Magnetothermodynamic Properties of MnCl2 from 1.3° to 4.4°K at 90 kG. A Zero Entropy Reference. The Magnetomechanical Process at Absolute Zero

The heat capacity of single‐crystal manganous chloride has been measured in a constant magnetic field, accurately stabilized at 90 000 G and parallel to the b magnetic axis (a crystallographic axis), over the range 1.4° to 4.35°K. The heat capacity approaches zero and magnetic saturation occurs near 1°K, thus providing a reference for the zero of electronic entropy which might otherwise be difficult to obtain due to possible irreversible behavior of this antiferromagnetic system below 1°K. It has been established that loss of magnetic moment from the saturated magnetic domain of the crystal occurs with increasing temperature through the loss of individual Bohr units of angular momentum. By extrapolating reversibly measured values of the magnetic moment, on isoerstedic paths to 0°K, the magnetomechanical process of magnetization at the absolute zero has been described in fairly complete detail. The magnetization curve at 0°K consists of two nearly straight lines joined by a small deviation near 10 kG and approximating saturation at about 32 kG. The value of the quantity ${\mathrm{(E}}_{\mathrm{sat}}{\mathrm{-E}}_{\mathrm{H}\text{=0}}{)}_{0^{\circ }\mathrm{K}}\text{=10.80\hspace{0.17em}}\mathrm{cal}/\mathrm{mole}$ . It has been shown that (∂E/∂T)_H for MnCl₂ at 90 kG is a negative quantity, as would be expected for an antiferromagnetic substance near magnetic saturation. Magnetic work is by far the dominant effect in determining the size of the quanta absorbed near magnetic saturation. The initial increments of demagnetization, with increasing temperature, are assisted by withdrawal of stored internal energy. Thus, the quanta with the smaller energies are those connected with the loss of angular momentum from individual atomic centers, rather than proportionally from the entire cooperative system of electron spins. The limiting heat content at 90 kG is given by $H_{\mathrm{H}}{\mathrm{–H}}_{\mathrm{lattice}}\text{=20.3}\exp \text{(−20.3/RT)\hspace{0.17em}}\mathrm{cal}/\mathrm{mole}$ , and the limiting magnetic moment is represented by $\mathbf{M}={\mathbf{M}}_{\mathrm{sat}}\mathrm{-Ng\beta /[}\exp \text{(20.3/RT)−1]}\mathrm{G}·{\mathrm{cm}}^3/\mathrm{mole}$ , where M_sat=27 988 and Ngβ=11 194 G·cm³/mole, with g taken as 2.004. The abuse of the word ``point'' in thermodynamics is discussed and the word ``pernt'' is defined.