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

Abstract

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, T=0.3170/(Xo−0.02221)−0.00075 °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 S1.22, leading to the equation, T=1.84 × 10−3 S 0.22 °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 2 × 106 °K.

Thermodynamic analysis of the quantum critical behavior of Ce-lattice compounds

“…As can be seen, all experimental results show a sort of envelope curve described by the S m (T ) dependence of CePd 3 B [71]. To our knowledge, the record of lowtemperature entropy accumulation in Ce compounds was found in Ce 2 Mg 3 (NO 3 ) 12 Á 24H 2 O reaching 0.5R ln 2 at T N ¼ 1.8 mK [78], 3 which exceeds the S m (T ) value of any other compound even after normalizing by R ln 4 because its CF splitting is D CF ¼ 50 mK. Ce-diluted (Ce 0.03 La 0.97 )B 6 [79] also exhibits very high entropy at low temperature (recall that CeB 6 has a N ¼ 4 GS) without magnetic order.…”

Properties of melting 3He: Specific heat, entropy, latent heat, and temperature

“…This requires the thermodynamic relations in Eq. (37) and an adjustment to the melting curve molar volume for which we use a magnetic Griineisen constant equal to -18. The points computed from specific heat as where the "hard core" molar volume is Vhc = 8 cm3/mole but can equally well be set to zero.…”

Appareil de mesure de capacité thermique et de transfert thermique jusqu'au mK

“…L'analyse des mesures de transfert thermique nécessite une bonne connaissance de la chaleur spécifique de chacun des deux milieux. L'ensemble des résultats précédemment obtenus sur le CMN présente une certaine dispersion à très basse température [7][8][9][10][11] ; ne disposant pas des résultats nécessaires pour les phases superfluides de 'He [4], nous avons effectué les mesures correspondantes qui sont présentées dans cet article, ainsi que les premiers résultats de transfert thermique à l'interface CMN-mélange 3He-4He.…”