Magnetothermodynamics of α–NiSO4·6H2O II. Heat Capacity, Entropy, Magnetic Moment, and Internal Energy, from 0.4 to 4.2°K, with Fields 0-90 kG Along the Bisector of the a, b Axes

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Abstract
The heat capacity and magnetic moment of a 4-cm-diam single crystal of α-NiSo4·6H2O have been measured with stabilized fields 0-90 kG along the bisector of the a, b axes. The temperature range was about 0.4 to 4.2°K. Magnetic saturation was not reached even at 0.4°K and 90 kG although there was no temperature coefficient of the magnetic moment below 0.7°K and any field. The heat capacity approached zero at the lower temperatures and all fields. Although idealized calculation indicates a nuclear-spin contribution to the heat capacity in a field of 90 kG between 0.4 and 1°K, there was no evidence that this component was in equilibrium with the lattice, such as was observed when the field was directed along the c axis. The rapid decrease of electronic heat capacity to zero near 1°K at 90 kG indicates that the electron system was unable to serve as a mechanism to bring the nuclear spins into equilibrium with the lattice at temperatures where the nuclear-spin heat capacity should have made a substantial contribution. With the assistance of temperature observations during adiabatic magnetization the entropy was evaluated over the experimental region. The entropy and its change with field, the isentropic change of temperature with field, the differential magnetic susceptibility, the internal energy, enthalpy, and magnetic work are tabulated.