The ac magnetic susceptibility of the spin-glass ${\mathrm{Eu}}_{0.4}$${\mathrm{Sr}}_{0.6}$S is found to have a power-law dependence on frequency near the freezing temperature. Extrapolating to zero frequency causes the peak in the real component to sharpen and shift to a lower temperature ${\mathrm{T}}_{\mathrm{c}}$. That ${\mathrm{T}}_{\mathrm{c}}$ marks a phase transition is supported by exponents describing the divergence of the relaxation time, development below ${\mathrm{T}}_{\mathrm{c}}$ of the order parameter ${\mathrm{q}}_{\mathrm{EA}(\mathrm{T}}$), suppression of ${\mathrm{T}}_{\mathrm{c}}$ by a magnetic field, and scaling of the nonlinear susceptibility.

• Received 2 July 1986

DOI:https://doi.org/10.1103/PhysRevLett.59.128