The appearance of homogeneous antiferromagnetism in URu₂Si₂under high pressure: a²⁹Si nuclear magnetic resonance study

Abstract

We have investigated the low-temperature phase appearing below To = 17.5 K in URu2Si2 by means of 29Si nuclear magnetic resonance (NMR) in a pressure range from 0 to 17.5 kbar across the pressure-induced phase transition at Pc = 15 kbar. At pressures below Pc, we have observed the 29Si NMR lines arising from antiferromagnetic (AF) and paramagnetic (PM) regions in the sample, giving evidence for a phase-separated AF ordering below To. The AF region increases in volume fraction with increasing pressure up to Pc. In the PM region, the temperature-dependence of the nuclear spin–lattice relaxation rate at Si sites shows a rapid decrease below To, strongly suggesting the occurrence of a phase transition driven by a hidden order parameter. As applied pressure exceeds Pc, the AF ordering appears uniformly at To 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 (~ 920 Oe), which indicates that the AF ordered moment is unchanged by pressure across Pc.