NONTRIVIAL MAGNETIC ORDER IN URU2SI2

We have searched for the hidden multispin order parameters that have b een proposed to explain the weak ordered moment observed in the heavy- fermion superconductor URu2Si2. Since the exotic spin order is predict ed to cause broken-symmetry Bragg peaks that develop rapidly with appl ied magnetic field, we have measured their strength in fields up to 8 T, for which a large easily observable moment is predicted. The Bragg neutron scattering is found to decrease rather than increase when a fi eld is applied along the easy tetragonal c-axis, while there is no cha nge for a field at right angles. The results rule out the proposed sym metric multispin order parameters. The zero-field transition is found to exhibit critical softening of the spin gap and a cusp in the suscep tibility at T-N, as expected for the ordering of magnetic dipoles.