NEAR-SURFACE EFFECTS AT THE ANTIFERROMAGNETIC PHASE-TRANSITION IN URANIUM PHOSPHIDE

A careful investigation of the resonant x-ray magnetic-scattering resp onse in uranium monophosphide reveals the crucial role played by magne tization density correlations in the discontinuous paramagnetic to ant iferromagnetic phase transition in this material. The magnetization de nsity correlations, which are measured on the short time scale typical of the resonant x-ray-scattering technique, appear highly anisotropic in their inferred spatial distribution. Such correlated regions exist in the near-surface regime probed by the low-energy x-ray beam (3.278 keV) and exhibit strongly temperature-dependent behavior at and above the bulk phase transition. For specular reflections this leads to wel l defined two length scale diffraction profiles in the vicinity of the phase transition and, when taken with the off-specular data, to a phe nomenological model which suggests that the extended length scale of n ear-surface antiferromagnetism may arise from a localized surface dist ortion. This type of model may be more generally useful both in the tw o length scale problem and in other systems where defects or fluctuati ons may, in a similar manner, drive pseudo-long-range order.