Anomalous increase of coercivity in iron oxide powder coated with sodium polyphosphate

An anomalous increase of coercivity in a non-stoichiometric acicular iron oxide powder which was treated with a chain like sodium metaphosphate was investigated. The coercivity increased by a factor of about 2 in the acicular iron oxides and about 3 in the granular one, respectively. The magnetization decreased in inverse proportion to the increased coercivity. The coercivity of the treated iron oxides which was washed with water to remove the sodium metaphosphate returned to the value or to less than the value of the untreated iron oxides. The large magnetic anisotropy was estimated from the magnetic hysteresis loop. A constricted hysteresis loop was observed in treated iron oxides. The experiment suggests that an origin of the increased coercivity is due to the surface magnetic anisotropy which is based on the electrostatic coupling, participating in a absorbed compound.