Antiferromagnetism in Pr3In: Singlet/triplet physics with frustration

We present neutron diffraction, magnetic susceptibility and specific heat data for a single-crystal sample of the cubic (Cu3Au structure) compound Pr3In. This compound is believed to have a singlet (Gamma1) groundstate and a low-lying triplet (Gamma4) excited state. In addition, nearest-neighbor antiferromagnetic interactions are frustrated in this structure. Antiferromagnetic order occurs below T_N = 12K with propagation vector (0, 0, 0.5 +/-\delta) where \delta \approx 1/12. The neutron diffraction results can be approximated with the following model: ferromagnetic sheets from each of the three Pr sites alternate in sign along the propagation direction with a twelve-unit-cell square-wave modulation. The three moments of the unit cell of 1 \mu_B magnitude are aligned so as to sum to zero as expected for nearest-neighbor antiferromagnetic interactions on a triangle. The magnetic susceptibility indicates that in addition to the antiferromagnetic transition at 12K, there is a transition near 70K below which there is a small (0.005 \mu_B) ferromagnetic moment. There is considerable field and sample dependence to these transitions. The specific heat data show almost no anomaly at TN = 12K. This may be a consequence of the induced moment in the Gamma1 singlet, but may also be a sample-dependent effect.