Using a novel magnetometer technique probing a ${(15-\mathrm{\mu }\mathrm{m})}^3$ 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, ${\mathrm{Eu}}_{0.4}$${\mathrm{Sr}}_{0.6}$S, 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.

• Received 18 April 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.905