Magnetic and thermodynamic properties of Mn(NH4)2(SO4)2⋅6H2O below 1°K and at fields up to 24000 G parallel to the b axis

The heat capacity of Mn(NH4)2(SO4)2⋅6H2O has been measured directly in fields of 0, 1400, 2800, 6800, 12600, 16800, and 22300 G, from 0.4 to 1°K, along with observation of magnetization both isothermally and isentropically. Temperatures, entropies, and heat capacities have been derived for the region between 0.1 and 0.4°K, where direct measurements were not possible. Deviations from ideal behavior have been interpreted in terms of nuclear spin polarization of 55Mn and of (electronic spin) exchange.