The direct measurement of the magnetization entropy of a paramagnetic substance: thermal and magnetic properties of ferric methylammonium alum at low temperatures

Abstract
A method is described for determining the magnetization entropy of a paramagnetic substance by the measurement of the heat absorbed by the sample during isothermal demagnetizations near 1 $^\circ$K. The method has been used with ferric methylammonium alum, which deviates appreciably from ideal behaviour because of the large Stark splittings of the ground state of the Fe$^{3+}$ ions. The results were found to agree within the experimental error with magnetization entropies computed from the energy levels of the ferric ions in a trigonal crystalline field. The computed entropies have been used to determine the absolute temperatures below 1 $^\circ$K reached by adiabatic demagnetizations of this salt.