  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 Tf = 2.89 +/- 0.05 K
for CuGa2O4 and Tf = 2.30 +/- 0.05 K for CuAl2O4. 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.
