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

Abstract

The specific heats of ${\mathrm{EuNi}}_{5}$${\mathrm{P}}_{3}$, an antiferromagnet, and ${\mathrm{EuNi}}_{2}$${\mathrm{P}}_{2}$, 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 ${\mathrm{EuNi}}_{5}$${\mathrm{P}}_{3}$ shows a \ensuremath{\lambda}-like anomaly with a maximum at 8.3 K. With increasing field in the range 0--2.5 T, the maximum shifts to lower temperatures, as expected for an antiferromagnet. In higher fields the antiferromagnetic ordering is destroyed and the magnetic part of the specific heat approaches a Schottky anomaly that is consistent with expectations for the crystal-field/Zeeman levels. In low fields and for temperatures between 1.5 and 5 K the magnetic contribution to the specific heat is proportional to the temperature, indicating a high density of excited states with an energy dependence that is very unusual for an antiferromagnet. The entropy associated with the magnetic ordering is \ensuremath{\sim}R ln8, confirming that only the ${\mathrm{Eu}}^{2+}$---with J=7/2, S=7/2, L=0---orders below 30 K. In zero field approximately 20% of the entropy occurs above the N\'eel temperature, consistent with the usual amount of short-range order observed in antiferromagnets. The hyperfine magnetic field at the Eu nuclei in ${\mathrm{EuNi}}_{5}$${\mathrm{P}}_{3}$ is 33.3 T, in good agreement with a value calculated from electron-nuclear double resonance measurements. For ${\mathrm{EuNi}}_{2}$${\mathrm{P}}_{2}$ 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 \ensuremath{\sim}100 mJ/mol ${\mathrm{K}}^{2}$.