Susceptibility and Entropy ofF-Centers in Alkali Halides below 1°K

The entropy and the paramagnetic susceptibility of $F$-centers in alkali halides below 1°K for alkali nuclei with spin $\frac{3}{2}$ have been computed using the model of Kip et al. in which the predominant perturbation of the electron in the $F$-center is the magnetic interaction with the neighbor alkali nuclei. The lower limits of temperature possible by magnetic cooling of such systems are set forth and considerations are given as to the practical possibility of such cooling of $F$-center assemblies.