Spectroscopy of RKKY spin glasses (abstract)

We have measured the nonlinear susceptibilities χ4, χ6, χ8 of dilute AgMn spin glasses above and below the transition temperature Tg as a function of temperature, magnetic field, and frequency. In the static limit, these quantities display very well-defined singularities at Tg with very weak magnetic field dependence. At very low frequencies, we show that the nonlinear susceptibilities χ2n can be related to 2n spin-spin correlation functions by Kubo formulas, and that their measured critical exponents are consistent with the usual scaling hypothesis. Thus, we can infer β=0.9, γ=2.1, and δ=3.0. The nonlinear susceptibilities scale above the transition as power laws of frequency with exponents which depend weakly on temperature. To relate these exponents by dynamic scaling require several ansatz which validity are investigated. In a magnetic field, only one singularity is observed at the same temperature Tg as in zero field, but the frequency dependence of the nonlinear susceptibilities weakens below the transition. We also discuss the frequency dependence of the linear and nonlinear susceptibilities below the spin-glass transition.