Magnetic irreversibility in the antiferromagnetic state of UPt2Si2

We present a detailed investigation of the magnetothermal hysteresis observed in the magnetically ordered phase of the moderately mass-enhanced U intermetallic UPt2Si2. Aside from the antiferromagnetically ordered state below TN=31K, in a susceptibility study we establish the presence and field dependence of two irreversibility temperatures Tirr and Tirr2. Comparing the susceptibility data to the result of a neutron-scattering study, we find the lower temperature Tirr2 to be accompanied by an anomaly in the temperature dependence of various magnetic Bragg peaks, viz., a reduction of the intensity. Tentatively, we attribute this behavior to a modification of the symmetry of the magnetically ordered phase. Altogether, our findings imply that the magnetic phase diagram of UPt2Si2 is much richer than previously thought, and similar to related uranium (122) intermetallic compounds.