  We demonstrate the detection of magnetic particles carried by water in a
continuous flow using an atomic magnetic gradiometer. Studies on three types of
magnetic particles are presented: a single cobalt particle (diameter ~150 um,
multi-domain), a suspension of superparamagnetic magnetite particles (diameter
\~1 um), and ferromagnetic cobalt nanoparticles (diameter ~10 nm, 120 kA/m
magnetization). Estimated detection limits are 20 um diameter for a single
cobalt particle at a water flow rate 30 ml/min, 5x10^3 magnetite particles at
160 ml/min, and 50 pl for the specific ferromagnetic fluid at 130 ml/min.
Possible applications of our method are discussed.
