Magnetothermodynamics of Ce2Mg3(NO3)12· 24H2O. II. The evaluation of absolute temperature and other thermodynamic properties of CMN to 0.6 m°K

The design and operation of a magnetocalorimeter for the highly accurate measurement of the enthalpy increase and magnetic properties of CMN over the range 0.0006 to 0.01°K are described. The essentials were as follows: A Pyrex calorimeter equipped with a Pt‐9%W noninductively wound heater and carbon thermometers, and containing a very small amount of ³He gas in contact with a spherical sample of CMN suspended as a pendulum. At temperatures above 0.3°K the gas assures very rapid equilibrium. At temperatures below 0.2°K the sample will be isolated by a vacuum, produced by adiabatic demagnetization, which simultaneously cools the sample to very low temperatures. The calorimeter remains at an intermediate guard temperature to block heat leak to the sample and collect it for later accounting. All measurements were corrected for entropy and enthalpy gains due to heat leak and vibration. On the basis of measurements of the magnetic moment of CMN on 64 ``isentropic'' demagnetizations, from 25 000 or 10 000 G to − 30 G, we could find no evidence of magnetic hysteresis when the low field rate was held below 1.5 G/sec under 150G. With an entropy of 0.032 gibbs/mole Ce³⁺, the magnetization from −30 G to 80% of saturation, near +50 G, leads to an adiabatic susceptibility which was found to remain constant at 85.7±0.2 cm³/mole Ce³⁺. This extrapolates to a critical field of 60.0 G. The initial susceptibility has been tabulated from 0.001 to 0.04°K. Between 0.007 and 4°K we propose the equation, $\text{T=0.3170/(}{\mathrm{X}}_o\text{−0.02221)−0.00075\hspace{0.17em}°}\mathrm{K}$ , where 0.02221 is the molal temperature independent initial susceptibility. On the basis of heat introduction at zero field the temperature is tabulated from 0.0006° to 0.01°K. Between 0.0006 and 0.0014°K the enthalpy varies as S^1.22, leading to the equation, ${\text{T=1.84 × 10}}^{\text{−3}}{\mathrm{\hspace{0.17em}S\hspace{0.17em}}}^{\text{0.22}}\mathrm{\hspace{0.17em}°}\mathrm{K}$ . Heat introduction was also used at constant fields of 1000, 2500, and 5000 G. Magnetic work measurements were evaluated on isentropes to determine temperatures and heat capacities on isoerstedic fields of 1000, 2500, 5000, 10 000, and 15 000 G. The various experimental data obtained in this research have defined the CMN temperature scale from 0.0006 to 4.2°K, a seven thousandfold range of temperature. The magnitude of the range may also be expressed by stating that a similar range starting at ordinary temperatures would exceed ${\text{2 × 10}}^6\mathrm{\hspace{0.17em}°}\mathrm{K}$ .