. On the basis of this formalism, an optimized experimental geometry is described and the potential and limits of this technique are explored, as are its applicability to X-ray and constant-wavelength neutron diffraction. © 2006 International Union of Crystallography Printed in Great Britain - all rights reserved.

Abstract:

The mixed-valent compound RBaCo4O7 (R=Rare earth, Y), hereafter abbreviated as R-114, is built up of Kagomé sheets of CoO 4 tetrahedra, linked in the third dimension by a triangular layer of CoO4 tetrahedra in an analogous fashion to that found in the known geometrically frustrated magnets such as pyrochlores and SrCr 9xGa12-9xO19 (SCGO). We have undertaken a study of the structural and magnetic properties of the Y-114 and Yb-114 compound using combined high resolution powder neutron and synchrotron X-ray diffraction. Both compounds undergo a first order trigonal -→ orthorhombic phase transition that breaks the trigonal symmetry of the structure. We show from Bond Valence Sum arguments that this transition occurs as a response to a markedly underbonded Ba2+ site in the high-temperature phase. The symmetry-lowering transition relieves the geometric frustration of the structure, and a long-range ordered 3-D anti ferromagnetic state develops at low temperature. The magnetic structure of the Y compound has been solved and shows a compromise between the well-known 120° structure of the Kagomé net and a collinear antiferromagnet in the third dimension. © 2007 Materials Research Society.

Abstract:

The crystal structure of Ca(2-x)Sr(x)RuO(4) with 0.2 < or = x < or = 1.0 has been studied by diffraction techniques and by high resolution capacitance dilatometry as a function of temperature and magnetic field. Upon cooling in zero magnetic field, the crystal structure and the octahedra shrink along the c direction and elongate in the a and b planes, whereas the opposite occurs upon cooling at high field (x = 0.2 and 0.5). These findings yield evidence for an orbital rearrangement driven by temperature and magnetic field, which accompanies the metamagnetic transition at low temperature. The temperature and magnetic-field dependencies are found to be governed by the same energy scale.