In earlier work, we obtained the higher-temperature melting properties of He3 with the solid in the paramagnetic range. The exchange-coupled solid was assumed to become antiferromagnetic below its spin-ordering temperature. The thermal excitations of the permanently paramagnetic liquid were described in terms of its spin and nonspin degrees of freedom according to a theory of this phase elaborated in early work. Recent accurate measurements of the melting properties of He3 permit a critical comparison of theory with experiment. With complete spin disorder in the solid at melting, at even the lowest temperature reached in the measurements, the latter confirm the satisfactory accuracy of the theoretical entropy of the liquid at melting. We now use the asymptotic molecular-field-theory model to extend the description of melting He3 into the antiferromagnetic range of the solid. The effect of the magnetic transition at melting allows one to locate the temperature at which this transformation begins. The low temperatures attained by adiabatic freezing, which we studied previously, are shown to have a theoretical lower limit of about 0.5 m°K. This is identical with the low-temperature maximum of the melting pressure or the end of the anomalous melting phenomenon which starts at the melting-pressure minimum around 325 m°K. The characteristic cooling on adiabatic freezing over the indicated temperature range is, nevertheless, far from exhausting the physical content of the remarkable melting anomaly of He3. It is thus proved that the cooling effect permits the verification of the theoretical melting pressure down to the upper reaches of its indicated low-temperature limit. This is shown to be feasible without the measurements of the still unknown very low temperatures. This verification procedure determines the latter unambiguously through the intermediary of the entropies of the liquid and solid at melting; the functional arguments of the accurately measured melting pressure changes between initial and final states of the adiabatic freezing process. A thermodynamic standard of temperatures, down to very low temperatures, has been obtained thereby. This should open up a new field of experimental investigations where, heretofore, in the absence of its direct and accurate accessibility, the concept of temperature seemed to lose some of its significance. The thermodynamic He3 melting-pressure thermometry, at the very low temperatures, may reasonably be expected to initiate and guide the charting of the submillidegree range, which at the present time and on the basis of the results obtained in the present work becomes accessible only by magnetic cooling. © 1969 The American Physical Society.
