Magnetization, resistivity, and specific-heat measurements have been performed on a high-quality single crystal of (tetragonal) ${\mathrm{UIr}}_2$${\mathrm{Si}}_2$. In all physical quantities anomalies are found related to an antiferromagnetic transition at $T_N$+4.9 K. No superconductivity was observed down to 40 mK. The magnetization and resistivity are strongly anisotropic. Below 4.9 K a metamagnetic transition occurs along the c axis at a field of 2.2 T. For T>$T_N$ the specific heat yields an electronic constant γ of 105 mJ/mol ${\mathrm{K}}^2$, whereas below $T_N$ a much higher value (290 mJ/mol ${\mathrm{K}}^2$) is found. The magnetic entropy related to the transition is too small to be explained by an ordering of localized moments. A crystal-electric-field level scheme involving three singlets and a doublet is used in an attempt to interpret the data, but clearly effects of the heavy electrons and their hybridization must be considered.

• Received 5 October 1989

DOI:https://doi.org/10.1103/PhysRevB.41.2589