H c enhancement of Co-adsorbed γ-Fe2O3 particles via surface treatment with sodium polyphosphate

Publisher Website
Google Scholar
Abstract
The coercive force HC of partially reduced, acicular γ‐Fe2O3 particles having an adsorbed Co layer (4 wt % Co; Fe+2/Fe+3≊0.09; 2000 Å×240 Å particle dimensions; 36 m2/g specific surface area) increases substantially following surface treatment with (NaPO3)n≊14⋅Na2O. The maximum observed effect occurs at a polyphosphate‐to‐iron oxide weight ratio P/Fe near 0.5, denoted (P/Fe)max, where HC of the Co‐adsorbed oxide increases from 687 to 1096 Oe immediately following treatment. The latter value increased to 1215 Oe after a four month storage in an argon environment containing less than 1 ppm H2O and O2, suggesting that the polyphosphate‐oxide interaction is sensitive to moisture or oxygen. Co‐adsorbed particles containing no Fe+2 in the oxide core also exhibit HC enhancement following surface treatment, but the change in HC is substantially less than that of particles containing Fe+2. The general behavior of polyphosphate‐treated Co‐adsorbed particles is observed to be similar to that of treated berthollide particles having no adsorbed Co layer: (i) the magnitude and sign of change in HC varies with P/Fe, reaching a peak value at (P/Fe)max, (ii) the magnitude of increase in HC at (P/Fe)max depends on the Fe+2 content of the particle core, (iii) saturation magnetization of the particles treated at (P/Fe)max is essentially unchanged from that of the untreated precursor, (iv) HC of specimens treated at (P/Fe)max returns to the value of the untreated precursor when the polyphosphate coating is removed via washing with water, and (v) particle dissolution occurs when P/Fe≫(P/Fe)max. Studies of anisotropy field distributions show that there is no obvious bimodal character to the very broad distribution of anisotropy fields found in randomly oriented specimens of surface‐treated particles.