X-RAY STUDY OF THE CRITICAL MAGNETIC SCATTERING IN NPAS

Magnetic critical scattering has been observed by resonant-enhanced X- ray diffraction at the transition to the incommensurate antiferromagne tic phase of neptunium arsenide. The observed critical behaviour is si gnificantly different to that observed with conventional neutron scatt ering techniques. Our analysis suggests that the critical scattering i s composed of two components, the sharper of which may be related to t he surface or be a result of long-range order nucleated by impurities.