Evidence for incommensurate antiferromagnetism in nonsymmorphic UPd$_{0.65}$Bi$_2$

The intersection between nonsymmorphic symmetry and electronic correlations has emerged as a platform for topological Kondo semimetallic states and unconventional spin textures. Here we report the synthesis of nonsymmorphic UPd$_{0.65}$Bi$_2$ single crystals and their structural, electronic, magnetic, and thermodynamic properties. UPd$_{0.65}$Bi$_2$ orders antiferromagnetically (AFM) below $T_N\simeq$ 161 K as evidenced by a sharp cusp in magnetic susceptibility, a second-order phase transition in specific heat, and an upturn in electrical resistivity, which suggests an incommensurate AFM structure that deviates from the A-type magnetism typically observed in this class of materials. Across $T_N$, Hall effect measurements reveal a change from electron-dominated to hole-dominated transport, which points to a sharp reconstruction in the electronic structure at $T_N$. Upon further cooling, a first-order transition is observed at $T_1 \simeq 30 $K in magnetic susceptibility and heat capacity but not in electrical resistivity or Hall measurements, which indicates a small change in the AFM structure that does not affect the electronic structure. Our specific heat data reveal a small Sommerfeld coefficient ($\gamma \simeq$13 mJmol$^{-1}$K$^{-2}$), consistent with localized 5$f$ electrons. Our results indicate that UPd$_{0.65}$Bi$_2$ hosts weak electronic correlations and is likely away from a Kondo semimetallic state. Low-temperature magnetization measurements show that the AFM structure is remarkably stable to 160 kOe and does not undergo any field-induced transitions. Neutron diffraction and magnetization experiments at higher fields would be valuable to probe the presence of unconventional spin textures.