H c enhancement in partially reduced γ-Fe2O3 via surface treatment with sodium polyphosphate (revisited)

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Abstract
The coercive force of acicular, nonstoichiometric (partially reduced) γ‐Fe2O3 particles is increased after surface treatment with solutions of sodium polyphosphate. The coercive force of 0.6×0.1 μm particles having Fe+2/Fe+3=0.134 increases from 450 to 670 Oe when treated with a 0.15 g/ml aqueous solution of (NaPO3)n≊14⋅Na2O. The magnitude of the change in the coercive force is dependent upon the relative amounts of polyphosphate and iron oxide present during treatment, but is independent of the polyphosphate solution concentration within the range 0.05–0.15 g/ml and the solution pH within the range 7–9. The enhanced coercivity values decrease slowly with time when the treated particles are exposed to air. In contrast to the partially reduced oxide, enhanced coercive forces are not observed in stoichiometric γ‐Fe2O3 after similar treatment. These results are qualitatively consistent with earlier reports that Hc can be increased in nonstoichiometric γ‐Fe2O3 via surface treatment with chainlike sodium polyphosphates. Possible mechanisms for this behavior are discussed.