High sensitivity SQUID-detection and feedback-cooling of an ultrasoft microcantileve

We measure the motion of an ultrasoft cantilever, carrying a ferromagnetic particle, by means of a superconducting quantum interference device(SQUID). In our scheme, the cantilever motion modulates the magnetic flux in the SQUID due to the coupling with the magnetic particle. For the cantilever fundamental mode, cooled to temperatures below 100 mK, we achieve a dimensionless coupling factor as large as 0.07, displacement sensitivity of 200 fm/Hz---?, and subattonewton force sensitivity. We demonstrate the outstanding combination of very low displacement and force noise by feedback-cooling the cantilever mode to an effective mode temperature of 160 ?K.