Presented is a novel high sensitivity magnetometer allowing us to measure the magnetization reversal of about 104&mgr;Bcorresponding to a sensitivity of about 10−16emu. The detector is a niobium micro‐bridge DC superconducting quantum interference device (SQUID), fabricated using electron‐beam lithography. It is operational in the temperature range of 0–7 K. Furthermore, we present a method to deposit on the SQUID loop a small number of Co clusters of about 2–5 nm in diameter. The first results obtained on these samples show that there is still a ferromagnetic coupling between the clusters and the magnetization reversal takes place by small avalanches. ©1995 American Institute of Physics.