Absence of a static in-plane magnetic moment in the 'hidden-order' phase of URu2Si2

We have carried out a careful magnetic neutron scattering study of the heavy fermion compound URu2Si2 to probe the possible existence of a small magnetic moment parallel to tetragonal basal plane in the 'hidden-order' phase. This small in-plane component of the magnetic moment on the uranium sites S-parallel to has been postulated by two recent models (rank-5 superspin/hastatic order) aiming to explain the hidden-order phase, in addition to the well-known out-of- plane component S-perpendicular to approximate to 0.01-0.04 mu(B)/U. In order to separate S-parallel to and S-perpendicular to, we take advantage of the condition that for magnetic neutron scattering only the components of the magnetic structure that are perpendicular to the scattering vector Q contribute to the magnetic scattering. We find no evidence for an in-plane magnetic moment S-parallel to. Based on the statistics of our measurement, we establish that the upper experimental limit for the size of any possible in-plane component is S-parallel to(max) <= 1 x 10(-3) mu(B)/U.