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

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.