Origin of the second length scale found above TN in UO2

We present the results of x-ray- and neutron-scattering studies of the temperature dependence of the magnetic scattering exhibited by the type-I, triple-Q antiferromagnet UO2. Our neutron-scattering results are consistent with those of earlier studies, including the observation of short-ranged magnetic correlations at temperatures near and above TN. However, it is found by x-ray diffraction that a second, longer length scale is induced near TN when the near-surface volume of the sample is mechanically roughened. The longitudinal and transverse widths of the additional scattering increase continuously with increasing temperature above TN, similar to that which has been observed near the magnetic ordering transitions of Ho, Tb, and NpAs and near the tetragonal-to-cubic transitions of various perovskites. Another unusual feature of the present results for UO2 involves the apparent shift with temperature of the magnetic scattering along the surface normal direction at the (1,1,0) reflection, but not at the (2,1,0) reflection. To our knowledge, this is the first observation of a second length scale near a first-order transition. © 1996 The American Physical Society.