Physica B 326 (2003) 418–421 Inhomogeneous magnetism in URu 2 Si 2 studied by muon spin relaxation under high pressure H. Amitsuka a, *, K. Tenya a , M. Yokoyama a , A. Schenck b , D. Andreica b , F.N. Gygax b , A. Amato c , Y. Miyako d , Ying Kai Huang e , J.A. Mydosh f a Graduate School of Science, Division of Physics, Hokkaido University, N10W8, Sapporo 060-0810, Japan b Institute for Particle Physics of ETH Z . urich, CH-5232 Villigen PSI, Switzerland c Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland d Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan e Van der Waals-Zeeman Institute, University of Amsterdam, 1018XE Amsterdam, The Netherlands f Kamerlingh Onnes Laboratory, Leiden University, 2300 RA Leiden, The Netherlands Abstract We have studied the low-temperature phase below 17:5 K in the heavy-fermion superconductor URu 2 Si 2 ; using the zero-field mSR technique under hydrostatic pressures P up to 0:85 GPa: The measurements qualitatively confirm the previous results of NMR and neutron scattering that pressure induces unusual evolution of an inhomogeneous antiferromagnetic (AF) phase which spatially competes with some nonmagnetic ‘hidden order’. The AF volume fraction, however, develops abruptly at around a pressure of 0:6 GPa; in contrast to the P-linear variations in the NMR results. Moreover, the onset temperature of the AF-volume evolution is clearly lower than 17:5K; whereas the nominal N! eel temperature seems to be about 20 K: These experimental results suggest that the AF phase exists as a metastable state. r 2002 Elsevier Science B.V. All rights reserved. PACS: 71.27.+a; 75.30.Mb; 76.75.+i Keywords: URu 2 Si 2 ; Hidden order; Pressure effects; Antiferromagnetism The heavy-electron material URu 2 Si 2 has at- tracted particular interest for its curious, second- order, phase transition at 17:5K ð T 0 Þ [1], where antiferromagnetic (AF) order with an extremely small ordered moment ðm ord ¼ 0:0370:01m B =UÞ is observed [2]. Recent experiments of neutron scattering [3] and 29 Si NMR [4] performed under hydrostatic pressure have revealed that inhomo- geneous AF order with moments of a normal size ðm ord C0:25m B =UÞ is induced by pressure, at least, in the range from 0.3 to 0:83 GPa: Simple extrapolation to ambient pressure strongly sug- gests that the tiny AF moment is ascribed to a 1% volume fraction of this AF phase. The true nature of the 17:5 K transition is thus attributed to an unidentified, nonmagnetically ordered phase which occupies the rest 99% of the crystal [5]. *Corresponding author. Tel.: +81-11-706-3484; fax: +81- 11-706-4926. E-mail address: amiami@phys.sci.hokudai.ac.jp (H. Amitsuka). 0921-4526/03/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4526(02)01654-X