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

Abstract

This work is a continuation of our previous magnetothermodynamic measurements on a 3.934‐cm‐diam spherical single crystal of MnCl2·4H2O in which case the field was directed along the c crystallographic axis. Here the heat capacity and magnetic moment have been measured between 0.3° and 4.2°K with stabilized solenoid fields of 0, 1, 5, 7.5, 10, 14, 18, 22, 25, 40, 65, and 90 kG, along the b crystallographic axis. At 0.716°K and fields of 90, 80, and 70 kG. the electron system was magnetically saturated. The derived temperature‐independent magnetic susceptibility was found to be − 1.6 × 10−4 cm3 mole−1 and the saturation moment of the temperature‐dependent system is 27 974 G·cm3 mole−1, equivalent to g b = 2.004 . At 90 kG the nuclear spins were found to contribute a heat capacity term of (5.9 × 10 −3 / T 2 ) gibbs mole−1. The amount of enthalpy required to remove quanta of angular momentum from the saturated condition at 90 kG was found to be 20.63 cal mole−1 of angular momentum. This is less than gβ H = 24.07 cal mole −1 , the magnetic work, by 3.44 cal mole−1, which is the amount of stored internal energy contributed to the saturated condition for this limiting process, and indicates antiferromagnetic interactions. Extrapolation of the magnetic moment–temperature curves to absolute zero yields the expression M = 1.056H + 9.0 × 10−7 H 2G·cm3 mole−1 at 0°K. On this slightly curved line the saturation value, 27 974 G·cm3 mole−1, is reached at a field of 25 920 G, and the work of magnetization, ∫ 0 H H d M , equals 8.73 cal mole−1 at 0°K. Temperature–field observations on 31 isentropes were used to correlate the entropies along 12 iso‐Oerstedic heat capacity series. The zero of electronic and lattice entropy was located from the low‐temperature values of the heat capacity series at 90 kG. The upper limit of the electronic entropy was found to be 3.559 gibbs mole−1, compared to R ln 6 = 3.561 gibbs mole −1 . Smoothed, correlated values of the heat capacity,entropy,enthalpy, internal energy, magnetic moment, iso‐Oerstedic temperature coefficient of the magnetic moment (which equals the isothermal derivative of entropy with respect to field), and the magnetic work have been tabulated.