Heat capacities and related thermal properties of DyNi5, HoNi5 and ErNi5 between 5 and 300 K

Heat capacity data and calculated thermodynamic functions are presented for DyNi₅, HoNi₅ and ErNi₅. λ-type thermal anomalies are noted at 12.0 K (DyNi₅), 4.1 K (HoNi₅) and 8.0 K (ErNi₅). Schottky-type anomalies are observed at higher temperatures. The λ and Schottky anomalies are ascribed to the destruction of ferromagnetic order and to crystal field excitation, respectively. A deficiency of magnetic entropy, compared to Rln(2J + 1), is noted corresponding roughly to Rln2. This suggests that the ground state in the ordered materials is a doublet. ErNi₅ is analyzed using a Hamiltonian containing terms representing the crystal field and magnetic interactions. The analysis shows that a doublet ground state can result with reasonable values of the crystal field parameters. The parameters are shown to be consistent with the heat capacity behavior of ErNi₅. Ordering temperatures are not proportional to the de Gennes function.