Coercivity enhancement in γ-Fe2O3 by surface treatment with phosphate ions

Abstract

Significant increases in coercivity were observed for stoichiometric γ-Fe2O3 pigments that were treated with different phosphate ions. The objectives of this investigation were to determine if the effect was specific to the phosphate anions as well as establishing the influence, if any, of induced compressive stress in coercivity enhancement. Stoichiometric γ-Fe2O3 pigments were treated with aqueous solutions containing the sodium salts of different anions and the changes in coercivity measured by vibrating sample magnetometry. The surface treatments were done in an aqueous suspension, followed by water evaporation, and vacuum treatment at 150–160 °C. Sodium phosphate, sodium pyrophosphate, sodium tripolyphosphate, sodium metaphosphate, poly(sodium acrylate), and sodium p-toluenesulfonate were used and the weight ratio of salt to pigment was fixed at 1.5. Within the range of anions in this work, the coercivity enhancement was specific to the polyphosphate anions. The coercivity for pigments treated with sodium p-toluenesulfonate (273 Oe) or poly(sodium acrylate) (280 Oe) was only slightly higher than 265 Oe for the as-received pigments. For the phosphate anions, the coercivity enhancement increased in the order sodium phosphate (268 Oe), sodium pyrophosphate (287 Oe), sodium tripolyphosphate (316 Oe), sodium polyphosphate (336 Oe). To evaluate the effect of stress on the coercivity, samples treated with sodium polyphosphate were ground to a powder and then pressed into pellets. The coercivity for the samples decreased linearly with increasing applied pressure during pellet formation. Therefore applied stress does not account for the coercivity increase.