Magnetic equilibrium noise in spin-glasses: Eu0.4Sr0.6S

Using a novel magnetometer technique probing a (15-m)3 volume of a sample directly on top of a dc-SQUID chip with integrated pickup loops and field coils, we measure simultaneously the magnetic fluctuation noise and the ac susceptibility of a spin-glass, Eu0.4Sr0.6S, as a function of temperature. The data demonstrate directly the validity of the fluctuation-dissipation theorem both above and below the freezing temperature. New results on low-frequency dynamics are presented for this sample. © 1986 The American Physical Society.