Zero-point entropy of the spinel spin glasses CuGa₂O₄ and CuAl₂O₄

The zero-point entropy of a spin glass is a difficult property to experimentally determine and interpret. Spin glass theory provides various predictions, including unphysical ones, for the value of the zero-point entropy, however experimental results have been lacking. We have investigated the magnetic properties and zero-point entropy of two spinel Cu²⁺ based spin glasses, CuGa₂O₄ and CuAl₂O₄. Dc- and ac-susceptibility and specific heat measurements show many characteristic spin glass features for both materials. The spin glass freezing temperature is determined to be T_f 2.89 0.05 K for CuGa ²O⁴ and T^f 2.30 0.05 K for CuAl ²O⁴. By integrating the specific heat data we have found that CuGa₂O₄ and CuAl₂O₄ have zero-point entropies of S₀ 4.96 JK^-1mol^-1 and S₀ 4.76 JK^-1mol^-1 respectively. These values are closest to the prediction for a Sherrington-Kirkpatrick XY spin glass, however they are notably higher than all of the theoretical predictions. This indicates that CuGa₂O₄ and CuAl₂O₄ have a greater degeneracy in their ground states than any of the spin glass models.