Thermal and Magnetic Properties of Ferromagnetic YbPt$_{2}$

We present the results of magnetization and heat capacity measurements carried out on needle-like single crystals of YbPt$_{2}$. Above 250 K, the magnetic susceptibility is well described by a Curie-Weiss expression with a moment of 3.35 $\mu_{B}$ per Yb and a Weiss temperature of 73 K, indicating ferromagnetic interactions among almost localized moments. At the lowest temperatures, the magnetization is increasingly nonlinear in field, approaching a saturation value of 3.5 $\mu_{B}$/Yb at 1.8 K. However, an Arrott plot analysis indicates that ferromagnetic order occurs at a much lower temperature, approximately 0.85 K. Above $\sim$10 K,we find that the heat capacity C=$\gamma$T+$\beta$T$^{3}$, with $\gamma$=0.358 J/mol-K$^{2}$, while between 10 K and 3 K, C/T drops by a factor of four, perhaps due to the opening of a gap at the Fermi surface. A large anomaly is found in the heat capacity at 0.83 K, suggesting that the transition to the ferromagnetic state is weakly first order. Our results imply that YbPt$_{2}$ is a rare example of a heavy fermion ferromagnet.