Morphology and porous texture of iron fine particles with relation to their magnetic properties

Abstract

The effects of silica film as thin as 0.4 nm on the variation of morphology of particles, in the process of reduction of microporous haematite particles with hydrogen gas, were investigated by means of gas adsorption, electron microscopy, X-ray diffraction and magnetic measurements. Microporous haematite particles were prepared by decomposition of the silica-coated goethite particles. A silica film of 0.4 nm thickness was coated on the surface of the goethite particles (length: 400 nm, width 25 nm). In the process of reduction, collapse and sintering of the particles were markedly suppressed by coating with ultra-thin silica film. In the reduced particles, iron crystallites were separated from each other forming iron particles of large porosity. By introducing heat-treatment of the silica-coated haematite particles at 700° C in air prior to reduction, highly acicular iron particles having the crystallites grown along the elongated direction of the particles were obtained. A marked increase in the coercivity of the iron particles from 1075 to 1530 Oe was attributed to the increase of the shape anisotropy.