Magnetothermodynamics of α-NiSO4·6H2O. I. Heat Capacity, Entropy, Magnetic Moment, and Internal Energy, from 0.4° to 4.2°K, with Fields 0–90 kG along the c Axis

Abstract

The heat capacity and magnetic moment of single-crystal α-NiSO4·6H2O have been measured over a range of about 0.4°—4.2°K in stabilized fields of 0–90 kG. The field was parallel to the c crystallographic axis of the 4-cm-diam spherical sample. Although magnetic saturation was not attained even at 90 kG and 0.4°K, there was essentially no temperature coefficient of the magnetic moment below 0.7°K at any field. The heat capacity approached zero for all fields except for a small calculable contribution due to nuclear polarization at high fields. At 90 kG it was found that the nuclear-spin heat capacity at 0.95°K and above was in general equilibrium. At 0.90°K equilibrium was quite slow indicating a rather rapid increase in relaxation time with decreasing temperature. Each heat capacity series provided a zero-entropy reference, above that due to nuclear spin. These references agreed with precise measurements of the temperature change with field on adiabatics joining the various isoerstedic heat capacity series. The entropy changes, in combination with the work and free energy of magnetization, enabled the evaluation of the changes of internal energy and enthalpy as a function of field and temperature. With very minor deviations it was found possible to represent the heat capacity data at all fields by means of three state Schottky functions. At zero field the two upper states are degenerate and are 4.74 cm−1 (13.55 cal/mole) above the ground state. The variation of the enthalpy of each of the individual levels with field is given. A detailed description of the magnetocalorimeter is included.