Dynamic scaling in spin glasses

We present neutron spin echo (NSE) results and a revisited analysis of historical data on spin glasses, which reveal a pure power-law time decay of the spin autocorrelation function s(Q, t)=S(Q, t)/S(Q) at the glass temperature T_g. The power law exponent is in excellent agreement with that calculated from dynamic and static critical exponents deduced from macroscopic susceptibility measurements made on a quite different time scale. This scaling relation involving exponents of different physical quantities determined by completely independent experimental methods is stringently verified experimentally in a spin glass. As spin glasses are a subgroup of the vast family of glassy systems also comprising structural glasses and other noncrystalline systems the observed strict critical scaling behavior is important. Above the phase transition the strikingly nonexponential relaxation, best fitted by the Ogielski (power-law times stretched exponential) function, appears as an intrinsic, homogeneous feature of spin glasses.