LONG-RANGE ANTIFERROMAGNETIC ORDER AND ITS COEXISTENCE WITH SUPERCONDUCTIVITY IN URU2SI2

Neutron scattering from a high quality single crystal of the heavy fermion superconductor URu2Si2 shows an abrupt onset of antiferromagnetic order at T(N) = 17 K, unlike the gradual onset previously seen in lower quality samples. The magnetic peak intensity increases linearly down to 5 K, indicating mean-field behaviour and long range RKKY interactions. The intensity remains constant to within 7% between 3 and 0.2 K with no change at the superconducting transition temperature of 1.3 K. The resolution limited Bragg peak shows the order is long-range. The coexistence of antiferromagnetism and superconductivity seen in a lower quality crystal is confirmed and thus is intrinsic.