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°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°K reached by adiabatic demagnetizations of this salt.