Phase transitions in $R_{5}$NiPb$_{3}$ ($R$=Ce,Nd,Gd)

We report magnetic and thermodynamic measurements for recently-synthesized $R_{5}$NiPb$_{3}$ ($R$=Ce,Nd,Gd) (hexagonal Hf$_{5}$CuSn$_{3}$-type structure), as well as non-magnetic La-based analogs. High-temperature Curie-Weiss fits yield effective moments of 2.43, 3.70 and 9 $\mu_{B}$ for Ce$_{5}$NiPb$_{3}$, Nd$_{5}$NiPb$_{3}$ and Gd$_{5}$NiPb$_{3}$ respectively. These are close to the $R^{3+}$ ionic moments, showing that Ni is nonmagnetic in all cases. For Ce$_{5}$NiPb$_{3}$ a peak seen in both the magnetization and specific heat at 48 K indicates an apparent ferromagnetic transition at that temperature, which is also confirmed by field dependent heat capacity and a positive Curie-Weiss temperature. Nd$_{5}$NiPb$_{3}$ exhibits two magnetic transitions, an antiferromagnetic transition at 42 K and an apparently weak ferromagnetic canting transition at 8 K. Ce$_{5}$NiPb$_{3}$ shows a kink in both the magnetization and specific heat at 68 K indicates a ferro- or ferrimagnetic transition at that temperature, which is also confirmed by a positive Curie-Weiss temperature. For this material, ZFC and FC measurements show irreversibility at transition temperature. For Ce and Nd samples $M$-$H$ curves show metamagnetism at low temperatures. We will compare the results with the non magnetic analog La$_{5}$NiPb$_{3}$.