Polarization dependence of magnetic Bragg scattering in YMn2O5

The polarization dependence of the intensity of elastic magnetic scattering from YMn2O5 single crystals has been measured at 25 K in magnetic fields between 1 and 9 T. A significant polarization dependence was observed in the intensities of magnetic satellite reflections, propagation vector τ= 1/2 ,0, 1/4 , measured with both the [100] and [010] axes parallel to the common polarization and applied field direction. The intensity asymmetries A observed in sets of orthorhombic equivalent reflections show systematic relationships which allow the phase relationship between different components of their magnetic interaction vectors to be determined. They fix the orientation relationships between the small y and z moments on the Mn4+ and Mn3+ sub-lattices and have allowed a further refinement of the magnetic structure, which determines the phases of the vector Fourier components with much higher precision. Systematic differences found between values of A(hkl) and A( h-bar k-bar l-bar ) suggest that there is a small modulation of the nuclear structure which has the same wavevector as the magnetic modulation and gives rise to a small nuclear structure factor for the satellite reflections. The magnitudes of the differences suggest shifts in the atomic positions of the order of 0.05 Å. (paper)