Critical dynamics of metallic spin glasses

We have measured the ac nonlinear susceptibilities χ₃', χ₅', and χ₇' of very dilute AgMn spin glasses above and below the transition temperature Tg as a function of frequency, temperature and magnetic field. In the static limit, these quantities display well-defined critical singularities at Tg and imply a divergence of the spin-glass correlation length. The nonlinear susceptibilities can be fitted above Tg to powers of frequency with weakly temperature-dependent exponents. While the effective exponents satisfy the relations imposed by static and dynamic scaling, the observed temperature and frequency roundings suggest that the spin-glass correlation length does not actually become infinite and is cut off at a length scale of 2000 A{}. Activated behavior in the vicinity of Tg with a logarithmic dynamic scaling also accounts for the measured dynamical nonlinear susceptibilities. The measured critical exponents ({β}=0.9, {γ}=2.3, {δ}=3.3, {ν}=1.3, z=5.4) are not mean field, and differ from the exponents obtained by Monte Carlo simulation of short-range Ising systems.