  We study TmB_4, a frustrated magnet on the Archimedean Shastry-Sutherland
lattice, through magnetization and transport experiments. The lack of
anisotropy in resistivity shows that TmB_4 is an electronically
three-dimensional system. The magnetoresistance (MR) is hysteretic at
low-temperature even though a corresponding hysteresis in magnetization is
absent. The Hall resistivity shows unconventional anomalous Hall effect (AHE)
and is linear above saturation despite a large MR. We propose that complex
structures at magnetic domain walls may be responsible for the hysteretic MR
and may also lead to the AHE.
