Origin of the Second Length Scale Found above TN in UO2

Abstract

We present the results if X-ray and neutron scattering studies of the temperature dependence of the magnetic scattering exhibited by the type I, triple Q antiferro-magnet UO2. It is found that a second, longer length scale is induced near the magnetic ordering transition when the near-surface volume of the sample is mechanically roughned. The longitudinal and transverse widths of the additional scattering increase continuously with increasing temperature above TN, similar to what has been observed near the magnetic ordering transitions of Ho, Tb and NpAs and near the cubic-to-tetragonal transitions of various perovskites. An unusual feature of the results for UO2 involves the apparent shift of the magnetic scattering along the surface normal direction at the (1,1,0) reflection, but no at the (2,1,0) reflection. To our knowledge, this is the first observation of a "second length scale" near strongly discontinuous ordering transition.JRC.E-Institute for Transuranium Elements (Karlsruhe