No evidence for "Small-Moment antiferromagnetism" under ambient pressure in URu2Si2:: Single-crystal 29Si NMR study

The still unidentified hidden order (HO) phase below T-0 = 17.5 K in the heavy-electron superconductor URu2Si2 has been investigated by Si-29 NMR on a single crystal under external magnetic field H-ext || c-axis and ambient pressure P. For a previously proposed model of the small-moment antiferromagnetism (SMAF), in which tiny-moment antiferromagnetism (AF) under ambient P is ascribed to spatially homogeneous AF with an ordered moment mu(SMAF) = 0.02-0.04 mu(B), splitting of the line would be expected on the basis of a scaling from the corresponding splitting in the P-induced AF. However, we observed at T = 4.5 K no splitting of the line. This clearly indicates that the SMAF model cannot describe the ambient-P properties. Below To down to similar to 14 K, the 29 Si NMR line shows additional broadening over the paramagnetic width. The additional contribution, however, disappears below similar to 14 K, which indicates that the HO itself produces almost zero internal field at the 29Si site along the c-axis at low temperatures in the HO phase.