The magnetic or magnetizable particles to be employed for the present invention may include, for example, gamma-hematite (??-Fe2 O3), magnetite (Fe3 O4), gamma-hematite or magnetite doped with transition metal elements, such as cobalt, iron oxides of non-stoichiometric oxidation compounds between gamma-hematite and magnetite, said iron oxide coated primarily with cobalt oxide or hydroxide, chromiu