Low-temperature specific heat of antiferromagnetic EuNi5P3 and mixed-valent EuNi2P2 in magnetic fields to 7 T

The specific heats of EuNi5P3, an antiferromagnet, and EuNi2P2, a mixed-valence compound, have been measured between 0.4 and 30 K in magnetic fields of, respectively, 0, 0.5, 1, 1.5, 2.5, 5, and 7 T, and 0 and 7 T. In zero field the specific heat of EuNi5P3 shows a -like anomaly with a maximum at 8.3 K. With increasing field in the range 02.5 T, the maximum shifts to lower temperatures, as expected for an antiferromagnet. In higher fields the antiferromagnetic ordering is destroyed and the magnetic part of the specific heat approaches a Schottky anomaly that is consistent with expectations for the crystal-field/Zeeman levels. In low fields and for temperatures between 1.5 and 5 K the magnetic contribution to the specific heat is proportional to the temperature, indicating a high density of excited states with an energy dependence that is very unusual for an antiferromagnet. The entropy associated with the magnetic ordering is R ln8, confirming that only the Eu2+ with J=7/2, S=7/2, L=0orders below 30 K. In zero field approximately 20% of the entropy occurs above the Néel temperature, consistent with the usual amount of short-range order observed in antiferromagnets. The hyperfine magnetic field at the Eu nuclei in EuNi5P3 is 33.3 T, in good agreement with a value calculated from electron-nuclear double resonance measurements. For EuNi2P2 the specific heat is nearly field independent and shows no evidence of magnetic ordering or hyperfine fields. The coefficient of the electron contribution to the specific heat is 100 mJ/mol K2.