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

The still unidentified hidden order (HO) phase below T₀ = 17.5 K in the heavy-electron superconductor URu₂Si₂ has been investigated by ²⁹Si 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 μ_SMAF = 0.02 -0.04 μ_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 T₀ down to ∼14 K, the ²⁹Si NMR line shows additional broadening over the paramagnetic width. The additional contribution, however, disappears below ∼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.