Magnetothermodynamics of CuK2(SO4)2·6H2O. II. Magnetic Moment, Heat Capacity, Entropy from 0.5 to 4.2°K with Fields to 90 kG along the γ Magnetic Axis

Abstract

This is a continuation of our magnetothermodynamic study of a 3.912‐cm‐diam spherical single crystal of CuK₂(SO₄)₂·6H₂O. The magnetic moment and heat capacity have been measured over the range 0.5–4.2°K, with fields of 0, 0.5, 1.0, 2.5, 5, 10, 15, 25, 40, 65, and 90 kG directed along the $\gamma$ magnetic axis. At 0.5°K the magnetic moment reached limiting values in the field range 45–90 kG. ${\mathbf{M}}_{\gamma }(\mathrm{limit}\mathrm{)\hspace{0.17em}=\hspace{0.17em}}{\mathbf{M}}_{\gamma }(\mathrm{sat}{\mathrm{)\hspace{0.17em}+\hspace{0.17em}\chi }}_{\gamma }(\mathrm{dia})\mathbf{H}{\text{\hspace{0.17em}=\hspace{0.17em}5782.3−2.45\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−4}}H\mathrm{G}·{\mathrm{cm}}^3/\mathrm{mole}$ ,where − 2.45 × 10⁻⁴ is the molal diamagnetic susceptibility, and 5782 is the molal saturation value of the temperature‐dependent paramagnetic system. Thus, $g_{\gamma }\text{\hspace{0.17em}=\hspace{0.17em}2.071}$ . The heat capacities at 90, 65, and 40 kG approached zero below 1°K, thus giving a common zero reference for the entropy of the lattice and electron systems. The heat required to remove quanta of $\text{h\hspace{0.17em}/\hspace{0.17em}2π}$ from the saturated condition was slightly larger than the magnetic work $\mathrm{g\beta H}$ as expected for a ferromagnetic system. No evidence of hysteresis was observed at temperatures above 0.5°K. Temperature‐field measurements on 31 isentropes, which connected the isoerstedic heat capacity series, evaluated the entropy over the experimental range. The total electronic entropy was found to be 1.379 gibbs/mole compared to the expected $R\ln \text{2\hspace{0.17em}=\hspace{0.17em}1.3775}\mathrm{gibbs}/\mathrm{mole}$ . Smoothed values of the heat capacity, entropy, magnetic moment, enthalpy, internal energy, differential isothermal magnetic susceptibility, differential isoerstedic temperature co‐efficient of magnetic moment, and the isothermal work of magnetization are tabulated.