Heat Capacity and Entropy of NiSiF6·6H2O in the Region 0.30°—4.25°K with Magnetic Fields 0–90 kG Parallel to the c Axis

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Abstract
The heat capacity of nickel fluosilicate hexahydrate has been measured over the range 0.30°—4.25°K in stabilized magnetic fields of 0, 0.25, 0.5, 1, 2, 3, 5, 10, 20, 40, 60, 80, and 90 kG. The field was directed along the crystallographic c axis of the 3.5‐cm‐diam, spherical, single crystal. The variation of temperature with magnetic field has been measured with a carbon thermometer in a calorimetric apparatus at nearly constant entropy. Very minor corrections for entropy leakage and the empty calorimeter have enabled the accurate tabulation of entropy changes in NiSiF6·6H2O with temperature and field over the area 0.30°—4.25°K and 0–90 kG. At the higher fields and lower temperatures it was necessary to consider a small amount of entropy removed by nuclear magnetization of the hydrogen and fluorine atoms. All of the data apply to a sphere in a homogeneous external field uncorrected for the demagnetization effect. The substance gave no indication of magnetic irreversibility. These results will serve as a base for a thermodynamic treatment of later observations in the region below 0.30°K. The total electronic entropy removed to magnetic saturation was Rln3.

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