Probing the Magnetic Order of UPt$_{3}$ by Magnetoresistance

It is believed that magnetic order has a strong influence on the unconventional superconducting state of UPt$_{3}$, both as a symmetry breaking field that splits two degenerate superconducting phases, and as an element of the pairing mechanism. There are conflicting reports of the behavior at 5 K, static antiferromagnetic order versus slow fluctuating magnetism. We present measurements of the temperature dependence of the magnetoresistance in bulk single crystals of UPt$_{3}$. Below 5 K, we observe an onset and subsequent rapid increase of a term linear in the applied field. With the field in the basal plane, this linear contribution is positive with current along the c-axis and negative with the current along the b-axis. We identify this behavior in the magnetoresistance with observation of magnetic Bragg peaks from neutron scattering that appear below 5 K. We discuss preliminary results of similar measurements at temperatures in the millikelvin range.