Electrostatographic carrier materials having low bulk densities and high magnetic permeabilities are obtained by providing an alumino-boro-silicate glass particle containing from between about 10 to about 15 molar percent Fe.sub.2 O.sub.3 in which superparamagnetic ferrite crystallites having an average...http://www.google.fr/patents/US4124735?utm_source=gb-gplus-shareBrevet US4124735 - Magnetic glass carrier materials
