Magnetic characteristics of ferrimagnetic microspheres prepared by dispersion polymerization

A magnetite based colloid was obtained by chemical co-precipitation of iron(II) and iron(III) salts in alkaline medium and stabilized with oleic acid. Magnetic micron-size poly(2-hydroxyethyl methacrylate) (PHEMA)-based latex particles of narrow size distribution were prepared by dispersion polymerization in toluene/2-methylpropan-1-ol in the presence of three kinds of ferrimagnetic nanoparticles; chromium dioxide, maghemite, and magnetite. Cellulose acetate butyrate and dibenzoyl peroxide were used as the stabilizer and the initiator, respectively. The magnetic characteristics were examined with respect to behavior in the magnetic field and thermal stability. Our results show that chromium dioxide and derived PHEMA particles are magnetically stable in moderate temperatures up to about 100degreesC. Maghemite particles are thermally stable up to 500degreesC. Measurements of the hysteresis loops and remanent magnetization showed that embedment of magnetic particles in organic polymer has practically no effect on their magnetic hysteresis. All the samples reached magnetic saturation in fields below 0.3 T (saturation of magnetite). Regarding separation by the magnetic field, ultrafine, superparamagnetic magnetite particles show the best performance because of their magnetic susceptibility, the highest measured here and the absence of coercive force.