Magnetothermodynamics of Nd(C2H5SO4)3⋅9H2O. II. Determination of absolute temperature scales and other properties below 0.5 °K with constant fields along the c axis

Abstract

The magnetic moment of a 3.920 cm diam spherical single crystal of Nd(C2H5SO4)3⋅9H2O has been measured below 0.5 °K after cooling by demagnetization from fields of 25, 15, 10, and 5 kG along the c axis. Small amounts of irreversibility, measured by ’’entropy gain,’’ enabled correction to true magnetic moments on isentropes. Equilibrium between the lattice and electron systems was lost at fields above about 40 kG and temperatures below 0.5 °K. Entropy gain values were recorded for 0, 1000, 2500, and 5000 G on isoerstedics. An ’’adiabatic demagnetization–sample isolation calorimeter’’ was used to measure enthalpy as a function of entropy by means of electrical heating, thus enabling the determination of the thermodynamic temperature scales along the above constant and zero fields as a function of entropy. The temperature dependent component of the initial susceptibility (χ0−χT−ind) obtained from the magnetic moment data leads to the relation (χ0−0.00341) =1.2114 [1+0.0135/ T −2.69×10−4/T2+1.02×10−6/T3] cm3/mole NES, which is valid down to 0.014 °K. Below this temperature the susceptibility is independent of temperature although both the entropy and temperature continue to decrease. It was shown that the magnetic component of the heat capacity tail, after correction for the known nuclear effects of 143Nd and 145Nd, agreed with theoretical calculations based on ideal dipole–dipole interactions, thus confirming that mechanism. CH=0=2.266×10−3/T2−8.45×10−5/ T 3+1.44×10−3 T3 gibbs/mole NES. The limiting low temperature susceptibility was found to be 85.6 cm3/mole Nd3+ which extrapolates to the temperature dependent saturation moment, 10 036 G⋅cm3/mole Nd3+, at a critical field of 117 G. A heat capacity maximum occurred at 0.014 °K. The temperature, relative enthalpy, relative internal energy, heat capacity, and magnetic moment were tabulated as a function of entropy, over the range 0.01–0.5 °K for zero field. Tabulations are also included for 1000, 2500, and 5000 G.