Studies of anisotropy mechanisms in polyphosphate-treated magnetic iron oxide particles

Abstract

The coercive force of small, acicular (⋍ 2000 Å length, ⋍8:1 length-to-diameter ratio) spinel-type iron oxide particles increases substantially following surface treatment with sodium polyphosphate. Regardless of Fe 2+/Fe 3+ cation ratio, H c always attains a peak value when the polyphosphate/iron oxide weight ratio P/Fe is in the range 0.5–0.6, denoted (P/Fe) max. The maximum change in H c is observed when Fe 2+/Fe 3+ ≈ 0.10–0.15. When P/Fe⪢0.5–0.6, however, both magnitude and sign of the change in coercive force show strong dependence on the Fe 2+ content of the oxide, suggesting that the H c changes are caused by a magnetostrictive mechanism at these high treatment ratios. Calculated anisotropy field distributions of treated specimens show that both the mean anisotropy field δ H kδ G and predicted H c reach a peak when particles are treated at (P/Fe) max, where the distribution becomes very broad. At high treatment ratios both δ H kδ G and predicted H c decrease to values below that of the untreated oxide. Mössbauer studies of treated and untreated particles show no significant change in the environment of surface iron ions following treatement at P/FE = 0.5, but indicate a small increase in the Fe 3+ concentration of the particle core. When P/Fe = 2.5, however, the Fe 2+ concentration of the core increases markedly.