Metamagnetic Transition in a Heavy Fermion Superconductor URu 2Si 2

We have carried out the magnetization measurements in high magnetic fields up to 60 T and in the wide temperature range from 1.3 to 77 K for a heavy femion superconductor URu 2 Si 2 . A sharp metamagnetic transition with three steps is found below 14 K for the field along the tetragonal [001] direction. This transition is changed into a sharp metamagnetic transition with two steps in a narrow temperature range from about 14 to 17 K and furthermore into a broad metamagnetic transition with one step above the Neel temperature T N =17.5 K. The broad transition persists up to a characteristic temperature T χ m a x (=55 K) where the magnetic susceptibility has a maximum in the temperature dependence. These results are discussed on the basis of a change of the electronic state of 5 f -electrons from itinerant to localized and the crystalline electric field model.