MAGNETIZATION REVERSAL MEASUREMENTS OF SIZE-SELECTED IRON-OXIDE PARTICLES PRODUCED VIA AN AEROSOL ROUTE

We report first measurements of the magnetization reversal of monodisp erse 30 nm and 50 nm ferromagnetic Fe3O4 particles. These particles ar e produced in a carrier gas as an aerosol by spray pyrolysis. After pr oduction and size selection, they are precipitated on a silicon chip w ith a niobium SQUID (superconducting quantum interference device) inco rporated on its surface. By changing a magnetic field in the plane of the SQUID, we can measure the magnetization reversal of the particles by the flux they induce into the SQUID. The angular dependence of this reversal is determined by rotating the magnetic field around the SQUI D. Scanning electron microscope (SEM) images have confirmed the partic le size and revealed the position of the collected particles. If the p article concentration is too high, we cannot detect changes in the mag netic moment of a single particle, but measure the magnetic properties of the whole assembly. If only a few particles are found on the SQUID loop the angular dependence of the magnetic reversal of a single part icle can be measured; this result is compared with a simple model of m agnetization reversal. (C) 1998 John Wiley & Sons, Ltd.