Hidden order and weak antiferromagnetism in URu2Si2

The origin and nature of the low-temperature please appearing below 17.5 K (equivalent to T-0) in URu2Si2 is still not fully understood. Since the weak antiferromagnetic state (WAF; mu(ord) similar to0.02-0.04 muB) developing below around To is not simply reconciled with a large reduction in the magnetic entropy (similar to0.2R ln 2), the existence of some hidden order parameter has been argued, where WAF is considered to be of a secondary nature. The paper reviews recent development of microscopic characterization of WAF based on neutron scattering and Si-29 NMR under high pressure. It is demonstrated that, at least, for pressures ranging from similar to0.3 to similar to0.83 GPa, there are spatially competing two different ordered states in the crystal: a normal-moment (similar to0.25 mu(B)/U) antiferromagnetic state and some unknown, non-magnetically ordered state. Simple extrapolation to ambient pressure suggests that WAF originates from one percent volume fraction of this antiferromagnetic phase. We also discuss the unusual magnetic properties at ambient pressure, presenting our latest results of zero-field and longitudinal-field muSR experiments. (C) 2002 Elsevier Science B.V. All rights reserved.