Activated dynamic scaling in spin glasses above the freezing temperature

Dynamic-susceptibility data on ${\mathrm{Eu}}_{0.4}$ ${\mathrm{Sr}}_{0.6}$S spin glass above its transition temperature are found to obey an activated (logarithmic) dynamic scaling form, implying a generalized Vogel-Fulcher relation $\tau \propto \exp \left[\frac{c}{{(T-T_g)}^{\nu \theta }}\right]$ with $\nu \theta \sim 0.65$. With use of a fractal-cluster model, a novel scaling relation for another of the exponents is derived and tested experimentally. The "$\frac{\pi }{2}$ rule" relating real and imaginary susceptibilities is derived for systems obeying activated scaling.