Professor Paolo G. Radaelli OSI

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.

Abstract:

Powder x-ray diffraction patterns of magnetite (Fe3O 4) have been recorded at 90 K, below the Verwey transition, at three wavelengths in the pre- to mid-edge region of the 7.1 keV Fe K absorption. Simultaneous fitting of these profiles has been used to refine the anomalous scattering coefficients for the octahedral B site iron atoms. The refined values give direct evidence for a significant degree (46%) of Fe2+/Fe 3+ charge ordering in magnetite, and provide new constraints on the number of possible charge ordered models. © 2005 IOP Publishing Ltd.

Abstract:

The commensurate and incommensurate magnetic structures of the magnetoelectric system YMn$_{2}$O$_{5}$, 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 magneto-elastic 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, in particular its sign reversal at the commensurate-incommensurate transition.