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 no
t 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.
Neutron scattering measurements show the ferromagnetic XY pyrochlore Yb2Ti2O7 to display strong quasi-two dimensional (2D) spin correlations at low temperature, which give way to long range order (LRO) under the application of modest magnetic fields.
Rods of scattering along < 111 > directions due to these 2D spin correlations imply a magnetic decomposition of the cubic pyrochlore system into decoupled kagome planes. A magnetic field of ~0.5 T applied along the [1-10] direction induces a transition to a 3D LRO state characterized by long-lived, dispersive spin waves. Our measurements map out a complex low temperature-field phase diagram for this exotic pyrochlore magnet.
