Adiabatic Demagnetization of Chromium Methylamine Alum

Adiabatic demagnetization experiments have been performed on a specimen of methylamine chromic alum, C${\mathrm{H}}_3$N${\mathrm{H}}_3$·Cr${(\mathrm{S}{\mathrm{O}}_4}_2$·12${\mathrm{H}}_2$O, comprising a spherical mass of small crystals. Using fields up to 23 000 oersteds and starting temperatures of the order of 1.15°K, the "free-spin" entropy of $R{log}_{e}4$ could be reduced to $0.26R$, and the magnetic susceptibility versus entropy relation has been obtained over this range of entropy. The susceptibility was measured at 210 cps with an ac mutual inductance bridge. For temperatures above 0.1°K very good agreement is obtained between these results and the Hebb-Purcell theoretical curve with a value of 0.275° (0.19 ${\mathrm{cm}}^{-1\mathrm{}}$) for the crystalline field splitting of the ground level of the ${\mathrm{Cr}}^{+++}$ ion. In the neighborhood of entropy $R{log}_{e}2$ the susceptibility begins to increase rapidly and goes through a maximum at $S=0.53R$. As in the case of other paramagnetic alums previously investigated, ac losses occur in the region of maximum susceptibility (the loss component also showing a maximum), and ballistic measurements show hysteresis effects occurring on the low entropy side of the susceptibility maximum.