This paper is aiming to review some of the neutron scattering studies performed on URu2Si2 in Grenoble. This compound has been studied for a quarter of century because of a so-called hidden order ground state visible by most of the bulk experiments b
ut almost invisible by microscopic probes like neutrons, muons NMR or x-ray. We stress on some aspects that were not addressed previously. Firstly, the comparison of the cell parameters in the 1-2-2 systems seems to point that the magnetic properties of URu2Si2 are leading by an U4+ electronic state. Secondly, a compilation of the different studies of the tiny antiferromagnetic moment indicates that the tiny antiferromagnetic moment has a constant value which may indicate that it is not necessary extrinsic. We also present the last development on the magnetic form factor measurement in which the magnetic density rotates when entering in the hidden order state. To end, the thermal dependence of the two most intense magnetic excitation at Q0=(1,0,0) and Q1=(0.6,0,0) seems to indicate two different origins or processes for these excitations.
New inelastic neutron scattering experiments have been performed on URu2Si2 with special focus on the response at Q0=(1,0,0), which is a clear signature of the hidden order (HO) phase of the compound. With polarized inelastic neutron experiments, it
is clearly shown that below the HO temperature (T0 = 17.8 K) a collective excitation (the magnetic resonance at E0 approx 1.7 meV) as well as a magnetic continuum co-exist. Careful measurements of the temperature dependence of the resonance lead to the observation that its position shifts abruptly in temperature with an activation law governed by the partial gap opening and that its integrated intensity has a BCS-type temperature dependence. Discussion with respect to recent theoretical development is made.
