We have studied the ultrasonic velocity and ac susceptibility of ${\mathrm{URu}}_2{\mathrm{Si}}_2$ in magnetic fields up to 45 T. The resulting phase diagram reveals new phase boundaries that place strong constraints on theories of hidden order for this material. Furthermore, a significant difference between the constructed $H-T$ phase diagram and that extracted from earlier pulsed field measurements is explained in terms of a large magnetocaloric effect. An offshoot of this analysis is that care should be taken in interpreting pulsed field measurements.

• Received 6 June 2003

DOI:https://doi.org/10.1103/PhysRevB.68.020406