Non-trivial magnetic order in URu2Si2?

We have searched for the hidden multispin order parameters that have been proposed to explain the weak ordered moment observed in the heavy-fermion superconductor URu2Si2. Since the exotic spin order is predicted to cause broken-symmetry Bragg peaks that develop rapidly with applied 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 field is applied along the easy tetragonal c-axis, while there is no change for a field at right angles. The results rule out the proposed symmetric multispin order parameters. The zero-field transition is found to exhibit critical softening of the spin gap and a cusp in the susceptibility at TN, as expected for the ordering of magnetic dipoles. (author)