H-T phase diagram of the metamagnetic transition in URu₂Si₂, measured in high d. c. magnetic fields

We have studied the ultrasonic velocity and ac susceptibility of $URu_2Si_2$ in d. c. magnetic fields up to 45T. A significant difference between the constructed H-T phase diagram and that extracted from earlier pulsed fields measurements can be explained in terms of a large magnetocaloric effect. A hysteresis of metamagnetic transition at low temperatures and a new phase separation line have been observed for the first time.