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

The zero-point entropy of a spin glass is a dificult 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 Tf = 2:89 0:05K for CuGa₂O ₄ and Tf = 2:30 0:05K for CuAl₂O₄. By integrating the specific heat data we have found that CuGa₂O ₄ 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 CuAl₂O ₄ have a greater degeneracy in their ground states than any of the spin glass models.