Anomalous Increase of Coercive Force of Acicular Iron Oxide Powders after the Treatment with Sodium Metaphosphate

Abstract

The surface magnetic anisotropy of the non-stoichiometric iron oxide powders treated with sodium metaphosphate, (NaPO₃)_n, was investigated. In the treated iron oxide powders, anomalous increase in the coercive force and the decrease in the magnetization which was inversely proportional to the coercive force were observed. The maximum coercive force was obtained at a mole ratio of 1.7 (NaPO₃/iron oxide). The coercive force and magnetization of the treated iron oxide powders were restored to the original values after the removal of the coating material by washing with water.
The peak value shown in Fig.3 suggests that the increase in the coercive force neither depends on the decrease in the particle interaction nor on the increase in the magnetostriction of the particle. It might be possible that a large increase in the surface magnetic anisotropy of a particle is caused by a bonding between phosphorous and iron via oxygen ion.