LOW-TEMPERATURE SPECIFIC-HEAT OF ANTIFERROMAGNETIC EUNI5P3 AND MIXED-VALENT EUNI2P2 IN MAGNETIC-FIELDS TO 7T

The specific heats of EUNi(5)P(3), an antiferromagnet, and EuNi2P2, a mixed-valence compound, have been measured between 0.4 and 30 K in mag netic 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 h-like ano maly with a maximum at 8.3 K. With increasing field in the range 0-2.5 T, the maximum shifts to lower temperatures, as expected for an antif erromagnet. In higher fields the antiferromagnetic ordering is destroy ed and the magnetic part of the specific heat approaches a Schottky an omaly that is consistent with expectations for the crystal-field/Zeema n levels. In low fields and for temperatures between 1.5 acid 5 K the magnetic contribution to the specific heat is proportional to the temp erature, indicating a high density of excited states with an energy de pendence that is very unusual for an antiferromagnet. The entropy asso ciated with the magnetic ordering is similar to R In8, confirming that only the Eu2+-with J=7/2, S=7/2, L=0-orders below 30 R. In zero field approximately 20% of the entropy occurs above the Neel temperature, c onsistent. with the usual amount of short-range order observed in anti ferromagnets. The hyperfine magnetic field at the Eu nuclei in EUNi(5) P(3) is 33.3 T, in good agreement with a value calculated from electro n-nuclear double resonance measurements. For EuNi2P2 the specific heat is nearly field independent and shows no evidence of magnetic orderin g or hyperfine fields. The coefficient of the electron contribution to the specific heat is similar to 100 mJ/mol K-2.