Strict limit on in-plane ordered magnetic dipole moment in URu2Si2

Neutron diffraction is used to examine the polarization of weak static antiferromagnetism in high quality single crystalline URu2Si2. As previously documented, elastic Bragg-like diffraction develops for temperature T<T_{HO}= 17.5 K at q=(100) but not at wave vector transfer q=(001). The peak width indicates correlation lengths \xi_c=230(12) \AA \ and \xi_a=240(15) \AA. The integrated intensity of the T-dependent peaks corresponds to a sample averaged c-oriented staggered moment of \mu_{c}=0.022(1) \mu_B at T=1.7 K. The absence of T-dependent diffraction at q=(001) places a limit \mu_{\perp}<0.0011 \mu_B on an f- or d-orbital based in-plane staggered magnetic dipole moment, which is associated with multipolar orders proposed for URu_2Si_2.