Adiabatic Demagnetization and Specific Heat in Ferrimagnets

The specific heat of yttrium iron garnet (YIG) has been measured at 1.45°K as a function of magnetic field from zero to 18 kilo-oersteds. It is found to drop about 30% over this field range as the field is increased. Adiabatic temperature changes, $\Delta T$, were observed during magnetic cycles over various field intervals in the range from 3 to 18 kilo-oersteds, and at various temperatures between 1 and 4°K. The experimental values of $\Delta T$ and the specific heat fit the predictions of spin-wave theory to within experimental error. The data are sufficiently extensive to provide a useful test of spin-wave theory as well as checks on the consistency of the data itself as between the two types of observations. Values of various parameters which characterize the thermal and magnetic properties of YIG are determined from this investigation to have the following values: Landau-Lifshitz exchange constant $A=4.3\mathrm{}\times {10}^{-7\mathrm{}}$ erg/cm; Debye temperature $\theta =510\mathrm{}°$K. The serious effect of magnetic impurities on investigations of this sort is pointed out.