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 mum, multi-domain), a suspension of superparamagnetic magnetite particles (diameter ~;1 mum), and ferromagnetic cobalt nanoparticles (diameter ~;10 nm, 120 kA/m magnetization). Estimated detection limits are 20 mum diameter for a single cobalt particle at a water flow rate 30 ml/min, 5x103 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.

Magnetic resonance imaging (MRI) is a noninvasive and versatile methodology that has been applied in many disciplines1,2. The detection sensitivity of conventional Faraday detection of MRI depends on the strength of the static magnetic field and the sample "filling factor." Under circumstances where only low magnetic fields can be used, and for samples with low spin density or filling factor, the conventional detection sensitivity is compromised. Alternative detection methods with high sensitivity in low magnetic fields are thus required. Here we show the first use of a laser-based atomic magnetometer for MRI detection in low fields. Our technique also employs remote detection which physically separates the encoding and detection steps3-5, to improve the filling factor of the sample. Potentially inexpensive and using a compact apparatus, our technique provides a novel alternative for MRI detection with substantially enhanced sensitivity and time resolution while avoiding the need for cryogenics.