Seminarium Oddziału Badań Magnetyków
sala nr 6 (bud. II)
Does Anisotropy in Atomic Wavefunctions affect the Interpretation of Diffraction Experiments?
Dr hab. Martin Rotter
Max Planck Institute for Chemical Physics of Solids, Dresden
In many magnetic compounds the charge, spin- and orbital moment densities of the the rare earth and transition metal ions are extremely anisotropic. Causes are the crystal field, short or long range order of dipoles or higher order multipoles, ligand fields and electronic correlations. The anisotropy of the total magnetization density of a magnetic ion leads to sizeable corrections to the dipole approximation for the magnetic form factor for neutron scattering. We have extended the magnetic modeling suite MCPHASE to enable a straightforward calculation of the higher-order anisotropic terms in the form factor for elastic and inelastic neutron scattering on rare earth, transition metal and actinide based systems with localized electrons.
The state of art method for the determination of magnetic structures is neutron diffraction on powder and single crystals. Data analysis is normally made using the dipole approximation for the magnetic form factor. We have made an analysis of neutron-diffraction data for a number of rare-earth and transition metal compounds to assess the use of the dipole approximation in magnetic structure refinements.
I will present several examples with emphasis on two special cases: (i) In the case of CePd2Si2, our analysis confirms the published magnetic structure, but we find that use of the exact magnetic form factor of Ce3+ gives a significantly improved description of the data. (ii) For NdBa2Cu3O6+x, however, we find that incorrect conclusions have previously been drawn about the magnetic structure through the use of the dipole approximation [1]. Polarized neutron scattering can be used to determine the magnetic form factor with higher accuracy. In YbInNi4 such data clearly excludes the possibility of a Gamma-7 doublet as the crystal field ground state [2].
References:
[1] Going beyond the dipole approximation to improve the refinement of magnetic structures by neutron diffraction", M. Rotter and A. Boothroyd Phys. Rev. B Rapid Comm. 79 (2009) 140405R
[2] "Crystal-fields in YbInNi4 determined with magnetic form factor and inelastic neutron scattering" A. Severing, F. Givord, J.-X. Boucherle T. Willers, M. Rotter, Z. Fisk, A. Bianchi,M.T. Fernandez-Diaz, A. Stunault B.D. Rainford,J. Taylor,and E. Goremychkin, Phys. Rev. B 83, 155112 (2011)
