Neutron-scattering and specific-heat measurements of the heavy-fermion superconductor URu2Si2 under hydrostatic pressure and with Rh-doping [U(Ru{0.98}Rh{0.02})2Si2] show the existence of two magnetic phase transitions. At the second-order phase transition Tm ≈ 17.5 K, a tiny ordered moment is established, while at TM < Tm, a first-order phase transition (under pressure or doping) gives rise to a large moment. The results can be understood in terms of a hidden OP Psi coupled to the ordered moment m, where m and Psi have the same symmetry.
@article{arxiv.cond-mat/0312206,
title = {Pressure dependence of magnetism in URu2Si2},
author = {F. Bourdarot and B. Fak and V. P. Mineev and M. E. Zhitomirsky and N. Kernavanois and S. Raymond and P. Burlet and F. Lapierre and P. Lejay and J. Flouquet},
journal= {arXiv preprint arXiv:cond-mat/0312206},
year = {2007}
}