Magnetothermodynamics of MnCl2·4H2O. I. Heat Capacity, Entropy, Magnetic Moment, from 0.4° to 4.2°K with Fields to 90 kG along the c Crystallographic Axis

The heat capacity and magnetic moment of a 3.934-cm-diam spherical single crystal of MnCl2·4H2O have been measured over the range 0.4°–4.2°K, with stabilized magnetic fields of 0, 5, 7.5, 10, 14, 18, 25, 40, 65, and 90 kG, along the c crystallographic axis. At the lower temperatures, magnetic saturation was attained at 90 and 65 kG. The temperature-dependent saturation value was found to be 28 076 G·cm3/mole, corresponding to a gc = 2.011. At 90 kG the several nuclear spins were found to contribute a heat-capacity term equal to (6.0 × 10−3) / T2gibbs/mole. The amount of energy required to remove a quantum of angular momentum from the saturated condition at 90 kG was found to be 22.9 cal/mole. This is less than gβH = 24.16 cal/mole, the equivalent required magnetic work at 90 kG, by 1.3 cal/mole, which is the amount of stored internal energy contributed by the saturated condition for this limiting process and indicates antiferromagnetic interactions. Using magnetic saturation at 90 kG to locate a zero entropy reference for the electronic and lattice systems, the entropy was evaluated over the above range of field and temperature. Temperature–field observations on a large number of isentropes were used to correlate the entropies along the various isoerstedic heat-capacity curves. The upper limit of the electronic entropy was found to be 3.559 gibbs/mole, compared to the amount R ln6 = 3.561 gibbs/mole expected for an S = 5 / 2 system. Smoothed values of the heat capacity, entropy, enthalpy, internal energy, magnetic moment, and the magnetic work have been tabulated.