X-ray magnetic scattering in antiferromagnetic URu sub 2 Si sub 2

X-ray-resonance magnetic scattering has been used to study antiferromagnetic ordering in the small-moment ({bar {mu}}{congruent}0.02{mu}{sub {ital B}}) heavy-fermion superconductor URu{sub 2}Si{sub 2}. The intensity of the magnetic (003) reflection develops abruptly at {ital T}{sub {ital N}}{congruent}17 K and grows linearly to {ital T}=3 K, where it saturates. Long-range antiferromagnetic order ({zeta}{sub 003}{sup {ital c}}{congruent}450 A) persists into the superconducting state at {Tc}=1.3 K demonstrating the microscopic coexistence of these two ground states. At saturation, a remarkable peak intensity of 8 counts/sec was measured.