Magnetothermodynamics of gadolinium gallium garnet. II. Heat capacity, entropy, magnetic moment from 0.5 to 4.2°K, with fields to 90 kG, along the [111] axis

Abstract

The magnetic moment of a 2.44 cm diam spherical single crystal of gadolinium gallium garnet has been measured with stabilized fields of 500, 1000, 2500, 5000, 10 000, 15 000, 25 000, 40 000, 65 000, and 90 000 G along the [111] axis, over the range 0.35–4.2°K. The heat capacity was measured at the same fields and over the range 0.5–4.2°K, except that 500 G was omitted and zero field included. The magnetic moment reached saturation at 0.35°K and 90 000 G. Within the limit of error, a zero of entropy for the electronic and lattice systems was located at 90 000 G. By means of temperatures measured on 39 adiabatics, corrected with respect to entropy, which intersected the various isoerstedic heat capacity series, the entropies were evaluated as a function of field and temperature. The sample had an excess of electronic entropy of 0.5% over the maximum R ln8 for Gd³⁺ ion. Thus the sample must have contained 0.5% excess gadolinium over the stoichiometric amount for Gd₃Ga₅O₁₂. Using the related correction factor 0.9950, the saturation value for Gd₃Ga₅O₁₂ is found to be M_sat[111] = 38 979 G · cm³/mole Gd³⁺, corresponding to a g_[111] = 1.994±0.002. A correction of M_sat[100], taken from J. Chem Phys. 59, 4652 (1973), using the correction factor 0.9950, gives M_sat[100] = 39 056 G · cm³/mole Gd³⁺, corresponding to g_[100] = 1.998±0.002. Smoothed correlated values of the heat capacity, entropy, enthalpy, internal energy, magnetic moment and its differential isoerstedic temperature coefficient, differential isothermal magnetic susceptibility, and the isothermal work of magnetization have been tabulated over the ranges 0–90 kG and 0.5–4.2°K. The problem of location of the zero of entropy in a system where nuclear spin contributes to the heat capacity has been discussed.