Magnetoresistance of UPt₃

In order to study antiferromagnetic order in the unconventional superconductor UPt₃, we have measured the magnetoresistance of high-quality bulk single crystals over a broad temperature range. We observe a linear magnetoresistance when the magnetic field is applied in the basal plane for temperatures below ≈ 5 K where magnetic Bragg scattering has been observed. However, when we apply a magnetic field along the c-axis, we find a negative magnetoresistance that appears below a characteristic field, H ₀ ≈2 T, which we associate with suppression of spin-dependent scattering and which changes abruptly near 5 K.