Hidden and magnetic order in powdered URu2Si2 found by NMR at ambient pressure

Below the hidden order transition temperature T-0 similar to 17 K, ambient pressure Si-29 and Ru-99 NMR spectra in an aligned powder of URu2Si2 reveal local internal field distributions at both Si-29 and Ru-99 sites. For a longitudinal field orientation, the Ru-99 spectral parameters also reveal a second transition temperature T* similar to 12 K, below which an anomalous increase in the linewidth is observed, accompanied by a kink in the Knight shift vs. magnetic susceptibility plot. From temperature and applied-field orientation studies, we can separate the two types of linewidth contributions, and find that the distribution of internal fields arising below T-0 is anisotropic at Ru-99 positions, with larger widths in the ab-plane. The second contribution to the linewidth appears below T* and has Ising symmetry. We discuss disorder in the non-magnetic sector as a way to understand our measurements and reconcile them with recent pressure experiments by other groups. (c) 2005 Elsevier B.V. All rights reserved.