Detailed measurements of the temperature and field dependence of the ac susceptibility χ for the spin glass Cu-${\mathrm{Mn}}_{\mathrm{x}}$ reveal a new cusp transition whose locus is a line in the χ-T plane, independent of x. We have formulated a modified mean-field theory incorporating spin clusters whose parameters are deduced from a fit to the data. We find a correlation length associated with the rotational coherence of the local field and identify the cusp line as a physical feature corresponding to the divergence of the correlation length at the transition temperature $T_g$. We find that our data are well represented by scaling relations both above and below $T_g$, and from these relations we evaluate the critical exponents δ, γ, δ’, and γ’.

• Received 28 October 1985

DOI:https://doi.org/10.1103/PhysRevB.34.4771