New heavy fermion compounds Yb$_4$Ni$_9$Al$_{24}$ and YbRh$_3$Si$_7$

Yb$_4$Ni$_9$Al$_{24}$ and YbRh$_3$Si$_7$ are newly discovered compounds, with triclinic (space group P $\overline{1}$) and rhombohedral (space group R $\overline{3}$c) crystal structure respectively, and two, and respectively one Yb site in the unit cell. Measurements on solution-grown single crystals of Yb$_4$Ni$_9$Al$_{24}$ and YbRh$_3$Si$_7$ indicate anisotropic susceptibility and field-dependent magnetization for both compounds. No apparent magnetic ordering was observed in Yb$_4$Ni$_9$Al$_{24}$ down to 1.8 K, whereas the YbRh$_3$Si$_7$ data are consistent with a ferromagnetic component of the ground state below $\sim~10$ K, for H $\parallel (ab)$. We are also presenting zero-field resistivity and specific heat data, based on which and YbRh$_3$Si$_7$ can be classified as new stoichiometric heavy fermion compounds.