Field-induced transition in UPdSn at 3 T

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Abstract
A single crystal of the intermetallic compound UPdSn has been studied by means of neutron diffraction on a two-axis spectrometer in magnetic fields up to 7 T and at temperatures between 6 and 46 K. The uranium moments have previously been shown to exhibit two antiferromagnetic phase transitions at approximately 25 and 40 K. The c axis is known to be the hard magnetic axis, and if the field is applied in the hexagonal basal plane, the magnetization measurements show a low-temperature spin-flop transition at approximately 3 T. In the present measurements, the field was applied along the [100] direction of the orthorhombic magnetic unit cell. We have studied the temperature dependence of the (010) antiferromagnetic reflection, which corresponds to the order parameter of the 25-K transition, at fixed fields; the ground state is stable for fields below 3.5 T and temperatures below 25 K, and we have established the magnetic phase boundaries in this regime. We also made a series of measurements in which the sample was zero-field cooled from high temperature and then the field was ramped up and down at fixed temperature. In this configuration, we observe significant irreversible domain repopulation effects below the spin-flop transition as the field is increased, but the curves obtained on reducing the field from above the spin-flop transition seem to be indicative of intrinsic single-domain properties; they are consistent with the field-cooled data. We have also studied the induced (ferromagnetic) moment, and have found that the behavior is very different above and below the 25-K transition. For T<25 K, there is a sharp transition at approximately 3 T in agreement with bulk magnetization measurements, while above 25 K, the magnetic intensity rises parabolically with fields as would be expected for a simple antiferromagnet; the induced magnetic moment in this case arises from the transverse magnetic susceptibility.

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