Particles (A) comprising a polycrystalline coat of magnetic ironoxide core with a diameter of 20 nm to 1 mu m, are new. Independent claims are included for: (1) preparation of (A) comprising mixing an aqueous ferric salt solution containing ferrous salt and ferric salt with an aqueous alkali solution in the presence of a synthetic (co)-polymer, starch or its derivative, dextran or its derivative, cyclodextran or its derivative, fatty acid, polysaccharide, lecithin and/or mono-, a di- or a triglyceride or its derivative to obtain a colloidal solution of (A), introducing (A) containing the solution into a magnetic gradient field, removing the magnetic field and extracting (A) in the gradient field； (2) a composition comprising (A)； and (3) a liquid comprising the composition.
