A Theoretical Study of Magnetic Cooling Experiments

Abstract

The experimental results obtained by several investigators on the magnetic cooling method are investigated in the light of (1) the electric crystalline fields acting on the magnetic ions of the paramagnetic salts used in the experiments, and (2) the magnetic dipole-dipole coupling between these ions. These two factors produce anomalous specific heats and departures of the magnetic susceptibility from Curie's law at temperatures below 1°K with which the magnetic method is concerned. Thus the empirical Curie temperature, proportional to the reciprocal of the susceptibility, deviates from the true Kelvin temperature in this region. By the methods developed and discussed in the preceding paper by Van Vleck, numerical calculations of the specific heat and susceptibility have been performed for several salts. The experimental specific heat measured on a Curie scale has been corrected to the Kelvin scale and compared with the calculated results. Fairly good quantitative agreement is obtained, although there is some uncertainty as to the effect of (2) on the susceptibility and hence on the relation between the empirical and Kelvin temperature scales.