The appearance of homogeneous antiferromagnetism in URu2Si2 under high pressure:: a 29Si nuclear magnetic resonance study

We have investigated the low-temperature phase appearing below T-O = 17.5 K in URu2Si2 by means of (29) Si nuclear magnetic resonance (NMR) in a pressure range from 0 to 17.5 kbar across the pressure-induced phase transition at P-c = 15 kbar. At pressures below P-c we have observed the 21 Si NMR lines arising from antiferromagnetic (AF) and paramagnetic (PM) regions in the sample, giving evidence for a phase-separated AF ordering below T-o. The AF region increases in volume fraction with increasing pressure up to P-c. In the PM region, the temperature-dependence of the nuclear spin-lattice relaxation rate at Si sites shows a rapid decrease below T-o, strongly suggesting the occurrence of a phase transition driven by a hidden order parameter. As applied pressure exceeds P-c the AF ordering appears uniformly at T-o throughout the sample. In the pressure range from 0 to 17.5 kbar, the magnitude of the internal field at Si sites in the AF region remains constant (similar to920 Oe), which indicates that the AF ordered moment is unchanged by pressure across P-c.