MAGNETOTHERMODYNAMICS OF MNCL2.4H2O .I. HEAT CAPACITY,ENTROPY,MAGNETIC MOMENT, FROM 0.4 DEGREES TO 4.2 DEGREES K WITH FIELDS TO 90 KG ALONG C CRYSTALLOGRAPHIC AXIS

The heat capacity and magnetic moment of a 3.934-cm-diam spherical single crystal of MnCl2·H2O 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·cm 3/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)/T2 gibbs/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.