Heat Capacity and Entropy of Mn(NH4)2(SO4)2·6H2O from 1° to 4°K in Fields up to 24 000 Oe Perpendicular to the b Axis; Direct Observation of Magnetic Imperfection

The magnetization behavior of a spherical single crystal of Mn(NH₄)₂(SO₄)₂·6H₂O has been investigated from 1° to 4°K in fields up to 24 000 Oe perpendicular to the $\text{2̄01}$ crystal plane, along 15 (approximately) isentropic paths for entropies ranging from about 1.6 cal mole⁻¹·deg⁻¹ to 3.5 cal mole⁻¹·deg⁻¹. The heat capacity of the specimen has been measured at 0, 4360, 6450, 12 900, and 17 760 Oe. The two sets of measurements have been combined to obtain the entropy as a function of field and temperature over the range covered. The heat capacity and entropy are in all cases close to the values predicted by a Brillouin function with S=5/2, g=2.000 with small but significant deviations due to departures from ``ideal'' paramagnetic behavior. The deviations are shown clearly in the behavior of (H/T) along isentropes, which is presented for 10 particular values of the entropy. They correspond qualitatively to that predicted by the crystalline field obtained from other sources.