Temperature dependence of the magnetic 1/f noise in Eu_0.4Sr _0.6S (invited)

We analyze in detail a novel integrated SQUID technique for studying magnetic noise and susceptibility. In particular, we show how the susceptibility, π/2 rule and the fluctuation-dissipation theorem (FDT) are affected by large demagnetizing factors and by the geometry of our pickup and field coils. We present data on the spin glass Eu0.4Sr0.6S as a function of temperature. We confirm the FDT and show evidence against the conventional power-law dynamics in the spin-glass phase.