NiSiF6·6H2O. Temperature and Other Thermodynamic Properties from Adiabatic Demagnetization without Low-Temperature Heat Introduction. H ∥ c Axis

Abstract

Values of field and magnetic moment have been measured on 40 magnetic isentropes of NiSiF₆·6H₂O. The field was directed along the crystallographic c axis of the 3.5‐cm‐diam spherical single crystal. Typical derived values of the magnetic work, differential isentropic susceptibility, relative heat content (enthalpy), and relative internal energy have been tabulated. A principal purpose of the measurements has been to investigate the evaluation of temperature and other thermodynamic properties in the presence of any available magnetic fields without heat introduction below 0.3°K. This avoids the difficulty of distributing heat under small temperature heads in poor thermal conductors. Absolute temperatures from the thermodynamic equation T = (∂H/∂S)_H, where H, S, and H are heat content, entropy, and field, respectively, and values of heat capacity C_H = T (∂S/∂T)_H have been derived at representative fields. At entropies below 1 gibbs/mole and within about ±200 G of zero field, the adiabatic magnetic susceptibility remains constant at 34.3 cm³/mole. At the same time the limiting temperatures of the adiabatics varied from about 0.05°—0.15°K. There was some evidence that the system was beginning to develop irreversibility near 0.05°K but it appeared to be reversible at higher temperatures.